P1635

Code

PEUGEOT P — Powertrain

Ignition relay control short circuit to earth or open circuit

Brand: PEUGEOT

Type: P — Powertrain

Views: UK: 8 EN: 12 RU: 5

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Page language: EN

Causes

Broken or disconnected wire in the ignition relay control circuit
Short to ground on the relay control feed (damaged insulation, pin pushed through)
Faulty ignition relay (coil open or internal short)
Blown fuse or poor fuse/relay socket connection
Corroded or loose connector at the relay or ECU
Failed ECU/engine control module driver transistor or output stage

Symptoms

Check Engine light (MIL) illuminated
Engine will not crank or crank but not start (no ignition/fuel pump power)
Intermittent no-start or stall while driving
No power to ignition coils or fuel pump when ignition is turned on
Blown fuse for ignition/ECU relay circuit

What to check

Read and record freeze-frame and live data; clear the code and attempt to re-create the fault to confirm current status
Visually inspect wiring and connectors at the ignition relay and ECU for damage, corrosion, or loose pins
Check related fuses and relay sockets for signs of overheating or poor contact
Swap the ignition relay with a known-good identical relay (if available) and retest
Measure voltage at the relay coil terminals with ignition ON and while cranking; backprobe if necessary
Check continuity/resistance between the ECU relay-control pin and the relay socket pin with ignition OFF

Signal parameters

With ignition OFF and key removed: relay-control line should be open/high-impedance to battery; continuity to ground should be absent (infinite/very high resistance)
With ignition ON or ECU requesting relay ON: control line should show the expected switching voltage (either ~12 V supply on one side of coil and ECU switching to ground on the other, or the ECU output pulled to battery depending on circuit design). Expect battery voltage (~11–14 V) across relay coil when energized
When ECU is not commanding the relay: voltage across the coil should be near 0 V
Resistance of a healthy relay coil typically measured in tens to hundreds of ohms (consult service data for exact value); an open or near-infinite resistance indicates a faulty relay

Diagnostic algorithm

Retrieve freeze-frame and related codes. Note whether code is current or historic and whether fault is permanent or intermittent.
Visually inspect relay, fuse, relay socket, and wiring for obvious damage, melted insulation or corrosion. Repair obvious issues and retest.
Swap the ignition relay with an identical known-good relay from the same vehicle (e.g., horn or fan relay with same part number) to check for relay failure.
With ignition OFF, disconnect battery negative. Measure continuity between the ECU relay-control pin and the relay socket pin; confirm there is continuity (low resistance) when connector wiring intact. Repair/open circuits if continuity is poor or absent.
Reconnect battery. Backprobe relay-control pin and monitor voltage with ignition ON and while cranking. Confirm expected switching behavior (see signal_params). If the control line is held at ground continuously or shows a short to ground, isolate by disconnecting harness sections until fault clears.
If a short to ground is present, visually and physically inspect harness for chafing or pin damage; disconnect sections and test each to locate short. Repair wiring (splice/replace) and protect with conduit/loom as needed.
If open-circuit is present, trace and repair broken wires or poor connectors; replace damaged relay socket or connector pins.
After repair, clear codes and perform functional test (start engine, cycle ignition, road test if needed) to ensure code does not return.
If wiring, connector and relay are confirmed good and fault persists, suspect ECU output driver failure and consult manufacturer repair procedures for ECU testing or replacement.

Likely causes

Damaged wiring harness or connector between ECU and ignition relay (most common)
Failed relay or poor relay socket contact
Corroded or loose ground or supply at the relay
Failed ECU output stage (less common, suspect after harness and relay checked)

Fault status

⚠️ Status

ECU detected ignition-relay control circuit either shorted to earth (ground) or open-circuit. Relay control signal not within expected range; ignition relay may not be switching reliably.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2 hours

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P1635 — Ignition relay control short circuit to earth or open circuit