2000 RX300 Check Engine light on after Battery Replacement

There are hundreds of possible trouble codes that would illuminate the check engine light, none that relate to battery replacement. Rather than making wild guesses, take the car to someplace that will check the trouble codes for free, or lend you a scan tool like Autozone. After you scan the codes, write down the numeric trouble code(s) and post them here.

Many thanks for your reply. I took the car back to the same mechanic and using a scanner he confirmed that this is due to two (or three) oxygen sensors which need to be replaced. As of now, he has reset the code so the light does not come up. He told me if it lights up again and stays on .. I need to drive it for 60 miles or so and then bring it in so he can replace the two sensors.

Does this sound right to you ? I do trust my mechanic .. have been going to him for around 7 years for my old 1987 Toyota Camry. Question .. what do these sensors do ?

Thanks again.

As I mentioned before, it is unlikely that a battery replacement would cause a check engine light to come on. It is certainly feasible that one or more oxygen sensors has failed. The diagnostic trouble code (DTC) for O2 sensors uses 2-trip detection logic, which means that the computer has to detect the same problem on 2 consecutive trips.

Most likely, the check engine light will come back on. It would help if you got and posted the specific trouble codes that your mechanic found.

Yes, I remember he tested twice and got the same code. I will ask my mechanic and post the actual trouble codes found.

While battery replacement shouldn't cause oxygen sensor codes, your mechanic was right: drive it for a while. Let the battery charge up and see how things work.

Normally batteries should be charged prior to installation, because they sat on the shelf and gradually lost charge. Otherwise it will take a car (especially a low miles, low speeds one a long time to fully charge the battery). A weak battery may mean the computer isn't working properly.