U0294 Lost Communication with PCMM
Diagnostic trouble code U0294 indicates a loss of communication on the CAN data bus between the powertrain control module (PCM) and the powertrain control module monitor (PCMM). The PCMM is a supervisory component that verifies proper PCM operation, typically used in advanced OBD-II emissions diagnostics. The code sets when the receiving module fails to detect valid messages from the transmitting module for a manufacturer-defined period, usually between 250 ms and 1 second.
1. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.
2. Most Common Causes
- Faulty powertrain control module monitor (PCMM)
- Open or shorted CAN bus wiring (high or low) between PCM and PCMM
- CAN bus terminal resistance out of specification (should be 60-120 ohms)
- Internal PCM failure preventing data transmission
- Damaged or corroded connector pins at modules or harness splices
- Electromagnetic interference on the communication bus wiring
- Low voltage or poor ground at the PCMM
3. Symptoms You Will Notice
- Check Engine Light illuminated
- Possible drivability issues such as rough running, stalling, or reduced power
4. Step-by-Step Diagnosis
Scan for all stored codes and freeze frame data
Inspect CAN bus wiring for damage, corrosion, or loose connections
Measure CAN bus resistance at the PCM or PCMM connector (should be 60-120 ohms)
Check voltage and ground at the PCMM
Perform a network communication test to isolate the fault
If wiring and power are good, suspect a failed PCMM or PCM
5. Frequently Asked Questions
Is it safe to drive?▼
No, this code indicates a serious communication fault that can affect engine and transmission control. Driving may lead to stalling, poor performance, or increased emissions. It is recommended to have the vehicle diagnosed and repaired promptly.
How much does it cost to fix?▼
Repair costs vary widely depending on the root cause. A simple wiring repair may cost $100-$300, while replacing a PCMM or PCM can range from $500 to $1,500 including parts and labor.