Why and when cycling ignition matters

Turning ignition off triggers the ECU’s software shutdown procedure.

This includes among other things:

  • Saving important variables (microcontroller RAM) into the extra data (EEPROM) chip – necessary to cope with potential power loss (disconnected battery).
  • Actuator testing (throttle, EGR valve)

Only after finishing shutdown the software cuts off the ECU’s own main power supply via relay. The resulting relay click sound was audible in my car, shutdown took around 15 seconds or so. I also tested this relay using own kernel software, confirming the I/O port number.

Here’s a problem I noticed with my Impreza Diesel during a dealership service where I had an oil change and software update done:

  1. Oil dilution reset – without ignition cycle
  2. ECU software update

Checking oil dilution parameter afterwards revealed that it actually had not been reset!

I am almost sure, the reason is that for the software update the ECU software switches into a very special mode. In this mode, it does not know anything about an engine, it only knows and responds to a few commands to transfer new software into the chip. Apparently it did not save variables (including the zeroed oil dilution amount) into EEPROM before switching modes.

At ignition-on, the software goes into normal mode, loading saved values from EEPROM into the chip’s RAM, ready to run the engine.


Either cycle ignition or do any maintenance/adjustment procedures after a software update.

In any case, verifying parameters after a such operations is always a good idea – takes only a minute whereas a failed procedure can cause the car owner troubles e.g. another dealership visit or much worse (DPF light, limp mode).


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