01 prizm. P0420 code. common causes?

[dizwiz24]

My check engine light came on my 01 prizm.

Autozone scanned it and it says P0420, reduced catalytic converter efficiency.

It says large vacuum leaks, engine oil consumption, etc could cause this.

I replaced the motor w/ one from an 07 corolla back in 08 due to heavy oil consumption. So I know engine oil consumption is not the issue here (also the oil level doesnt change on the dipstick).

So what can this be?

Is my cat possibly messed up from thousands of miles of burning oil (before engine swap)?

Could this be a simple o2 sensor?

Any help is appreciatd

[sam333]

How many miles on your cat? They loose efficiency over their life span. 150000 miles is a good run for a cat. If you were burning a lot of oil, it will shorten the life of the cat , how much depends on how much oil you were burning. Either 02 sensor could be bad also. The computer compares the front 02 sensor signal to the rear 02 sensor signal, and expects to see a different wave form depending on the amount of oxygen present( should be less oxygen and a lower frequency) after the cat. Outside air(bad gaskets/holes/cracks) entering the exhaust anywhere before the 02 sensors can also cause false readings and throw a code.

[leakyseals]

Originally Posted by dizwiz24

Is my cat possibly messed up from thousands of miles of burning oil (before engine swap)?

More than likely what happened. The ECU is set to trigger a CEL somewhere around 80% efficiency for environmental standards. That means if you dont feel restriction on the highway, you can drive that way till inspection. suggest not to use aftermarket cats, the ECU is programmed for specific results between the upstream and the downstream 02 sensors. Use of aftermarket cats with more flow and less media "might" produce a CEL in weeks/months time. Might means about 50/50 you have no problem. Expensive odds if it doesn't work out.

Originally Posted by dizwiz24

Could this be a simple o2 sensor?

about 10% possible its an o2. Can try cleaning downstream o2 with throttle body cleaner, followed by MAF cleaner. DO NOT use seafoam deep creep or any cleaner with heavy lubricant on an air sensor.