Check engine light

My wife took her 2002 Ford Focus to the dealer because of a check engine light. The dealer replaced the battery and the check engine light went out. How are these connected?

Possibly related to battery voltage being out of range. There are a number of codes for this kind of problem but we don’t know what codes were present.

If the battery is 4 or 5 years old it’s had a good life. If it’s the original battery then it’s had a great life.

Was this the original 8 year old battery? What problems had there been starting the engine, recently? I’m presuming there were warning signs that the battery was getting weak.
This, probably, turned out to be an expensive diagnostic (connecting a trouble code reader to the engine), and battery installation fee, and battery cost.
This is another reason for people to change their battery before it “gets too tired to carry on”.

Replacing or disconnecting the battery will disable the code. Also their equipment will disable it if an occasional error condition.

The battery likely needed to be changed however that CEL may illuminate again. Bring it back ASAP to them if it does no charge to diagnose for real this time. Charge for fix thoguh.

The most obvious connection is that disconnecting the battery clears the computer memory. That will turn the Check Engine light off temporarily. But it won’t keep it from coming back on if the condition that caused the light is encountered again. (Some of these conditions on some cars are notoriously intermittent)

Mandated trouble codes codes are emission related and therefore don’t include direct checks of the electrical system. The manufacturer could add checks of the battery and charging of course, but I don’t think Ford has. I don’t see any for the focus here . But if the battery/charging system is bad enough I would imagine that one of the sensor voltage checks would fail or the computer itself would start to do odd things.

Anyway, there is a good chance that the dealer knows what he is doing. They see Focuses (Foci?) with lit Check Engine lights all the time.