What it means
The Transmission Control Module communication circuit reports a communication circuit low fault. Data exchange between TCM and ECM is impaired.
At a glance
- Usually lights the check-engine light
- The engine or gearbox may go into limp mode
How it works
The Transmission Control Module (TCM) exchanges transmission data with the Engine Control Module (ECM) over the vehicle network. This code sets when the TCM communication circuit is detected at a low electrical level, which can prevent reliable data exchange.
Common causes
Most likely first.
- Communication bus fault
- TCM internal fault
- Gateway module routing CAN messages incorrectly
Symptoms
- Check engine or transmission warning light on
- Harsh, delayed, or missing shifts
- Possible limp mode (locked in one gear)
- Slipping or shuddering during gear changes
How to diagnose it
- Save freeze-frame data and scan all modules to see whether other controllers also report a low network condition or lost TCM communication.
- Check whether the scan tool can access the TCM and review network status for missing messages.
- Inspect the TCM network connector and wiring for a conductor shorted to ground, damaged insulation, corrosion, or bent pins.
- With the circuit safely isolated as specified by the manufacturer, test the network line for a short to ground.
- If wiring checks pass, test TCM output and gateway routing using the manufacturer's network diagnostic procedure.
Common fixes
- Repair a network wire shorted to ground.
- Clean, secure, or repair the TCM connector and wiring.
- Correct gateway routing or configuration if it disrupts TCM messages.
- Repair or replace the TCM if testing confirms its output is holding the circuit low.
Can I keep driving?
If shifts are harsh, delayed, or the vehicle is in limp mode, avoid continued driving except to reach a safe place; the low communication condition can interrupt transmission data exchange. Stop if drive is lost or the vehicle shifts unpredictably.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.