Allen-Bradley PowerFlex 755 Aux Input Fault Fix

Quick answer

PowerFlex 755 Aux Input Fault (F2) means a digital input on the drive’s I/O — one that has been configured as the auxiliary fault input — went to its programmed inactive state. In plain terms: an external safety device wired to the drive (motor thermostat, vibration switch, low-oil pressure switch, conveyor pull-cord, e-stop chain pass-through) opened a contact that the drive is monitoring as an external fault interlock. The most common field cause is exactly what the device is telling you — the protected condition tripped, and the drive is correctly reporting that an external signal said “stop now.” A close second is a wiring problem: a broken interlock loop, a damaged dry-contact source, or a digital input parameter (DI Aux Fault) configured to monitor a contact that isn’t actually wired.

What PowerFlex 755 Aux Input Fault means

The PowerFlex 755 has six standard digital inputs on the main control board (DI 0 through DI 5), plus additional inputs available via option modules in slots 4, 5, and 6 (typically the 20-750-2262C-2R or similar I/O cards add 8 more digital inputs). Each digital input can be assigned a function in parameter group 155 DI A Function through 165 DI 5 Function (and the equivalent ranges for option-card inputs). One of the available functions is 18 Aux Fault — when a digital input is assigned function 18, the drive monitors that input and trips F2 Aux Input Fault when the input transitions from active to inactive.

The “active” state depends on parameter 175 DI Active Lvl for that input — most installations use the default of Active High, meaning 24 VDC on the input terminal is “OK” and 0 V is “fault.” But some installations invert this (Active Low) for fail-safe wiring schemes — a broken wire becomes a fault, not a runaway.

F2 is not a fault generated by the drive’s internal monitoring — it’s a fault the drive is reporting on behalf of an external device. So diagnosing F2 always starts at the external device wired to the configured input, not inside the drive cabinet.

Distinguish F2 from related faults: F3 Power Loss is the input-voltage / DC bus fault. F70 Control Sync is the cassette-to-power-module communication fault. F81 SI Comm Loss is a network communication fault. F2 is specifically the auxiliary external-input fault — it tells you a digital input did something, not that the drive itself misbehaved.

Read the fault history first

Read the fault log before pressing Clear Faults. Capture which digital input tripped so you know which external device to investigate.

Via Studio 5000 Logix Designer:

1. Open project, expand I/O tree, right-click PowerFlex 755 → Properties
2. Navigate to Drive → Faults tab
3. Most recent 32 faults appear with code, descriptor, and timestamp
4. Click the most recent F2 entry to see snapshot
5. Cross-reference with parameter 220 Digital In Status — bit field showing the state of every digital input at the moment of trip; the bit that is “off” is your fault source (if you wired Active High)
6. Also read parameter 271 Drive Logic Result for the overall logic state at trip

Via Connected Components Workbench or the HIM:

1. Press Esc to main menu
2. Navigate to Diagnostics → Fault Queue
3. Highlight the most recent F2 entry, press Enter for snapshot
4. Pull these parameters:
   • N003 — Last Fault Code (will read 2)
   • 220 — Digital In Status at trip
   • 221 — Digital Out Status at trip
   • 271 — Drive Logic Result
   • 155 through 165 — DI Function assignments (which input is configured as Aux Fault, function 18)

Then walk the parameter table for 155 DI A Function through 165 DI 5 Function — find the one set to value 18 Aux Fault. That’s the input you need to trace.

Rockwell documents this in PowerFlex 750-Series Programming Manual publication 750-PM001 chapter 3 (I/O configuration) and chapter 4 (Diagnostics).

Field insight — the F2 history trap: parameter 220 shows the digital input states at trip, but if your external device “bumped” briefly (e.g., a vibration switch flickered, a thermostat dithered around its setpoint) and then closed back up, you’ll see the bit reading “active” by the time you read it post-trip. The trip happened, but the snapshot shows current state, not trip state. Use parameter 226 DI Cumulative State Changes (if your firmware exposes it — newer firmware does) or set up a Studio 5000 trend on DI status to capture transient events going forward.

Common causes (ranked by frequency)

1. The protected condition actually tripped — motor thermostat opened on overtemperature, vibration switch tripped on bearing failure, low-oil-pressure switch tripped on lubrication failure, pull-cord activated, e-stop pressed
2. Broken or pinched interlock wire — wire in the interlock loop has a damaged conductor or insulation rub causing intermittent open; common in conveyor systems with cable trays subject to mechanical stress
3. Failed dry contact — the contact itself has worn out, oxidized, or developed high contact resistance; common in old motor thermostats and reed-switch vibration detectors
4. Loose terminal on drive digital input — wire backed out of the DIx terminal on the drive’s main control board or option-card I/O
5. Wrong DI configuration — parameter 155–165 set to Aux Fault for an input that isn’t actually wired, or the active level (175) is set wrong for the wiring scheme
6. Loss of 24V common to the interlock loop — the +24V source for the dry contacts failed; without source voltage every Active-High input reads inactive and the drive trips F2
7. EMI / induced voltage on a long interlock run — induced noise on a 100+ foot interlock cable trips a low-threshold input; address with twisted-pair shielded cable or relay isolation

Step-by-step diagnosis

Before opening anything: lock and tag the upstream disconnect only if you need to work inside the drive cabinet. For external-device troubleshooting (the more common F2 case) you may keep the drive de-energized for safety while the external device circuit is on a separate control transformer. If you do open the drive cabinet, wait the PowerFlex 755 capacitor discharge time — 5 minutes for frames 1–4, 15 minutes for frames 5–8. Verify zero DC bus with a CAT-IV meter rated 1000 VDC and respect the NFPA 70E arc-flash boundary on the cabinet label.

1. Read the fault history. Pull N003, 220, 221, 271 and review parameters 155–165 to identify which DI is set to function 18 Aux Fault. Note the timestamp — was it during a startup attempt, mid-run, or unprompted while idle? Each tells a different story.

2. Identify the external device. Follow the wiring from the drive’s DIx terminal back to the source. Use as-built drawings or your panel schematic. Common Aux Fault sources by application:

   • Conveyors: pull-cord switches, belt-misalignment switches, blocked-chute detectors
   • Pumps: flow switches, dry-run sensors, low-suction pressure switches
   • Fans: vibration switches (often on the bearing housing), belt-tension monitors
   • Mixers/agitators: motor thermostats (klixon-style PTC or NTC), bearing thermocouples
   • General: motor klixon overtemperature switches, oil-level switches

3. Test the external device. With drive de-energized (or with the external circuit isolated from the drive’s input), use a continuity meter on the device’s contact. Check expected state: should it be closed (continuity) when the protected condition is normal, open when tripped? Most safety devices are normally closed (NC) so a broken wire fails to “open” = fault, fulfilling fail-safe principles.

4. Inspect the wiring path. Walk the cable from device to drive cabinet. Look for visible damage, pinch points at panel entries, missing strain reliefs, or moisture intrusion at outdoor junction boxes. A multimeter on continuity mode along the interlock run will isolate a broken conductor — go through each junction.

5. Verify the +24V common. The drive’s DI inputs need a +24V common from somewhere — typically the drive’s internal 24V output (terminal X4 24V Aux Out on the main control card), the 20-750-APS auxiliary power supply, or an external 24VDC source. Measure the +24V at the input terminal block — if it reads 0V or significantly below 24V, the source has failed and every Active-High input will trip.

6. Check input terminal torque. With drive de-energized, verify the wire at the configured DI terminal is properly seated and the screw is torqued (typical spec 0.5 Nm / 4 in-lb for control terminals, but check the back of the drive cover for the exact value). A backed-out wire is a common cause of mysterious F2 events that came on after a panel build.

7. Verify DI Function assignment is correct. If the application has been recommissioned recently, someone may have changed parameters 155–165 and inadvertently assigned function 18 Aux Fault to an unused digital input that now floats. Read each DI Function parameter, confirm the assignment matches the actual wiring, change as needed.

8. Test for EMI on long interlock runs. If the device wiring runs more than 50 feet in a conduit shared with motor cables or other high-frequency sources, use a scope on the DI input to look for induced noise. The fix is twisted-pair shielded cable, ferrite cores on the cable at the panel entry, or relay isolation (use a separate interposing relay near the device and run only short clean wires to the drive).

9. For chronic intermittent F2: set up Studio 5000 trending on parameter 220 Digital In Status to capture the exact moment of each future trip. The trend resolution will tell you whether the dropout is a clean transition (real device trip) or a noise spike (EMI / vibration / loose contact).

Parts that may need replacement

The PowerFlex 755 itself is rarely the source of F2 — the fault is reporting an external condition. If you replace the drive without finding the external cause you will get F2 on the new drive too.

When to call a controls engineer

Bring in senior support when: F2 trips happen with no apparent external device transition (suggests EMI or hardware-level drive issue requiring oscilloscope diagnosis); the application is safety-critical and SIL-rated (any modification to the auxiliary fault chain requires re-validation by a competent engineer per IEC 61508 / 61511); F2 is chronic on a system with many interlock devices and you need a systematic approach to isolate which one is intermittent; or when the drive has a 20-750-S5 Safe Speed Monitor cassette and F2 may be intertwined with the safety functions in a way that requires careful diagnosis to preserve safety integrity.

FAQs

Can I just unassign the Aux Fault function to make F2 go away? Technically yes — set the DI Function parameter to 0 (Not Used). But you’re now operating with a disabled safety interlock. If the interlock was there because someone (an engineer, an OEM, an inspector) decided you needed it, you need to keep it. Diagnose and fix the cause.

My drive trips F2 every time it rains. Why? Moisture in an outdoor junction box on the interlock loop is shorting wires to ground or creating high contact resistance. Inspect every junction box on the interlock path — replace gasket seals, add silica gel desiccant packs, or upgrade to NEMA 4X (sealed) boxes.

Difference between F2 and F8 (Aux Fault Input — older designation)? On PowerFlex 750-class drives, F2 is the current designation. Older PowerFlex 7-class drives used F8 with a similar function. They are equivalent in concept — external interlock input — but live in different parameter spaces.

Should I add interposing relays between every external device and the drive? For long runs (over 50 ft), yes — interposing relays isolate the drive from cable noise and from any source-side voltage transients. For short runs in clean wiring environments, direct dry-contact wiring is fine. The drive’s digital inputs are rated for 24V industrial signals and are decently noise-immune up to 50 feet of twisted shielded cable.

Can I configure auto-reset on F2? Yes via parameter 145 Auto Rstrt Tries and 144 Auto Rstrt Delay, but think hard before doing so. Auto-reset on a safety interlock fault defeats the purpose of the interlock. Use auto-reset only on truly nuisance-class auxiliary inputs (e.g., a process-control “permissive to run” signal that may glitch during normal operation) and never on a safety-critical device.

Related guides

• Allen-Bradley PowerFlex 755 Power Loss Fault Fix
• Allen-Bradley PowerFlex 753/755 Control Sync Fault Fix
• Allen-Bradley PowerFlex F081 Fault — Communication Loss Fix
• Allen-Bradley PowerFlex F029 Fault — Analog Loss Fix
• Allen-Bradley PowerFlex F122 Fault — I/O Card Fix

See Also

• Allen-Bradley PowerFlex F012 Fault — HW Overcurrent Fix
• Allen Bradley PowerFlex 700 F7 Fault — Causes & Fix
• Allen Bradley PowerFlex Fault Codes — Complete Reference
• Allen-Bradley MicroLogix 1400 Common Fault Codes