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My 2017 Civic’s check engine light came on last week, and the code reader says it’s a P0420 for catalyst efficiency below threshold. The weird part is I don’t notice any drop in power or rough idle, and my fuel economy seems normal. I’m just trying to understand if this is something that needs immediate, expensive attention or if I have some time to figure it out.

I had a P0420 once on my Civic and there was no power drop, no rough idle, and mileage didn’t look off. It felt like something mild was nagging the ECU. I figured it could be a tired downstream O2 sensor or a marginal catalyst reading from a long highway run. I checked the gas cap, reset the light, and kept driving for a few days to see if it cleared; it didn’t.

I've learned that this code sometimes isn't urgent. Often it’s an O2 sensor edge case or a small exhaust leak. If you’ve only had short trips lately, the car might not show obvious symptoms yet. In my case, after replacing the sensor, the light stayed off for a while before coming back, so I treated it as a long-term fix.

One quick question: is the light steady or flashing?

During one stretch I did a quick under-hood check, made sure the gas cap clicked, and cleared codes, then drove roughly 60 miles to see if it would reappear. It did, so I parked the issue for a proper diagnostic rather than chasing it on the cheap.

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