What does it mean?
P1690 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: A/T (TCM) TCSTB Data Line Circuit Malfunction .
Symptoms
- MIL on plus transmission and sometimes traction control warnings
- Transmission stuck in limp mode with one fixed gear and high engine speed on the motorway
- Scan tool cannot reach the transmission module, or communication drops out
- Erratic shift behaviour or a refusal to shift at all
- Related communication codes stored in several modules at once
- Fault that appears with vibration, rain or heat and then clears
Likely causes
- Open, shorted or high-resistance data line between the two modules
- Corroded or water-damaged pins in the data line connectors
- Chafed harness section rubbing against the body, a bracket or the transmission case
- Failed module at either end of the link holding the line high or low
- Loss of power or ground to one module on the link so it never answers
- Poor splice or crimp left behind by earlier harness repair work
- Aftermarket accessory tapped into the data circuit
How to fix it
- Try to communicate with each module individually to find which one is missing from the network
- Check power and ground at the non-responding module before touching the data line itself
- Measure the data line for continuity, shorts to ground and shorts to power with both modules disconnected
- Inspect connectors for corrosion, pushed-out pins and moisture, then clean and reseat them
- Wiggle-test the harness with live data running to expose an intermittent break
- Repair the damaged wire section with a soldered and sealed joint rather than a twist connector
- Replace the module that fails to communicate once the wiring and supply are confirmed good