Q: Check Engine Light is on - 2003 Honda Accord

Hello. If the issues appeared when using the AC and defroster, then they are likely related to that system. In most cases, the AC / Heater does not use vacuum lines as a part of it’s operation, so a vacuum leak is not a likely a cause for your symptoms. However, there are some exceptions. There are also a wide variety of issues that can produce the exact same symptoms as a vacuum leak.

If you believe you do have one, however, I would closely inspect the engine for any worn or damaged hoses and listen for any hissing sounds that may hint as to where the leak is coming from.

Finally, it may be possible that the catalytic converter is part of the issue, again though, this is less likely due to the fact that the catalytic converter and emissions system usually functions completely independently of the AC system. In order to give more insight on whether or not the catalytic converter is a part of the issue, this would require the trouble codes that came on when the check engine light came on. If you would like to get a more definitive diagnosis, a certified professional from YourMechanic can come to your car’s location to pinpoint the cause of the Check Engine Light and proceed with the proper repairs.