Rep Power: 0

I bought a 2003 Civic LX Sedan, 1.7L SOHC, engine code D17A1. It has the cat that is built into the exhaust manifold on the front of the engine. After buying the car, it had no engine codes. I drove it for a week or so with no problems and no check engine light.

I then did some maintenance on it, including a new engine air filter. I did not change the spark plugs, they looked fine. However, I did clean the throttle body, which was a little dirty, using CRC Throttle Body Cleaner. I sprayed it in the throttle body and wiped it clean, but I didn't soak it. I also changed the PCV valve and replaced the leaking cam plug on the passenger side of the engine (common leak).

Anyway, after doing these things and driving it to the store, I got a check engine light with code P0420. The code keeps coming back when I clear it.

I understand that the code indicates a bad cat. However, I'm just wondering if the above maintenance somehow caused a problem which triggered the code. I replaced the front oxygen sensor with an OEM NTK one after getting the code, it was pretty easy to do. The code still persists. I haven't tried replacing the rear oxygen sensor yet.

Any ideas?

 
Rep Power: 119

First idea, you have a bad cat.
Second idea you have a bad cat.
There is a remote possibility there is a issue with the down stream 02 sensor, but most likely you have a bad cat.

Likely reason, seized oil control rings and or valve stem seals.

Have you been driving it enough to judge oil consumption?

Very likely one of the reasons the car sold in the first place.

 
Rep Power: 0

I have put a around 400 miles on it since changing the oil and the oil level appears to be the same.

 
Rep Power: 244

The cat is probably not cleaning as well as the car thinks it should. The sensor could just be dirty so you can try cleaning it and or replacing it but it is very possible the cat is bad and why it was sold becaue they are expensive!!

You can put paint thinner or something like that in the gas and it will shut the CEL off but it usually returns.

 
Rep Power: 0

I forgot to mention the car has 136k miles.

 
Rep Power: 119

Gently whack on the resonator under the passenger side before rear wheels with a rubber mallet. Does it sound like it’s full or rocks? If so you definitely will need a new cat.