Powertrain, generic · Vehicle speed, idle control and auxiliary inputs

P05F4 Exhaust Aftertreatment Power Relay Control Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

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.

Likely causes

Control line shorted to ground
Corroded connector or relay socket
Harness damaged
Output driver in the engine module faulty

Symptoms

  • Check engine light on
  • DEF/AdBlue warning, possibly countdown to no-restart
  • Follow-up codes for heaters, pump or NOx sensor supply

Repair estimate

DifficultyEasyDIY possible
Parts and labour€20–400
Time0.3–2 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

aftertreatment power relayrelay control circuitengine control module

Related codes

P21C2P21C3P21C4P21C7P20EAP20EB

References: law.cornell.edu, fordservicecontent.com, data.epo.org.