I’ve experienced and heard about many cases where a vehicle’s computer needs to “reset itself” after a battery change or loss of electrical current to the computer. This usually means rough and uneven idling and, on my Mazda6, a refusal to idle and stalling every time I step on the clutch pedal. This reset procedure usually takes a couple of days and a couple of hundred kilometres to rectify. Why would some vehicles “reset” so quickly and others much longer?
DEON THERON, Wellington
Answer: As there are so many software suppliers to carmakers, it’s difficult to comment on the strategy of a specific case like yours. It’s true that in some ECUs, injector offsets, cylinder-balancing fuelling and so on have to be “relearned” once the battery has been removed. In most cases, however, the values are stored in permanent memory so that they remain when the battery is removed.
If some values need to be relearned, the entry conditions to the strategy need to be met before the learning process can commence. This includes engine temperature, speed and load, which explains why it sometimes takes longer.
When an ECU is swapped at the dealership, it follows a procedure informally known as “breathe in, breathe out”, where all the learnt values in the old module will be copied to the new module to eliminate a relearning process. The idling issues in your Mazda6 should have taken place only after you replaced the battery. If not, there may be another problem unrelated to the ECU.