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Originally Posted by JTrujillo86
Do you think that if it were the cat that it would set off the SES light?
Jeremy
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Yes and no. I've seen bad CATs and no SES/CEL light. Normally the rear 02 sensor would generate a DTC such as P0420 after a few drive cycles and eventually generate a SES light. However, a CAT DTC error code shows up as a pending error and not a fatal error on your typical odb-ii scanner/code reader. I have a top of the line Actron scanner and the Autotap program running on a laptop and the indications are similar. It doesn't cause the ECU/PCM into limp home mode. Pending codes do not always trigger the SES light.
I mentioned CAT as a last resort because of the mileage on the car. Most GM CATs go bye bye after 80K miles which is when the emissions warranty expires. They've got over 100K on that Alero.