Lost Communication with Shift Control Module B
Diagnostic trouble code U0291 indicates a loss of communication between the powertrain control module (PCM) and the shift control module 'B'. This means the PCM is not receiving the necessary data from the transmission or shift lever module over the vehicle's communication bus, typically the CAN bus. Without this signal, the electronic system cannot verify the selected gear position, which is critical for safe engine starting and transmission control. This fault is usually related to electrical or network issues, not necessarily an internal mechanical transmission failure.
1. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.
2. Most Common Causes
- Shift control module 'B' lacks power or is damaged
- CAN bus communication wiring is open or shorted
- Corroded, bent, or loose connectors at the shift module
- Blown fuses or relays supplying the shift control module
- Internal failure of the powertrain control module (PCM)
- Electromagnetic interference (EMI) affecting the communication bus
3. Symptoms You Will Notice
- Check Engine Light illuminated
- Transmission may not shift or may be stuck in one gear
- Engine may not start or start in a default mode
- No crank or no start condition
- Transmission warning message on the dashboard
4. Step-by-Step Diagnosis
Scan for all stored codes and freeze frame data
Check power and ground at the shift control module 'B'
Inspect CAN bus wiring and connectors for damage or corrosion
Test continuity and resistance on the CAN bus lines
Verify fuses and relays related to the shift module
If all else is good, suspect a faulty PCM or shift module
5. Frequently Asked Questions
Is it safe to drive?▼
No, it is not safe to drive. The vehicle may not start or may shift unexpectedly, increasing the risk of an accident. Have the vehicle towed to a repair shop.
How much does it cost to fix?▼
The cost can range from $100 to $1,200 depending on the root cause. Simple wiring repairs or fuse replacements may cost under $200, while replacing a shift module or PCM can exceed $1,000 including parts and labor.