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DTC codes / P05xx · Vehicle speed, idle control and auxiliary inputs / P05F3

P05F3Exhaust Aftertreatment Power Relay Control Circuit/Open

P05F3 indicates that the powertrain control module (PCM) has detected an open circuit or excessive resistance in the control circuit for the exhaust aftertreatment power relay, which supplies power to the exhaust aftertreatment system. The PCM monitors the relay control circuit and sets this code when the expected voltage or current is not present, indicating a wiring fault, a failed relay, or a faulty PCM driver.

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) with no other noticeable symptoms.
  • The exhaust aftertreatment system may not activate, leading to reduced efficiency of the diesel particulate filter (DPF) regeneration or selective catalytic reduction (SCR) system.
  • Possible limp mode or reduced engine power if the aftertreatment system is disabled.
  • In some cases, a message on the dashboard indicating exhaust system service required.
  • Rough running or poor fuel economy if the aftertreatment system affects engine management.
  • Often no symptoms besides the warning light, especially if the relay is only intermittently faulty.

Common causes, most likely first

  1. Open or damaged wiring in the relay control circuit, such as a broken wire or corroded connector.
  2. Failed exhaust aftertreatment power relay (open coil or stuck contacts).
  3. Blown fuse or fusible link supplying power to the relay or control circuit.
  4. Poor ground connection for the relay or the aftertreatment system.
  5. Faulty powertrain control module (PCM) driver for the relay control circuit.
  6. Corrosion or water intrusion in the relay socket or wiring harness.
  7. Aftermarket accessories or improper wiring repairs that interfere with the circuit.

How to diagnose and fix it

  1. Verify the code with a scan tool and check for any related codes in other modules.
  2. Inspect the fuse and relay for the exhaust aftertreatment power circuit; replace if blown or faulty.
  3. Perform a visual inspection of the wiring harness and connectors for damage, corrosion, or loose pins.
  4. Use a multimeter to check for continuity and voltage on the relay control circuit between the PCM and relay.
  5. Test the relay by applying power and ground to the coil and checking for continuity across the contacts.
  6. If the circuit and relay are good, check the PCM driver output with a test light or oscilloscope; repair or replace PCM if faulty.
  7. Clear codes and road test to verify the repair, ensuring the aftertreatment system operates correctly.

Relay circuit faults often misdiagnosed

  • A common oversight is assuming the relay itself is always at fault, but the control circuit wiring or PCM driver is frequently the culprit.
  • The code can set intermittently due to a loose connector or chafed wire that only loses contact under certain conditions, such as vibration or temperature changes.
  • Some vehicles use a solid-state relay or a module integrated into the aftertreatment system, so the 'relay' may not be a traditional plug-in component.
  • A weak battery or charging system issue can cause the PCM to misreport a relay control fault, so always check battery voltage and grounds first.
  • This code does not always mean the aftertreatment system is completely disabled; it may still function but with reduced performance, so don't jump to replacing expensive components.

Can I keep driving with P05F3?

You can generally continue driving with this code, but avoid long trips or heavy loads that rely on the aftertreatment system until it is repaired. If the vehicle enters limp mode or you notice reduced power, have it diagnosed promptly to prevent further damage.

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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