U0100critical severityNetwork
Lost Communication with ECM/PCM
on 2004 Subaru Legacy 3.0L H6 · AWD
What this code means
The diagnostic system cannot communicate with the Engine Control Module.
Symptoms
- Multiple warning lights
- Engine may not start
- Many systems inoperative
- Cannot read engine codes
Common causes
- PCM power or ground failure
- CAN bus wiring issue
- PCM failure
- Fuse blown
- Wiring harness damage
Diagnostic steps
- Check PCM fuses
- Test PCM power and ground circuits
- Check CAN bus wiring to PCM
- Try communicating with other modules
- Inspect wiring harness
Repair notes
Check fuses and power supply to PCM first. CAN bus issue is second most common.
Other Network codes on this vehicle
U0001
High Speed CAN Communication Bus
U0002
High Speed CAN Communication Bus Performance
U0003
High Speed CAN Communication Bus (+) Open
U0004
High Speed CAN Communication Bus (+) Low
U0005
High Speed CAN Communication Bus (+) High
U0006
High Speed CAN Communication Bus (-) Open
U0007
High Speed CAN Communication Bus (-) Low
U0008
High Speed CAN Communication Bus (-) High
U0009
High Speed CAN Communication Bus Shorted Together
U0010
Medium Speed CAN Communication Bus