P1636

Code

CITROEN P — Powertrain

Supply relay control short circuit to positive

Brand: CITROEN

Type: P — Powertrain

Views: UK: 5 EN: 6 RU: 2

AI status

Completed

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Page language: EN

Causes

Damaged or chafed wiring harness shorting the relay control wire to battery positive
Corroded or damaged relay socket or connector supplying permanent 12V to the control pin
Faulty supply/power relay stuck closed or welded contacts
Aftermarket accessory wired incorrectly to the relay/control circuit
Internal failure in the ECU/BCM driver transistor causing leakage to positive
Poor repairs, pin misrouting, or water ingress in connectors

Symptoms

Malfunction Indicator Lamp (MIL) illuminated
Vehicle may not shut off or may have intermittent power to systems
Parasitic battery drain when vehicle is off
No-crank or no-start if relay function is affected
Intermittent electrical faults or components remaining powered

What to check

Read stored freeze frame and related codes; note when the code sets (key on, running, after off).
Visual inspection of relay, relay socket, connectors and wiring for damage, corrosion or signs of heat.
Check fuse(s) related to supply relay and associated circuits for correct rating and condition.
Measure voltage at the relay control pin with ignition off and with key on/engine running; note presence of battery voltage when it should be off.
Unplug the supply relay and re-check voltage on the control circuit to isolate relay vs wiring/ECU.
Perform continuity/ohm checks between relay control pin and battery positive with ignition off (only with battery connected as needed for voltage checks; remove power when measuring resistance).

Signal parameters

Battery voltage reference: ~12.0–14.5 V with engine off/on.
Expected control-line voltage (ignition off): ≈ 0 V (open/high impedance). If shorted to positive, will read ≈ battery voltage.
Expected control-line voltage (relay commanded on): ≈ battery voltage (12–14.5 V).
Continuity: control pin to battery positive should be open when relay not commanded; measured resistance should be high/infinite unless relay energized.
Current draw: abnormal parasitic current (can be measured with ammeter) when vehicle off if relay remains powered.

Diagnostic algorithm

Read and record codes and freeze-frame data. Note when code occurs (ignition position, running, after shutoff).
Perform a visual inspection of the supply relay, relay socket, nearby harness, and ground points for damage, corrosion, or aftermarket taps. Repair obvious damage.
With ignition off, remove the supply relay. Measure voltage at the relay control terminal. If you still see battery voltage with the relay removed and key off, there is a short to positive in wiring or ECU.
With ignition off, measure continuity between the control pin and battery positive. A low resistance indicates a short. If continuity is open, the wiring is likely OK.
Reconnect harness and back-probe control pin with ignition on and command relay on/off (if possible). Compare voltages to expected behavior (0 V off, battery voltage on).
Disconnect the ECU/BCM connector (or isolate the ECU driver) and re-check the control wire. If the short disappears when the ECU is disconnected, suspect internal ECU/BCM driver failure and consider ECU replacement or repair.
If the short remains with ECU disconnected, trace wiring along harness to locate chafes, pin-to-pin shorts or accessory connections. Repair or replace damaged wiring/connector or relay socket as necessary.
Replace faulty relay or repair harness/connectors. After repairs, clear codes and perform functional test and road test to confirm code does not return.
If all wiring and relay are good and the short returns only when ECU is connected, consult manufacturer guidance for ECU replacement or repair; consider verifying with a known-good ECU if available.

Likely causes

Wiring chafe/short to battery positive at or near the relay/connector (most common)
Corroded/damaged relay socket or relay stuck closed
Aftermarket accessory or recent work created a miswire
Faulty ECU/BCM output driver (less common)

Fault status

⚠️ Status

Supply relay control short to positive — control circuit is receiving battery voltage when it should be off. Inspect wiring, relay socket and ECU driver for a short to battery.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours

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P1636 — Supply relay control short circuit to positive