P05F4Exhaust Aftertreatment Power Relay Control Circuit Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- Needs a drive cycle to confirm a fix
- How the car detects it
- A power relay, switched by the engine module through an output driver, feeds aftertreatment consumers such as the DEF pump, tank and line heaters and possibly NOx sensors or the SCR module. CARB requires such outputs to be checked for circuit faults and function. P05F4 means the relay control circuit is shorted to ground or its voltage is too low. Ford's comparable heater relay check flags a current above 6 A with the driver on, within 5 s. No manufacturer document available to us uses this exact code; GM and Ford monitor the same function under their own codes. If the relay fails, heaters, pump and SCR control drop out and dosing stops.
- What you may notice
- Check engine light on
- DEF/AdBlue warning, possibly countdown to no-restart
- Follow-up codes for heaters, pump or NOx sensor supply
- Likely causes
- LikelyControl line shorted to ground
- PossibleCorroded connector or relay socket
- PossibleHarness damaged
- RareOutput driver in the engine module faulty
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Easy · Often possible to do yourself.Typical cost: €20 – €400
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.