Autozone found P0155 code for the check engine light about 3 months ago. I found that I had cracked exhaust manifold. I replaced exhaust manifold and cat converter (they were one piece) and the O2 sensor on the maniford but did not change the second O2 sensor. Check engine light went away. After 3000 miles (about 3 months) it has come back with a P0420 code. I have 256000 miles and car is running perfect. What should I do.

 

P0420 is code for inadquate catalytic converter efficiency. It can be caused by exhaust leaks, forward and rear o2 sensors, and failed catalytic converter.

Since you had exhaust work done, I would carefully check for leaks. Not familar w/ Civics, but a good source for leaks is the flexible joint between exhaust manifold and catalytic converter.

If you have a scanner, you can scan the rear O2 sensor to see how it's performing. I had a P0420 on a Subaru Forester, and would see fairly repaid changes in voltage between 0.1-0.9V. This indicates the converter is not storing O2 and smoothing performance.

Interesting thread on subaruforester.org about cleaning the catalytic converter w/ a mixture of citric acid and water. For the truly curious and thrifty.

good luck

 

This area is for questions about how to work the website.

Moving it over to General Tech.