PowerFlex 4 Fault F2 — Auxiliary Input

Overview

The F2 Auxiliary Input fault on an Allen-Bradley PowerFlex 4 variable frequency drive (VFD) indicates that an external interlock or safety device wired to a programmable digital input has opened, triggering a safety shutoff. When any digital input terminal is configured as an "Auxiliary Fault" (typically designated in parameters t051-t054 as Option 1) and the electrical signal across that terminal is interrupted, the drive halts the motor immediately. Unlike internal electrical protection faults, the F2 code exists solely to respond to safety loops, external thermal overload units, or system-level interlocks that require a complete motor shutdown.

Symptoms

When light or heavy industrial machinery triggers an F2 fault, the drive and your automation panel will show the following symptoms:

• Flashing Keypad Display: The integrated keypad display flashes "F2" continuously.
• Immediate Motor Coast: The VFD cuts electrical output immediately, causing the motor and connected loads to coast to a stop rather than ramp down.
• Red Fault LED: The dedicated red fault indicator light on the VFD fascia turns on.
• Inability to Restart: The drive rejects all run commands (local or network) until the dry contact is closed, continuity is restored to the input, and a manual or remote reset command is executed.
• Upstream PLC Notifications: If connected via a communications module or a configured output relay, status signals report a faulted state to the supervising PLC.

Possible Causes

Because the auxiliary input is generic and can be adapted to many plant designs, several mechanical and electrical factors can initiate this fault:

• Tripped External Interlock: A physical device in the safety loop—such as a pull-cord emergency stop, a machine guard safety switch, or a conveyor drift switch—has physically tripped and opened the circuit.
• Motor Thermal Overload Relay Activation: An external thermal overload block (frequently used to monitor ex-rated or high-vibration motors) has overheated or failed, breaking the control circuit.
• Damaged or Loose Low-Voltage Control Wiring: A loose terminal screw, corroded junction point, or wire severed by machine motion has broken the physical 24V loop running back to the drive's control block.
• Incorrect Parameter Directives: A digital terminal parameter (e.g., parameter t051 for input 1 or t052 for input 2) is accidentally programmed to Option 1 ("Aux Fault") when no auxiliary device is actually wired to the terminal strip.
• Interrupted Control Power Supply: Loss of the internal VFD 24V DC control power (Terminal 11) or a failure of an auxiliary external supply powering the digital input circuitry.
• Electrical Noise (EMI) Interference: Lack of shielded cabling or proper routing on long control wire runs can inject transient noise spikes, causing the optocouplers on the drive's control board to register a momentary open circuit.
• Defective Relay Contact Blocks: Wear, carbon buildup, or oxidation on the contact blocks of external switches and relays, preventing low-current continuity from passing cleanly.

Step-by-Step Troubleshooting

Follow these systematic steps to isolate the root cause of an F2 fault and return your machinery to operation:

Step 1: Identify Which Input is Configured as the Auxiliary Input

You must determine which physical terminal is monitoring the auxiliary safety loop. Inspect the drive's current terminal configurations using the integrated keypad:

• Press the Escape (Esc) button to scroll to the parameter groups.
• Access the t (Terminal Relation) Group.
• Look through parameters t051 (Digital In1 Sel), t052 (Digital In2 Sel), t053 (Digital In3 Sel), and t054 (Digital In4 Sel).
• Note which parameter is programmed to value 1 (Aux Fault). For example, if t052 is set to 1, Terminal 06 is your auxiliary fault monitoring input.

Step 2: Track the Input Wiring

Trace the physical wire starting from the identified input terminal on the control block. On a typical PowerFlex 4 drive set to "Source" mode, the circuit runs from Terminal 11 (+24V DC Out), passes through your external safety contact series, and loops back into the assigned digital input terminal (e.g., Terminal 05 or 06). If configured in "Sink" mode, it runs from an external positive rail, through the circuit, and returns to the input terminal with a common ground loop.

Step 3: Perform a Voltage Check

Use a calibrated digital multimeter (DMM) to diagnose the physical circuit:

• Set your meter to DC Volts (VDC).
• Place your black (Common) probe on Terminal 04 (Digital Common).
• Place your red probe on the assigned helper digital input terminal (e.g., Terminal 06).
• Analysing the results:
  • If you read 24V DC: The fault signal is reaching the board, indicating that either the fault condition has already cleared (and just needs to be reset) or the VFD's internal input board optocoupler is failing to read the high level.
  • If you read 0V DC (or a very low float voltage): The circuit is physically open. There is an active trip on your external device or a break in your safety line.

Step 4: Verify the External Interlocking Devices

Trace along the loop wiring to each external circuit breaker, safety gate contact, or emergency stop button. Inspect each terminal junction box and device visually. Use the continuity checking mode (Ohm meter) of your multimeter at each device's terminal junction block to confirm whether the device contacts are closed. Always shut down power to the control circuit before executing point-to-point resistance checks.

Step 5: Test the Input Locally (Diag Jump)

To pinpoint if the fault is inside the VFD panel or out in the field:

• Power down the drive and tag out the disconnect switch.
• Insert a temporary copper jumper wire directly between Terminal 11 (+24V DC) and the problematic digital input terminal.
• Energing/Powering back on the VFD. If you can reset the F2 fault using this direct jumper, the drive's internal electronics are healthy, and your fault lies solely within your field instrumentation wiring.
• Warning: Never leave a diagnostic jumper permanently in place, as this bypasses critical physical machinery safety protection mechanisms.

Step 6: Review Sink/Source Switch Alignment

Verify that the physical Sink/Source selection switch (located on the control board directly above the terminal blocks) is properly aligned. If this switch is toggled to Sink when your wiring expects Source, the logic voltages will reverse, automatically triggering an F2 fault due to missing bias voltage.

Recommended Actions

Once you have located the failure point, take these corrective actions to resolve the issue permanently:

• Restore and Secure Field Connections: Clean any oxide buildup on external relay contact blocks with contact cleaner or replace worn contact blocks. Retorque any loose wire screws on terminal blocks inside the panel.
• Correct Configuration Settings: If a terminal is not physically wired down, re-program the corresponding parameter (t051-t054) to Option 0 (Not Used) to terminate the monitoring function.
• Re-route Signal Sourcing: If electrical noise is suspect, replace the input loop run with shielded, twisted-pair cable (Minimum 18 AWG / 0.75 mm²). Ensure you ground the cable's drain shield at the panel-end chassis ground only.
• Replace Unstable Limit Switches: Mechanically worn limit switches or position sensors that flicker under machine vibration should be replaced with high-vibration rated inductive sensors or robust mechanical contacts.

Recommended Replacement Parts

Should your components be worn or defective beyond cleaning, keep these replacement items on hand to minimize downtime:

• Main Drive / Control Board: If diagnostic jumps across Terminal 11 and the Inputs fail to clear the error, replace the VFD (e.g., 22A-A1P5N104, 22A-B2P3N104, or 22A-D1P5N104).
• External Auxiliary Contact Blocks: Heavy-duty snap-action blocks for thermal overload interfaces or limit switches (Allen-Bradley Bulletin 193 or Bulletin 800F lines).
• Industrial Power Supplies: A stable independent source of 24V to prevent control sag across long auxiliary runs (Allen-Bradley Bulletin 1606 series).
• Premium Shielded Control Cabling: 2-wire or 4-wire twisted-pair shielded cabling designed specifically for safety signals in VFD noise zones.

Related Articles

• /knowledge/replacement/allen-bradley-powerflex-4-to-525-migration
• /knowledge/compatibility/powerflex-digital-input-sink-source-selection
• /knowledge/guide/clearing-hard-faults-powerflex-4-series

FAQ

Q: Can I run my VFD with the F2 fault bypassed using a permanent jumper wire?

No. While jumpering the input is a valid diagnostic step to prove the VFD's hardware is operating correctly, it must never be done during normal production. Safeties, door lock sensors, and thermal guards are critical for plant safety and motor longevity. Bypassing them permanently invites catastrophic hardware failures and severe safety code violations.

Q: Does the F2 fault indicate a blown internal fuse or capacitor failure inside the VFD?

No. The F2 fault code indicates that an external safety circuit is open. If you have an internal drive bus failure or blown output transistors, the VFD would throw hardware faults such as F4 (UnderVoltage), F5 (Overvoltage), or F33 (Auto-Restart Tries).

Q: Why does the F2 fault occur immediately upon startup when my field devices are healthy?

This is typically because the field cabling is running in parallel with high-voltage motor output lines, inducing enough high-frequency electrical noise to drop the input signal momentarily. Ensure control and AC power runs are spaced at least 6 inches (15 cm) apart and cross at 90-degree angles.

Q: Is there a parameter lock that prevents me from changing digital input values t051-t054?

Yes. If parameter d012 [Control Source] or parameter P036 [Start Source] dictates active running configurations, ensure the VFD is completely stopped before writing changes to Terminal Parameters. Certain active configurations lock parameter editing in standard program modes.