What Does P0865 Mean?
The TCM communication circuit voltage is reading below the expected threshold. This low-voltage condition on the communication line prevents reliable data exchange between the TCM and the PCM or other control modules. The TCM may be partially or completely unable to receive shift commands or transmit status information.
Safety Warning
A low communication signal can cause partial or complete loss of electronic transmission control. The transmission may operate in emergency limp mode or fail to shift entirely. Avoid driving except to reach a nearby repair facility.
Common Causes
30%
Short to ground on the CAN bus or communication wiring to the TCM
25%
Faulty TCM pulling the communication line low
20%
Corroded connector at the TCM allowing moisture-induced shorts
15%
Damaged wiring harness near the transmission from road debris or heat
10%
Another module on the same CAN bus segment pulling the bus low
Diagnostic Steps
1
Measure the CAN bus voltage levels at the TCM connector — CAN-H should idle around 2.5V and CAN-L around 2.5V when no communication is active.
2
Disconnect the TCM and recheck bus voltage — if the voltage returns to normal, the TCM is causing the low signal.
3
If bus voltage remains low with the TCM disconnected, disconnect other modules on the same CAN bus segment one at a time to identify which module is pulling the bus down.
4
Inspect the TCM connector and wiring harness where it exits the transmission for damage from heat, road debris, or transmission fluid leaks.
5
Check for proper CAN bus termination — an extra terminating resistor or a failed resistor can alter bus voltage levels.
Estimated Repair Cost
$300 - $1,500
Parts + labor, varies by vehicle and location
The TCM communication circuit voltage is reading below the expected threshold. This low-voltage condition on the communication line prevents reliable data exchange between the TCM and the PCM or other control modules. The TCM may be partially or completely unable to receive shift commands or transmi...
The most common cause of P0865 (TCM Communication Circuit Low) is: Short to ground on the CAN bus or communication wiring to the TCM
Typical repair costs for P0865 range from $300 to $1,500, depending on the vehicle, location, and whether you do it yourself or go to a shop.
A low communication signal can cause partial or complete loss of electronic transmission control. The transmission may operate in emergency limp mode or fail to shift entirely. Avoid driving except to reach a nearby repair facility.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0865 to identify the root cause.
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Quick Info
Category
Powertrain
System
Transmission Control / CAN Bus Network
Difficulty
Type
Generic (SAE)
Recommended Tools
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