P05F6

Code

Generic P — Powertrain

Exhaust Aftertreatment Power Relay Control Circuit Stuck On

Brand: Generic

Type: P — Powertrain

Views: UK: 26 EN: 36 RU: 26

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Page language: EN

Causes

Short to battery voltage on the relay control wire
Stuck or welded relay contacts (relay mechanically stuck closed)
Faulty relay driver inside the powertrain control module (PCM/ECM)
Corroded, damaged, or bridged connector or wiring at the relay or module
Aftertreatment component or harness fault pulling the control circuit high
Software/firmware error or ECM internal fault

Symptoms

DTC P05F6 stored and MIL may or may not be illuminated
Aftertreatment devices remain powered when they should be off
Parasitic battery drain when vehicle is off
Intermittent or continuous faults related to aftertreatment system operation
Burnt connector smell or visible heat damage at relay or harness

What to check

Read freeze frame and related codes; note ignition state and conditions when code set
Visually inspect relay, relay socket, and wiring for corrosion, melting, or damage
Use a scan tool to monitor relay control status and related inputs/outputs
Measure parasitic draw with ignition off (compare to specification)
Backprobe the relay control terminal to verify voltage with ignition OFF and ON
Measure relay coil resistance and compare to specification or swap with known-good identical relay

Signal parameters

Control circuit voltage: ~0 V (off) or battery voltage (~11–14.5 V) (on) depending on ignition/command
Relay coil resistance (typical): 50–200 ohms (refer to component spec)
Parasitic current draw with relay energized: depends on aftertreatment load; check manufacturer spec
When not commanded on, expected continuity between control terminal and battery positive should be open

Diagnostic algorithm

Verify the code and note any associated/related DTCs. Record conditions (key on/off, engine running) when the code set.
Perform a visual inspection of the relay, socket, and wiring. Look for melted plastic, corrosion, or water entry.
With ignition off, measure parasitic current. If excessive, remove relay and see if draw drops. This isolates load vs. control circuit fault.
Backprobe the relay control terminal. With ignition off and no command, confirm voltage is not present. If voltage is present, suspect short to battery or ECM driver stuck high.
Unplug the relay and check for voltage on the control pin. If voltage disappears with relay disconnected, inspect relay socket for battery feed shorted into control pin.
Measure relay coil resistance. Compare to spec or swap with an identical known-good relay to see if the condition follows the relay.
Check continuity between the control wire and battery positive with ignition off and BCM/ECM disconnected to detect a short to battery feed.
Check continuity between the control wire and ECM/PCM driver pin. Inspect connector pins for bent or pushed-through pins that might contact battery pins.
If wiring and relay test good, test or bench ECM/PCM output driver (use oscilloscope or manufacturer procedures). Consider ECM replacement only after wiring and relay are verified.
Repair wiring/connectors or replace relay as indicated. Clear codes and confirm the fault does not return during proper vehicle operation.

Likely causes

Relay control wire has insulation damage and is contacting battery voltage
Defective/externally welded aftermarket or failed relay
ECM output transistor shorted high
Water intrusion or corrosion in relay connector causing high resistance/short
Incorrect replacement relay or improper relay pin fit

Fault status

⚠️ Status

Relay control circuit remains energized (battery voltage present) when command indicates OFF. Possible short to battery, stuck relay, or module driver fault.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-3.0 hours