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Check engine code P0420

I have a 2003 Toyota Corolla, mileage app. 106,000. Check engine light first came on when driving to destination 40+ miles away (most of my driving is shorter, around-town trips). My mechanic said code is P0420 and so they reset it and said if it comes on again that I need a new cat converter. It did not come on until about 6 weeks later when I was on a longer trip again (had driven about 40-50 miles). Can you tell me if the P0420 code absolutely means that a new cat. conv. is needed or if it could possibly be a sensor or something else hopefully less costly than a new cc? Thanks!

You might need a new cat converter, or you might need new oxygen sensors, or you might have a problem with the wiring running to the O2 sensors, or…

If I were you, I would consider asking friends for recommendations on a new mechanic, as yours sounds like he may be a “parts replacer” rather than a mechanic who searches for the simplest, cheapest effective solution for a problem.

VDC is correct as usual. All that code really means is that your “upstream” (before the cat) and “downstream” (after the cat) oxygen sensors are reading too similarly. If the cat is working properly they should be reading very differently and the computer is comparing those readings in order to keep an eye on the cat.

So one common reason that they read too much like each other is that you do have a bad cat. However, all you have right now is a report about the comparison between those sensors. Before replacing the cat it is best to eliminate the possibility that this is happening for some other reason (e.g. exhaust leak, O2 sensor, sensor wiring, car running rich).

It also could be as simple as replacing the exhaust pipe to manifold gasket (aka, doughnut). On the troubleshooting chart fro this code, that is the first step listed.

Toyota’s have a problem with this code, the cat rarely fixes it.