My gf '99 ES (1.8 liter) has thrown the two subject codes. She had the P0420 for a while, I tried cleaning both O2 sensors and clearing the code but it came back. Can anyone tell me the likeliest candidate for causing these codes (pre-cat, O2 sensor, etc)? I am in the middle of a move and don't have the time to be troubleshooting. Thanks.

 

The P0171 is a lean code. I mainly see this if the MAF is dirty. These cars are really suspect to set a code if you use an after market air filter.

The P0420 is the converter. The front O2 monitors pre-cat....the back O2 monitors post-cat.

When the readings mirror each other that means the converter is doing nothing. P0420 sets when the readings are to close to each other. The converter is covered for 8/80.

 

Thanks. Unfortunately her car has 94,000 so I'll have to get a cat on my own. I'll clean the MAF once we get settled in.

 

I had the same code on my 2000 and it was a leaking intake gasket.