Code
P1631
CITROEN
P — Powertrain
Inside ECU (CAN)
Views:
UK: 1
EN: 37
RU: 2
AI status
Completed
Completed
100%
Causes
- Internal ECU hardware or processor fault
- Faulty CAN transceiver inside ECU
- Loss of ECU power supply or intermittent battery feed
- Poor or corroded ground at ECU or chassis
- Open, shorted or damaged CAN bus wiring or connectors
- Missing or incorrect termination (bad/missing resistor) on CAN bus
Symptoms
- MIL (check engine) lamp illuminated
- Intermittent or total loss of functions controlled via the inside ECU (comfort, body, doors, lighting)
- Communication errors logged for multiple modules on scan tool
- Multiple related DTCs/U‑codes present (loss of communication)
- Random behavior of interior systems, fuses tripping or electrical feeds dropping
- Possible engine management derate if engine controller loses required CAN data
What to check
- Read all stored DTCs and freeze frame data from all modules; note time stamps and related U‑codes
- Check vehicle battery voltage and charging system; ensure stable supply (12.0–14.5 V)
- Visually inspect ECU connector(s) and nearby wiring for corrosion, water ingress, crushed wires or damaged insulation
- Check fuses and power feeds to the inside ECU
- Using a scan tool, check live CAN network status, module addresses and presence of the inside ECU on the bus
- Measure CAN termination resistor across CAN_H and CAN_L at the network (approx. 60 Ω for two 120 Ω terminators in parallel)
Signal parameters
- CAN idle voltages (recessive): CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V at ECU connector (key on, engine off)
- CAN dominant state: CAN_H ≈ 3.5–3.9 V, CAN_L ≈ 0.9–1.5 V during message transmission (differential ≈ 2 V)
- Termination: two 120 Ω resistors in parallel at ends → ≈ 60 Ω between CAN_H and CAN_L
- Supply voltage to ECU: typical battery range 12.0–14.5 V; check for voltage drops during cranking or operation
- Ground resistance: chassis ground at ECU should be low (
Diagnostic algorithm
- Record all DTCs from all modules and attempt to reproduce the fault; note any U‑codes indicating lost communications.
- Clear codes and perform a key‑on/drive cycle to see if P1631 returns and capture freeze frame data.
- Verify battery condition and charging system; ensure stable supply to ECUs.
- Inspect the inside ECU connector and harness for corrosion, water, pin damage, or chafing; repair any physical wiring damage.
- Check fuses and power pins at the ECU connector for correct voltage with key on; check ECU ground integrity (measure resistance to chassis).
- Measure CAN_H and CAN_L voltages at the inside ECU connector with key on; verify idle and dominant voltages and compare to expected values.
- Measure resistance across CAN_H and CAN_L at vehicle harness to confirm ~60 Ω termination; isolate sections if value incorrect to locate open/short.
- Use an oscilloscope or high‑speed CAN capable scan tool to view bus waveforms and detect noise, dominant stuck states or corrupted frames.
- If a specific module is driving the bus to a fault state, disconnect suspect modules one at a time (as allowed by safety/service procedures) to isolate the faulty node.
- If wiring, connectors, power and grounds are good, consult manufacturer service information for ECU reprogramming or known TSBs; consider replacement of the inside ECU and reprogramming/coding as a last step.
- After repairs or replacement, clear codes and perform full system verification including network scan to ensure all modules communicate correctly.
Likely causes
- Bad CAN transceiver inside the inside ECU
- Open or short to battery/ground on CAN_H or CAN_L at ECU connector
- Poor ground at inside ECU or main ECU ground strap
- Damaged harness where wires pass through bulkheads or hinges
- Failed internal power regulator in ECU
- Loose or corroded ECU connector pins
Fault status
Status
Internal ECU detected fault on CAN bus — communication/inside ECU error. Further diagnosis of ECU power, ground, CAN wiring and the ECU itself required.
Repair difficulty: Hard
Diagnostic time: 1.0-3.0 hours
Similar codes
Your experience will help others
+100 karma for a short comment :)
Was this AI description helpful?
Your feedback helps improve AI descriptions.
👍 Like
0
👎 Dislike
0
Send to email