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Hello, does any one know if a bad battery can cause ECM to throw check engine codes? I believe I have a bad battery. I'm getting the p0420 code bad cat converter. But I replaced converter at bank one back in January. If my cat is defective at least I have a warranty. But will have to pay labor to replace it.

I had 2 ignition coils go bad, not at the same time. Driving trying to get home with bad coil could have screwed up catalytic converter. Because it dumps unburned fuel into exhaust.
 

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It's easy enough to get the battery checked at your local auto parts store. And yes, weak battery can cause false ECM codes. Its kinda random, could be any code.
On the other hand, you're also correct that misfire that puts unburned fuel into the exhaust can mess up the cat. Tailpipe sniffer should be able to verify if the cat is still doing its job.
 

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I had 2 ignition coils go bad, not at the same time. Driving trying to get home with bad coil could have screwed up catalytic converter. Because it dumps unburned fuel into exhaust.
I'm not sure about the cat getting damaged by unburned fuel. Once a misfire is detected I believe the ECM shuts off that cylinder's injector.
 
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