Resolving SM 332 6ES7332-5HF00-0AB0 SF LED Fault on S7-300

1. Problem Statement

The SIMATIC S7-300 analog output module SM 332 with order number 6ES7332-5HF00-0AB0 shows a continuously lit red group fault LED (SF) on a station built around a CPU 313 DP. STEP 7's online module information shows the following diagnostic events:

• External error
• Faulty module
• No external auxiliary voltage

The user has already eliminated the obvious suspects: the two modules have been swapped (the working one stays good, the suspect one stays bad regardless of slot), the backplane bus connector has been swapped, and the front connector with the backshell wiring has been moved from the working module to the suspect module with no change in behavior. The fault follows the module; it does not follow the slot, the wiring, or the backplane connector. The diagnostic event points to a load-voltage problem, but the affected module was already moved, so the load-voltage problem is on-board the module rather than in the field wiring.

That interpretation is almost always wrong. The "No external auxiliary voltage" diagnostic on the SM 332 is a group-level diagnostic that fires whenever the on-board voltage monitor at the L+/M terminals of the front connector sees a load voltage outside the allowed window — typically below 20.4 V or above 30 V. It is reported as an external error because the firmware treats the load supply as externally provided. The accompanying "Faulty module" classification is the diagnostic class assigned by STEP 7 to the group fault; it does not mean the module's electronics are damaged.

The field observations in this case (fault follows the module, not the slot or wiring) are consistent with a load-voltage measurement error caused by:

1. Worn or oxidized front connector pins on the suspect module that drop voltage under load.
2. A defective on-board load-voltage supervisor (rare).
3. An intermittent wiring fault that was not transferred with the connector swap (e.g., broken wire inside the backshell strain relief, loose screw that vibrates loose).

The procedure below identifies which of these applies, fixes the cause, and verifies the fix.

2. Module Identification and Hardware Properties

The 6ES7332-5HF00-0AB0 is a member of the SIMATIC S7-300 SM 332 analog output family. The Siemens Industry Online Support product page for this part number documents the high-level properties:

Table 1 — SM 332 (6ES7332-5HF00-0AB0) Hardware Properties

Property	Value
MLFB / Order number	6ES7332-5HF00-0AB0
Product family	SIMATIC S7-300 / SM 332
Function	Analog output module
Number of outputs	8
Output group structure	1 group of 8 channels
Resolution	12 bits
Diagnostic capability	Programmable, group-level and channel-level
Diagnostic interrupt	Yes (configurable in HW Config)
Galvanic isolation	Yes (to backplane and to channel outputs)
Load voltage (L+/M)	24 V DC nominal, 20.4–28.8 V permitted
Front connector	20-pin (6ES7392-1AJ00-0AA0 screw, 6ES7392-1BJ00-0AA0 spring)

Source: Siemens Industry Online Support — 6ES7332-5HF00-0AB0 (ID 8859629).

The 8 outputs share a single group; the group has one set of load-voltage terminals (L+ and M) on the front connector. A loss or droop of the load voltage on those terminals is the only hardware condition that can produce the "No external auxiliary voltage" diagnostic. The module's other diagnostics (wire break, short circuit, parameter error, internal fault) have their own distinct event texts and are not confused with this one.

3. SF LED, Diagnostic Events, and the Diagnostic Buffer

The SM 332 faceplate has two functional LEDs:

Table 2 — SM 332 Faceplate LEDs

LED	Color	Meaning	Off when	On when
24 V	Green	Load voltage at L+/M is present and within tolerance	L+ missing, reversed, or below 20.4 V; or L+ and M reversed with reverse-polarity protection active	20.4 V ≤ VL+−M ≤ 28.8 V
SF	Red	Group fault; one or more diagnostic events are active	No active diagnostic events	At least one group-level, channel-level, or internal fault is active

The two LEDs are independent signals. The 24 V LED is a hardware-level indication of the load voltage. The SF LED is driven by the module's firmware based on the active diagnostic events. In a healthy module with no faults, both LEDs are in their expected state for the corresponding condition: specifically, the 24 V LED is lit and the SF LED is dark.

When the load voltage is missing, both LEDs indicate the fault: the 24 V LED is off because there is no L+ supply, and the SF LED is on because the firmware has raised the "No external auxiliary voltage" diagnostic. The user's report — SF LED on continuously — is consistent with the load voltage being either missing or out of tolerance. The green 24 V LED state is the first measurement point in the diagnostic flow.

The diagnostic buffer entry is reported to the CPU as part of the module's diagnostic data record (DS0 / DS1). The CPU's diagnostic buffer shows the same text as the module's online diagnostic in STEP 7, prefixed with the slot number and the rack/stack position. A typical entry looks like:

16:42:18.407  2019-03-14
  Station failure (module removed/inserted)
  Slot 4   SM 332 (6ES7332-5HF00-0AB0)
  External error
  Faulty module
  No external auxiliary voltage
  Incoming event

Three details matter here:

1. The event is "External error" by classification. This means STEP 7's diagnostic class is "external", which corresponds to a process-side or wiring-side cause, not an internal module fault.
2. The "Faulty module" tag is the diagnostic-level indicator (it indicates a fault is pending on the module), not a statement that the module is broken.
3. The "Incoming event" / "Outgoing event" pair tells you whether the condition is currently active or has cleared.

The "External error" + "Faulty module" combination, with "No external auxiliary voltage" as the cause, should be read as "the module has reported an external fault (load voltage), and that fault is still active". It should not be read as "the module is faulty". This semantic distinction is the source of most of the confusion in the original problem report.

4. Root Cause Analysis

Working from the diagnostic event, the root cause space is limited to the L+/M load-voltage path. Excluded by the diagnostic text are:

• Channel-level wiring faults (those would report as wire break or short circuit per channel).
• Parameter assignment errors (those would report a different diagnostic text).
• Backplane communication faults (those would report "Module faulty" or "Module does not exist").
• CPU-side configuration errors (those would not generate a module-level diagnostic at all).

The remaining causes, in declining order of frequency in the field:

Table 3 — Root Cause Matrix for "No External Auxiliary Voltage"

Rank	Root cause	How it presents	Field check
1	L+ or M wire loose, broken, or landed on wrong pin of the front connector	SF on continuously; 24 V LED off; 0 V at L+/M on the module	Pull the front connector and verify each wire's pin assignment with a continuity tester
2	External 24 V supply off, tripped, or fuse blown	SF on; 24 V LED off; 0 V at the supply output; SITOP/PSU LED off or red	Measure the supply output and check its status LED
3	Polarity reversed (M on L+, L+ on M)	SF on; 24 V LED off; meter reads −24 V across the module's L+/M	Measure with a DMM on DCV; reverse the two wires
4	Voltage drop in long, undersized cable between supply and module	SF on; 24 V LED off or dim; supply reads 24.0 V; module reads < 20.4 V	Measure at the supply and at the module; calculate the drop
5	Worn or oxidized front connector pins on the suspect module (intermittent contact under load)	SF on; 24 V LED off; 0 V or unstable V at the module; works on a different module in the same slot	Replace the front connector; inspect the suspect module's pins for oxidation
6	Defective on-board load-voltage supervisor (rare)	SF on; 24 V LED off; 24.0 V confirmed at L+/M; no other cause found	Module replacement; return to factory for repair

Items 1–5 are wiring or supply issues and account for the overwhelming majority of cases. Item 6 is rare and is only concluded after items 1–5 have been measured and excluded.

5. Prerequisites

Before starting the diagnostic, confirm the following are available:

• STEP 7 V5.5 or V5.6 installed on the engineering station (TIA Portal with the S7-300 HSP is also acceptable, but the menu paths below are written for STEP 7 classic).
• Online connection to the CPU 313 DP via MPI, PROFIBUS, or Ethernet (CP 343-1).
• Multimeter with DC voltage range, 0.1 V resolution, true RMS not required.
• Small flat-blade screwdriver matching the front connector screws (0.5 × 3.0 mm for the 6ES7392-1AJ00-0AA0 screw-type connector).
• Continuity tester or buzzer for the wiring check.
• Spare 20-pin front connector (6ES7392-1AJ00-0AA0 screw or 6ES7392-1BJ00-0AA0 spring).
• Project file (HW Config + symbol table + program) for the station, in case the configuration must be reviewed.
• Siemens Industry Online Support account, free, for downloading the latest module manual and firmware information.

If the station is part of a safety-relevant system, follow your site's lockout / tagout (LOTO) procedure and obtain a work permit before removing the module from the rack.

6. Step-by-Step Diagnostic Procedure

The procedure above is a decision tree that ends in one of three outcomes: the load voltage must be restored (Section 7), the front connector or the module's pin block must be replaced (Section 9), or the module itself must be replaced (Section 10). Run the steps in order; do not skip the wiring check just because the front connector was already swapped.

1. Open STEP 7 and go online to the CPU 313 DP. In HW Config, double-click the suspect SM 332 slot. In the "Diagnostics" tab of the module's Properties dialog, confirm that "Group diagnostics" is enabled for the output group and that "Diagnostic interrupt" is enabled. If both are disabled, the module's diagnostics are masked; enable them and download the hardware configuration.
2. Open the module's online diagnostic in STEP 7: PLC > Module Information. Click the "Diagnostic Buffer" tab. Confirm that the most recent entry matches the "No external auxiliary voltage" text from the field report. Note the timestamp and the "incoming" / "outgoing" status.
3. With the station powered, look at the suspect module's faceplate. The green 24 V LED state is the first measurement:
   • 24 V LED on, SF LED on: load voltage is present, but the module's on-board supervisor has flagged a problem. Go to step 5.
   • 24 V LED off, SF LED on: load voltage is missing or out of tolerance. Go to step 4.
   • 24 V LED off, SF LED off: no fault, transient cleared. Go to step 7 (verification).
4. Measure V_L+−M at the suspect module's front connector terminals using a DMM on DC voltage. The probes go on pin 1 (L+) and pin 20 (M). With the module powered:
   • If V = 0 V: open circuit. Check the wiring from the 24 V supply to the module (Section 7).
   • If V is negative: polarity reversed. Swap L+ and M at the supply end of the cable (Section 7).
   • If V < 20.4 V: voltage drop in the cable. Measure at the supply output and calculate the drop; check the cable gauge and length (Section 7).
   • If V is 20.4 V to 28.8 V: load voltage is in tolerance; the fault is on the module. Go to step 5.
5. Remove the front connector from the suspect module. Inspect the backshell wiring for broken strands, loose screws, or wires that have pulled out of the strain relief. Inspect the module's pin block for oxidation, contamination, or bent pins. Clean oxidation with a pencil eraser or isopropyl alcohol; replace the pin block (Section 9) or the module (Section 10) if pins are damaged.
6. Reinstall the front connector. Power the station and re-check the diagnostic buffer. If the diagnostic clears, the issue was a poor connection at the front connector. If it persists, go to step 7.
7. Replace the suspect module with the spare. Power the station and re-check. If the diagnostic clears on the spare, the original module is faulty (Section 10). If the diagnostic persists on the spare, the issue is in the wiring or the supply (Section 7).

Most cases of "No external auxiliary voltage" clear at step 4 or step 5. Only a small fraction reach step 7.

7. Wiring and Load-Voltage Verification

The 24V DC load supply for the SM 332 output group is fed to pins 1 (L+) and 20 (M) of the 20-pin front connector. The exact pin assignment is printed on the wiring diagram inside the hinged front door of the module and in the module manual. Both pins must be on the same group; the module does not have multiple load-supply inputs for the single 8-channel group.

Table 4 — SM 332 Front-Connector Pin Assignment (Output Group)

Pin	Signal	Function
1	L+	24V DC load supply, positive
20	M	24V DC load supply, return
3	M	AO group reference (must be tied to PE-side ground in the cabinet)
4	AOV0+	Channel 0 voltage/current output positive
5	AOV0−	Channel 0 voltage/current output return
…	…	…
18	AOV7−	Channel 7 return

The wiring verification procedure is:

1. Open the front connector's hinged cover. Identify the screws for pins 1 (L+) and 20 (M). Use a multimeter on continuity to verify each wire goes back to the 24V supply's positive terminal and ground terminal, respectively.
2. With the system powered, measure V_L+−M directly at the screw terminals of the front connector. A reading of 20.4 V to 28.8 V indicates the wiring and supply are good. A reading outside that range points to the supply (if 0 V or negative) or to the cable (if the supply shows 24 V but the module shows less).
3. Calculate the voltage drop in the feeder cable:

   V_drop = I_total × 2 × ρ × L / A
        = I_total × 2 × 0.0175 Ω·mm²/m × L / A
   

   where I_total is the total current drawn by the module on the L+/M feeder (quiescent + all channel loop currents), L is the one-way cable length in metres, A is the conductor cross-section in mm², and the factor of 2 accounts for the round trip on the positive and return conductors. For a 6ES7332-5HF00-0AB0 the quiescent draw on L+/M is in the order of 250 mA; add the sum of the channel loop currents to get I_total.

   For a typical installation with eight 4-20 mA channels at full scale, I_total ≈ 0.25 A + 8 × 0.02 A = 0.41 A. With L = 20 m of 0.75 mm² copper cable, the drop is:

   V_drop = 0.41 × 2 × 0.0175 × 20 / 0.75
          = 0.38 V
   

   This is well within the 3.6 V budget (24 V − 20.4 V). For a 100 m run on the same gauge, the drop would be 1.9 V — still acceptable. Beyond that, the gauge must be increased. The 24 V supply tolerance is 20.4–28.8 V, so the maximum allowed drop on the cable is:

   V_drop_max = V_supply_min − V_module_min
              = 24.0 V − 20.4 V
              = 3.6 V
   

   This budget must cover the entire cable, including the round trip.

4. If the calculated drop is greater than 3.6 V, replace the cable with a larger gauge. Re-measure to confirm V_L+−M ≥ 20.4 V at the module.
5. If the drop is within budget but the module's on-board supervisor still flags a fault, the supervisor itself is suspect. Proceed to Section 10 (module replacement).

8. STEP 7 Hardware Configuration Review

The "No external auxiliary voltage" diagnostic is reported only if "Group diagnostics" is enabled for the SM 332 output group in the hardware configuration. The default for a freshly inserted SM 332 in STEP 7 is "Group diagnostics = enabled", but it is possible (in older projects, in projects imported from other tools, or after a manual edit) for the option to be disabled. If group diagnostics is disabled, the module will not report the load-voltage fault at all — the SF LED will not come on, but the fault condition is still present.

Conversely, if group diagnostics is enabled but the diagnostic interrupt is disabled, the event is logged in the module's diagnostic buffer but OB82 is not called. The SF LED still comes on, but the CPU does not get a synchronous interrupt.

To verify the configuration:

1. In HW Config, double-click the SM 332 slot. The module's Properties dialog opens.
2. Click the "Outputs" tab. The output group is shown with checkboxes for "Group diagnostics" and "Diagnostic interrupt".
3. Confirm both are enabled. If either is disabled, enable it, save the project, and download to the CPU. The change is active after the next STOP-to-RUN transition or after a power cycle of the station.
4. Click the "Addresses" tab. Note the start address of the output word(s). The 6ES7332-5HF00-0AB0 occupies 8 output words (one 16-bit word per channel) for a total of 16 bytes. The start address is set in HW Config and must match the address used in the STEP 7 program.
5. Click the "Basic Parameters" tab (if visible). Confirm the output type for each channel is set correctly (voltage or current, with the correct range). An incorrect range is not the cause of the "No external auxiliary voltage" diagnostic, but it can mask other faults and should be verified while you are in the dialog.

For stations on TIA Portal, the equivalent path is Project tree > Devices > [CPU 313 DP] > [SM 332 slot] > Properties > Outputs. The options are identical in name and effect.

9. Module Replacement and Verification

If the diagnostic persists after the wiring, supply, and configuration checks, the suspect module is replaced. The replacement procedure is:

1. Bring the affected output group to a safe state in the STEP 7 program. The SF LED on a SM 332 with "No external auxiliary voltage" means all 8 output channels have been driven to their configured substitute value (typically 0 V or 0 mA). Confirm the downstream actuators (valves, drives, regulators) are in a safe state before removing power.
2. Power down the S7-300 station (or the affected expansion rack) per the site's LOTO procedure.
3. Open the front connector's cover. Loosen the two screws that lock the connector to the module. Pull the connector straight out. Do not pull on the wires.
4. Loosen the module's two mounting screws (top and bottom of the faceplate). Pull the module straight out of the rack by the faceplate handle.
5. Inspect the new module (the spare). Confirm the order number is 6ES7332-5HF00-0AB0 and the firmware version is compatible with the rest of the station. The firmware version is printed on the module's side label and is also reported in HW Config.
6. Insert the new module into the slot. Tighten the two mounting screws. Reinsert the front connector; tighten the locking screws.
7. Power up the station. The CPU goes through its restart sequence. The new module is parameterized from the project's HW Config. The SF LED should be off after the restart; the green 24 V LED should be on.
8. Go online in STEP 7 and check the module's diagnostic buffer. The new module's buffer should be empty (or contain only the initial "module initialized" entry). Force each output channel through the test program to confirm correct operation.

The replaced module should be sent to a Siemens repair center or to a qualified third-party repair shop. Do not discard the module without recording the fault — the diagnostic history is useful for the repair technician.

10. Verification Tests

After the fix, run the following verification sequence to confirm the SF LED stays off and the load voltage is stable:

1. Power the station and let it run for at least 15 minutes. The SM 332 has internal thermal protection; some load-voltage faults only present after the module warms up.
2. Measure V_L+−M at the module's front connector. Confirm the reading is stable and within 20.4–28.8 V.
3. Open STEP 7's module information. Confirm the diagnostic buffer is empty (or has no "No external auxiliary voltage" entries).
4. Force each of the 8 output channels to 0%, 50%, and 100% of range using the STEP 7 variable table (VAT). Confirm the corresponding actuator responds correctly at each step.
5. Disconnect L+ at the supply end for 1 second and reconnect. The module should raise the diagnostic, the SF LED should come on, and the diagnostic should clear when L+ is restored. This confirms the on-board supervisor is functional.
6. Document the fix in the station's maintenance log. Record the date, the failure mode, the corrective action, and the verification results.

11. Common Pitfalls and Field Notes

• "Faulty module" means "module has a fault", not "module is broken". STEP 7's diagnostic buffer text can be misleading. The SM 332 reports an external error (the load supply), and the "Faulty module" classification indicates that the fault is module-level, not that the module needs replacement.
• The green 24 V LED is the first measurement. If the 24 V LED is on, the load voltage is present and the module's on-board supervisor has a problem. If the 24 V LED is off, the load voltage is missing or out of tolerance. This narrows the search immediately.
• Polarity reversal does not destroy the module. The SM 332 has reverse-polarity protection on the L+/M input. A reversed L+ and M will not damage the module, but it will not power the analog outputs. The 24 V LED stays off; the SF LED comes on.
• The fault follows the module, but the cause does not. If the suspect module is moved to a different slot and the fault follows, the module's on-board supervisor is suspect. If the wiring was moved with the module and the fault stays, the wiring is suspect. The original poster's report — fault follows module, not slot, with connectors swapped — is consistent with a defective front-connector pin block on the suspect module or with a defective on-board supervisor.
• The 100 ms vs 20 ms module-removal response time is normal OB82 behavior. When a module is removed, the CPU detects the loss, calls OB82 (diagnostic interrupt), and updates the process image. The first time, the cycle is longer (typically 100 ms) because the CPU performs the full module-removal detection and OB82 invocation. On subsequent removal/restoration cycles, the cycle is shorter (typically 20 ms) because the module is already in the CPU's known topology. The 20 ms figure is the OB1 scan time minus the I/O update time and is not a fault indicator.
• Group diagnostics is not the same as channel diagnostics. The "No external auxiliary voltage" event is a group-level event. It is enabled by the "Group diagnostics" checkbox in HW Config. The "Diagnostic interrupt" checkbox, also on the same dialog, controls whether OB82 is called. Both must be enabled for the full diagnostic behavior.
• The S7-300 SM 332 is not the same module as the S7-300 SM 332 with HART (6ES7332-8TF01-0AB0 etc.). The HART variants have additional diagnostic events. The 6ES7332-5HF00-0AB0 is the non-HART 8AO/12-bit version. Mixing up the part number in HW Config will cause the module to refuse to start.
• CPU 313 DP, CPU 313C DP, and CPU 314 all support the SM 332. The maximum number of SM 332 modules per station depends on the addressing space of the CPU and the number of available slots in the rack. For a CPU 313 DP with a single expansion rack, the typical limit is 8 SM 332 modules. For larger configurations, verify against the CPU's manual.
• The 6ES7332-5HF00-0AB0 is the 12-bit version. The 6ES7332-5HB01-0AB0 and similar variants are the older 11-bit versions and are not drop-in replacements. The new module must be re-parameterized and re-commissioned.

12. Related Diagnostic Behaviors

The "No external auxiliary voltage" event is one of five group-level diagnostic events on the SM 332. The other four, and the differences between them, are useful for distinguishing the cause when the diagnostic is ambiguous:

Table 5 — SM 332 Group-Level Diagnostic Event Texts

Diagnostic text in STEP 7	Trigger condition	Effect on outputs
No external auxiliary voltage	Load voltage at L+/M is missing or out of tolerance	All channels drive to configured substitute value
Channel fault (wire break)	Current loop on a channel drops below 3.6 mA	Affected channel drives to substitute value
Channel fault (short circuit)	Voltage output on a channel is shorted	Affected channel drives to substitute value
Parameter assignment error	HW Config parameter does not match the module's actual range	Module refuses to start; SF on
Internal module fault	Module's self-test fails (rare)	Module refuses to start; SF on; CPU enters STOP if OB82 is not loaded

If the diagnostic text does not match the "No external auxiliary voltage" string, the troubleshooting procedure is different. Wire break, short circuit, and parameter assignment error each have their own specific fixes.

13. Frequently Asked Questions