Code
P1633
CITROEN
P — Powertrain
Ignition-on ECU supply control short circuit to earth or open circuit
AI status
Completed
Completed
100%
Causes
- Blown fuse or fusible link in the ECU/ignition supply circuit
- Faulty ignition/ECU supply relay
- Open circuit (broken/damaged wiring or connector) in the ECU ignition supply feed
- Short to ground in the ignition supply wiring (chafed insulation, pinched harness)
- Corroded/poor connector or pin at ECU, relay, fuse box or ignition switch
- Internal ECU power supply fault
Symptoms
- Engine cranks but does not start or intermittent no-start
- Loss of ECU power when ignition is ON (engine stalls or will not crank)
- Multiple ECU-related warning lights or gauges behaving erratically
- Battery drain or blown fuses when ignition is ON
- Intermittent communication with diagnostic scanner or no ECU comms
What to check
- Verify battery voltage and state of charge (should be ~12.0 V with engine off, ~12.6–14.5 V when running).
- Inspect relevant fuses and fusible links for continuity and signs of overheating.
- Check ignition/ECU supply relay operation and swap with known-good relay if possible.
- Visually inspect ECU connector, pins and wiring for corrosion, bent pins, or damage.
- Measure voltage at the ECU ignition supply pin with key ON and engine OFF; compare to battery voltage.
- Perform continuity/resistance check between ignition relay output, fuse, ignition switch and ECU supply pin.
Signal parameters
- Ignition ON, ECU supply terminal voltage: approx. battery voltage (~11.5–14.5 V)
- Ignition OFF, ECU supply terminal: near 0 V (unless permanent battery feed present)
- Continuity from fuse/relay output to ECU supply pin: low resistance (< a few ohms) when harness disconnected
- Resistance from supply wire to ground (with circuit isolated): should be high/open; short-to-ground shows very low resistance
Diagnostic algorithm
- Confirm and record freeze-frame data and any additional codes. Disconnect battery if doing wiring repairs.
- Check battery voltage and charge if low. Poor battery voltage can cause similar symptoms.
- Inspect and test the fuse/fusible link protecting the ECU ignition supply; replace if blown and re-test.
- Locate and test the ignition/ECU supply relay: verify coil activation and continuity of switched contact when energized; swap with identical relay to confirm.
- With ignition ON, measure voltage at the ECU supply pin and at the relay/fuse output. Note any large voltage drop or intermittent loss.
- If voltage is missing or intermittent, visually inspect wiring and connectors between relay, fuse box, ignition switch and ECU. Repair any corrosion, loose pins or broken wires.
- With power disconnected, check continuity of the supply circuit and measure resistance to chassis ground to detect shorts. Trace and isolate any shorted section.
- Wiggle-test wiring while monitoring voltage/continuity to reproduce intermittent faults; check areas prone to chafing (engine bay, firewall, harness clips).
- If wiring checks are good and proper voltage is present but the code persists, test/bench the ECU power supply or substitute a known-good ECU per manufacturer procedure.
- After repairs, clear codes and perform a road/test cycle to confirm the fault does not return. Re-scan and monitor supply voltage under load.
Likely causes
- Blown fuse or failed relay supplying ignition-on power to the ECU
- Loose, corroded or damaged connector at the ECU power pin
- Wiring harness abrasion causing short to chassis ground near firewall or engine
- Faulty ECU internal supply circuitry (less common)
Fault status
Status
Ignition-on ECU supply control: short to earth or open circuit detected (ECU ignition supply abnormal).
Repair difficulty: Medium
Diagnostic time: 1.5-3.0 hours
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