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Corolla Check Engine Light

I met another gentleman today at the Advance Auto. His 04 and my 03 corolla both decided to need batteries on the first cold day of the year.

During the installation of the batteries he posed this question;

“Does your car have a check engine light on”

Of course I normally would of thought that to be an odd question but, as I have been trying to diagnose why my engine light has been on for months now; not so odd.

After 2 mechanics with the normal replace your O2 sensor, if that does not work replace your cat. convertor; both of which were already done (cat from salvage yard because dealer price of 1200.00 a little high) no solution. Everyone I know with car knowledge not a help. Now have been driving around for 15K and no change in performance.

Went to local Valvoline for quick oil change and he mentioned an Exhaust recirculating valve problem. I have looked in my trusty repair manual without finding solution. Does anyone have ideas on this issue.

Whoever sold a used catalytic converter from a salvage yard is in violation of EPA laws.

I may have to bring this post to the attention of the EPA!


I assume the light has been on for nearly a year. What is the exact codes? They should be in the form of Pxxxx, like P0130. This is important, because the O2 sensor, catalytic converter, and EGR all have different codes that will point straight to them if they are the problem. We are guessing just like you until we know the codes. And throwing parts at it blindly will not fix the problem, only lighten your wallet. Advance Auto should be able to read them for free. They do around here.

Have the fault codes downloaded and post them here. We’ll try to hep.

Check engine these days is not enough to go on. If you did the o2 sensor and the cat(used does not matter) then maybe a mechanic did not quite get the code right. Post the codes and also post if you can a 02 sensor test of the downstream vs the up stream o2 sensor (these provide a check on the cat converter). To the previous poster a recycle cat only needs to be good no law broken(but you gotta test it).

Waiting for code to re-appear after changing battery, Had code written down but lost it. When I asked the dealer and advance auto the code pointed to front O2 sensor and cat. which was the sensor I replaced. Same code before and after replacement. Replaced cat. also without change. As soon as code comes up I will post it. With driving 100 miles/ day it should not take long.


P0420 - has nothing to do with the EGR system. Don’t even go to quickie oil change places for oil changes, let alone diagnostics.

This will give you a brief run down for the code P0420:

Do you suppose any of these mechanics have checked the exhaust system for leaks? Have they checked the wiring to the O2 sensors? Checked the coolant temp sensor? Anyway - avoid shops who replace parts as a diagnostic procedure. Find one that will actually test stuff to find out what the problem is. There’s a really good chance that your problem is not in the Os sensor or the cat - but this was an expensive way to find out.