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I Replaced My Battery And My Car Won't Idle Properly And The Engine Light Is On

You'd be surprised at how often this specific problem comes up for our Kashy mechanics. A battery replacement service or a roadside crew comes out to replace the battery and all of a sudden the engine light is on and/or the car won't idle properly.

What causes the engine light and poor running problem after a battery replacement?

Well, modern cars are very complicated bits of machinery that use a vast array of computers to control everything from the A/C temperature to what gear you're in.

Over the last 40 years, these computers have made cars significantly better by reducing emissions, making ABS (anti-lock brakes) possible and increasing reliability (amongst other things).

One of the most interesting features that computers have added to cars is machine learning (aka. fuzzy logic). The computers in these cars take ongoing measurements for everything from the oxygen (O2) sensors to the idle control valve and make long term adjustments to their running profile to accommodate for changes as the car (or part) ages.

The idle control valve is where we want to focus our attention today.

You see, the big problem with all of this is that these learned values tend to suddenly get lost if the car loses electrical power for any reason. Because the memory from these specific maps is lost, a whole raft of issues can suddenly come up that you wouldn't normally associate with each other.

A loss of electrical power such as when the battery is taken out for replacement.

The most common things we see (and the reason you're reading this blog post) is the engine warning light and poor idling. However, this can also include the radio forgetting its channel settings, the windows forgetting their automatic down and up function and a whole bunch of diagnostic trouble codes in every system.

So, how do you fix this?

The best solution is always prevention, and the way to prevent this is to use a memory saver device when changing the battery. This allows enough electricity to stay in the system to keep the memory up and running.

Beyond that, the best thing to do is an idle relearn procedure and check (and clear) the diagnostic trouble codes. There are a lot of ways to do this but each car will be slightly different. If you can't figure it out yourself, you can always give us a call at Kashy.

And remember, if you're going to be leaving the car for any period of time, always take precautions to keep the battery in good health.


If you have any questions about this blog post, would like a second opinion from a mechanic or would like to find an honest mechanic in Brisbane, check out Kashy here.

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