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Reply To: Mazda3 P0421 Diagnostic Help Request

Home Forums Stay Dirty Lounge Service and Repair Questions Answered Here Mazda3 P0421 Diagnostic Help Request Reply To: Mazda3 P0421 Diagnostic Help Request

Jason Weremy
Jason Weremy

So I am having a similar issue with my 2008 Mazda 3, 2.0 with 240,000 kms(150,000 miles) in the last couple of weeks. We had a big cold snap around Christmas time and it was around -30C, not adding windchills. I wasn’t able to start the car under it’s own power or using a booster pack. I had left it until Boxing Day to where it would turn over with a booster pack. I drive the car around around the block a few times to let it warm up. The engine light of the P0421 remained. Over those 2 days of above freezing temps the engine code had went away. The car started fine for those few days. Another cold snap came in -30 to -40’s just after Christmas and the code reappeared.

The car runs at idle of 800 RPM once the car is completely warm. Driving it at highway speed it’s just under 2000 RPM. The p0421 engine code also stated that it wasn’t coming up to temp(The CAT). To me what would make sense since that it’s been in the -30’s for weeks. I have a Bank 1, Sensor 1 O2 sensor that I am going to replace anyways. But I wonder, if I change the O2 Sensor and I reset the light and it comes back, do I just assume that the CAT is messed and needs to be changed? It is 14 years old after all. Is there a way to check the CAT without just having to replace it? Can you even get a used CAT?

Before you answer. Just so you know. The Bank 1, Sensor 2 was replaced back in the Fall of 2021 as well as the spark plugs in the Summer of 2021. Everything on the car is completely stock. I do have an cheap OBDll scanner, no multi meter and the car is completely stock. Everything is original, just the Air/fuel sensor, spark plugs are new.

Any help would be greatly appreciated everyone! Thanks.

Much love from weekender DIY Canadian.

  • This reply was modified 1 year, 10 months ago by Jason Weremy Jason Weremy. Reason: Missed some letters