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DTC codes / P05xx · Vehicle speed, idle control and auxiliary inputs / P05F4

P05F4Exhaust Aftertreatment Power Relay Control Circuit Low

P05F4 indicates that the powertrain control module (PCM) has detected a low-voltage condition on the control circuit for the exhaust aftertreatment power relay. This relay supplies battery power to the aftertreatment system's heater, pump, or control module, and a low circuit means the PCM sees voltage below the expected threshold when the relay is commanded on.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaVehicle speed, idle control and auxiliary inputs

Symptoms you may notice

  • Illumination of the check engine light (malfunction indicator lamp).
  • Often no symptoms besides the warning light, especially if the aftertreatment system is not immediately needed.
  • Reduced fuel economy or power due to the aftertreatment system not operating efficiently.
  • Possible regeneration inhibit or derate if the aftertreatment system cannot function.
  • In some cases, a no-start condition if the relay also powers other critical circuits.
  • Intermittent operation of the aftertreatment system leading to secondary codes.

Common causes, most likely first

  1. Blown fuse or fusible link supplying power to the relay.
  2. Open or high-resistance wiring in the relay control circuit (PCM to relay coil).
  3. Faulty exhaust aftertreatment power relay (coil open or contacts worn).
  4. Poor ground connection for the relay coil or aftertreatment system.
  5. Corroded or damaged connector at the relay, PCM, or aftertreatment component.
  6. PCM internal driver failure for the relay control circuit.
  7. Short to ground in the control circuit causing low voltage.

How to diagnose and fix it

  1. Check the fuse and relay for the aftertreatment power circuit; replace if blown or faulty.
  2. Inspect the relay control circuit wiring for opens, shorts, or corrosion; repair as needed.
  3. Test the relay control circuit with a multimeter to verify voltage and ground signals from the PCM.
  4. Check the ground connections for the relay and aftertreatment system; clean and tighten.
  5. If the control circuit is intact, test the relay by applying power and ground to the coil and checking continuity across the contacts.
  6. If the relay and wiring are good, diagnose the PCM driver circuit for internal failure.
  7. Clear codes and verify the repair with a test drive and scan tool.

Relay Control Circuit Low Voltage Checks

  • A low control circuit code often results from a poor ground at the relay coil, not a failed relay, so always verify ground integrity before replacing parts.
  • The aftertreatment power relay may share its control signal with other relays, so a fault in a related circuit can set this code without any aftertreatment component being defective.
  • Intermittent low voltage can be caused by a loose relay terminal that heats up and loses contact, which may not show up during a static test.
  • Some vehicles use a solid-state relay integrated into the aftertreatment control module, so the 'relay' may not be a serviceable separate component.
  • A low control circuit code can be set by a short to ground in the wiring harness where it rubs against a metal bracket, often near the aftertreatment system.

Can I keep driving with P05F4?

If the only symptom is the check engine light and the vehicle drives normally, you can continue driving but should schedule a diagnosis soon to avoid aftertreatment system damage. Avoid long trips or heavy loads if the vehicle enters a derate mode, and do not ignore the light if regeneration is inhibited.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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