What does it mean?
P1605 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Internal Control Module Malfunction .
Symptoms
- MIL (check engine lamp) on, and the code usually comes straight back after clearing
- Long relearn behaviour: rough, hunting idle for the first minutes after every start
- Shift quality worsens where transmission adaptations share the same memory
- Readiness monitors will not complete, so the vehicle fails an emissions inspection
- Occasional no-start or unexplained stalling
- Radio presets, clock and other stored settings lost at the same time on some vehicles
Likely causes
- Interrupted keep-alive battery feed to the module: blown fuse, corroded terminal or a broken wire
- Battery disconnected, replaced or run flat repeatedly without following the correct procedure
- Poor or high-resistance module ground
- Voltage spike from jump starting or arc welding on the body
- Failed internal memory chip after long-term heat cycling
- Software corrupted by an interrupted programming session
- Aftermarket wiring spliced into the constant power circuit
How to fix it
- Check the constant, always-live fuse for the module and confirm it has power with the key off
- Voltage-drop test the module constant supply and its ground under load
- Load test the battery and verify charging output; a tired battery triggers most of these faults
- Look for corroded splices or previous repairs in the constant power circuit, and for accessories tapped into it
- Clear the code, leave everything connected, drive the vehicle and see whether it resets
- Reflash the module with the current calibration if the software level is out of date
- Replace and program the module only after power, ground and battery all test good