TCM Communication Circuit Low
The P0865 code indicates that the Transmission Control Module (TCM) has detected a low voltage condition on its communication circuit, typically the Controller Area Network (CAN) bus. This can be caused by a short to ground, open circuit, or faulty module, leading to disrupted data exchange between the TCM and other control units.
01. Most Common Causes
- Faulty TCM
- Short to ground in CAN bus wiring
- Open circuit in TCM communication line
- Corroded or loose connectors
02. Symptoms You Will Notice
- Check Engine Light illuminated
- Transmission shifting erratically or stuck in gear
- Reduced fuel economy
- Transmission fails to shift
03. Step-by-Step Diagnosis
Step 1: Scan for additional codes and check freeze frame data.
Step 2: Inspect TCM connectors and wiring for damage, corrosion, or loose connections.
Step 3: Measure voltage and resistance on the CAN bus lines (CAN High and CAN Low) with a multimeter.
Step 4: Perform a TCM communication test using a scan tool to verify module response.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended as the transmission may shift unpredictably, potentially causing a safety hazard or further damage.
How much does it cost to fix?▼
Diagnosis typically costs $50-$150. Repair costs vary; a new TCM can be $200-$800 plus labor, while wiring repairs are usually under $200.