The Dreaded Check-Engine Light - Which Can Cost You Big Bucks


My check-engine light came on yesterday, about a week after I had filled gas. I felt it coincided with the hot weather and my increased driving. So, this morning, when it was still cool, I started the car and the check-engine light was still on. My car is a 2007 Prius which burns a lot of oil - usually needing about a minimum of four quarts at oil changes.

This morning, I changed the oil (from Feb 7 2025, it had burnt one quart) at around 7:45 AM and then filled gas at around 8 AM. The check-engine light persisted.

I got the code read at Autozone after Autonation Toyota refused to "just read a code, but there's an Autozone nearby." It was P0420 - Catalytic Converter System efficiency below threshold.

I searched online and learned about Cataclean - which says you need to add it to a tank that's one quarter full.

So I decided to burn some gas by driving - starting around 6 PM.

When I started the car, the check-engine light was off.

Is there any explanation?

chat-Ji

2. Oil Burning

  • You mentioned your 2007 Prius burns a lot of oil. That is very relevant.

  • Burning oil can coat the catalytic converter, degrading its efficiency and triggering P0420.

  • Fresh oil this morning means less burning and less contamination short-term → slightly better readings → light goes off.

3. Drive Cycle Helped Clear It

  • Driving after changing the oil and filling fresh gas may have improved combustion.

  • Long, continuous drives help the ECU retest the catalyst under optimal conditions.

  • The P0420 code was probably in a "pending" or "historical" state, and enough "good" data has since come in to clear the light.

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Follow up - I had an appointment with Autonation - Anthony M. service advisor.

I give him a call today, tell him it's P0420 and ask what they'll do. "It's pretty much the whole manifold system and the O2 sensors". How much. "They run anywhere from 3-4k" *d! And the tone! Not one iota of conveyance of concern for the customer and trying to save the customer money. Why? 
I tell him, it it's the O2 sensor, there would have been an additional code on the OBD2. Do they first try cleaning the cat and replacing the O2 sensors and seeing of the code goes away? "Not really. You can't really clean it. Once you get a P0420, it pretty much means the catalytic converter isn't really working anymore." *d! 
Long story short, I'm out $17 for the emissions inspection - which I passed this morning before I called AM (absolute moron) and canceled the appointment. "Let me shop around. Let's cancel for now."

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