Code
P1631
DS
P — Powertrain
Inside ECU (CAN)
Views:
UK: 3
EN: 35
RU: 5
AI status
Completed
Completed
100%
Causes
- Internal ECU hardware failure (processor, CAN transceiver)
- Corrupted or failed ECU firmware/software
- Supply voltage or ground fault to the inside ECU
- CAN bus wiring short, open, or high resistance
- Faulty connector, corrosion, or water ingress at ECU or harness
- Missing or incorrect bus termination
Symptoms
- MIL (check engine) or dashboard warning illuminated
- Loss or intermittent loss of interior functions controlled by the inside ECU (lights, HVAC controls, door functions, etc.)
- Multiple communication-related DTCs present for other modules
- No response from the inside ECU on a diagnostic scan tool
- Erratic behavior of systems that rely on CAN messages
What to check
- Read and record all stored DTCs and freeze-frame data from all available modules
- Check battery voltage and ground integrity (battery at rest and during cranking)
- Inspect ECU connectors and wiring for corrosion, damage or water ingress
- Verify ignition and supply voltages at the inside ECU pin(s)
- Scan CAN bus status for bus-off flags or high error counters
- Measure CAN bus idle voltages on CAN_H and CAN_L with DMM (approx. 2.5 V idle) and check differential
Signal parameters
- CAN idle voltage: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (differential ~0 V when recessive)
- CAN dominant state: CAN_H rises toward ~3.5 V, CAN_L drops toward ~1.5 V (differential ~2.0 V)
- Expected end-to-end bus resistance (terminators): ~60 Ω (two 120 Ω in parallel) or manufacturer value
- Common bus speeds: 250 kbps or 500 kbps (verify vehicle-specific bitrate)
- No continuous dominant level on the bus; normal traffic shows regular message frames
Diagnostic algorithm
- Retrieve and document all DTCs from all modules; note timestamps and freeze frame data.
- Check battery condition and measure supply and ground at the inside ECU connector with ignition on and off.
- Visually inspect ECU and harness connectors for corrosion, bent pins, water, or mechanical damage; repair as needed.
- With ignition on, measure CAN_H and CAN_L voltages at the ECU connector; confirm expected idle and differential voltages.
- Measure resistance between CAN_H and CAN_L with ignition off to verify termination (~60 Ω).
- Use an oscilloscope or good-quality CAN analyzer to inspect waveform quality, bus loading, error frames, and to identify a permanently dominant device.
- Disconnect suspected modules (start with easily accessible interior modules) one at a time to see if bus returns to normal; observe for bus-off recovery.
- Verify no short to battery or ground on CAN wiring using back-probing and continuity checks; repair wiring faults.
- Attempt communication with the inside ECU via dealer-level tool; attempt a soft reset or firmware reflash only if procedure available and stable power supply is ensured.
- If module fails self-tests or cannot be reprogrammed and all bus wiring and other modules test good, consider replacement of the inside ECU and then reflash/configure and recheck system.
- Clear codes and perform a road or function test to confirm repair; monitor for reoccurrence.
Likely causes
- CAN transceiver failure inside the inside ECU
- Low battery or intermittent power/ground to ECU during operation
- Damaged CAN_H/CAN_L wiring or pin corrosion at the connector
- One module in a bus-off state holding the bus low or high
- ECU firmware checksum error after a failed update
Fault status
Status
Inside ECU detected internal CAN/bus communication fault — possible internal hardware or software error, or network level fault preventing normal CAN communication.
Repair difficulty: Hard
Diagnostic time: 1.0-3.0 hours
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