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

P05F6 Exhaust Aftertreatment Power Relay Control Circuit Stuck On

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. P05F6 is a function check on the load side: the relay stays closed although it is switched off, so the consumers keep their supply, e.g. during after-run. A patent by Delphi describes this kind of check via the voltage at the heater node. 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

Relay contacts welded or sticking
External voltage on the load side (short to positive)
Corroded connector or relay socket
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
  • Consumers stay powered after switch-off, battery drains

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.