What does it mean?
P1681 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Control Unit Programming, Programming Not Finished .
Symptoms
- MIL on and, in most cases, fault codes in several modules at once
- The affected module runs in a bootloader or failsafe state with reduced or no function
- Engine may not start, or runs in limp mode with severely reduced power
- Scan tool reports the module as not programmed or with an invalid software version
- The code returns immediately after clearing, because the programming is genuinely incomplete
Likely causes
- Programming interrupted before it finished: tool disconnected, cable unplugged or session timed out
- Supply voltage dropping during the flash because no battery support unit was used
- Ignition switched off, a door opened or the vehicle disturbed during a session that must run undisturbed
- Wrong or corrupted software file selected for this vehicle
- CAN communication error or bus interference during programming
- Module hardware not compatible with the software version attempted
- A second diagnostic tool or an accessory on the bus interfering with the session
How to fix it
- Do not simply clear the code and release the vehicle; the programming has to be finished properly
- Connect a battery support unit or charger that can hold the voltage steady through the whole flash
- Use a stable, wired diagnostic connection and repeat the programming session from the beginning
- Verify the software file matches this vehicle's module hardware and part number before starting
- Disconnect any other diagnostic tool or aftermarket device from the bus during programming
- If the tool cannot reach the module, check its power, ground and CAN wiring first
- After successful programming, carry out the required coding and adaptations, clear all memories and re-scan every module