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Mechanic couldn’t fix my 2005 Honda Accord

Have a Honda Accord 2005 over 220,000 miles and the engine light is on. Have taken it to a mechanic and they have not been able to fix, any ideas?

Sure. Connect a code reader and post the exact codes here. Also, note that sometimes codes are generated by a defective PCM, or that the engine might be worn-out and a proper repair too expensive. In that case, the smart thing to do might be to try clearing the codes/disconnecting the battery to see if you can get the light to stay off long enough to get through your state’s inspection.

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first of all, we need a little more information?

Why was the check engine light on . . . what codes?

How long has the car been there?

Has the mechanic thrown a lot of parts at the car, yet the problem still persists?

Or does the guy not know where to start?

No one here will have a clue unless you post diagnostic codes. Otherwise, guessing could go on all day long.

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Diagnostic code P0420

This isn’t quite at the level of rocket science, even though it appears to be beyond your mechanic’s expertise.

That code could–hopefully–indicate that one of the O2 sensors has failed, or it could mean that the catalytic converter has given up the ghost. Clearly, the first possibility is the one that is far cheaper, but a mechanic who is bamboozled by that code is not one to whom you should return.

It looks like they already replaced the catalytic converter.

So, apparently they have been throwing parts–albeit very expensive parts–at the problem, despite having very little skill with diagnosis. PLEASE do yourself a favor and don’t return to that Charlatan Shop.

Edited to add:
No, it appears that they replaced an O2 sensor, and not the catalytic converter.

Thank you and appreciate the advice and fast response!

They didn’t. The invoice says “replace bad oxygen sensor at catalytic converter,” not “and catalytic converter.” None of the parts you were charged for is a catalytic converter. That said, if I’d poured $1200+ into a car and the problem was still there I’d be upset. I agree that it’s probably time to find another mechanic.

They also replaced the air flow sensor and the spark plugs. Check the engine air filter. That can also cause a P0420 code. Since they replaced the O2 sensor at the catalytic converter and the code is still there, look at that O2 sensor’s electrical connector to see if there is dirt or corrosion inside. Poor electrical contact can throw that code too.

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And all that can be done by a different mechanic since this one is at a loss.

One of the most common causes of the P0420 is exhaust leaks… I wont go in to the ways this works…just trust me on this one. Do you have any exhaust leaks? Certainly possible with this many miles.

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Every time a vehicle came into my shop with a DTC P0420, it was a bad cat.


This is actually not that complicated

Make sure there are no exhaust leaks

Make sure all the oxygen sensors are working correctly

Carefully look at the freeze frame- and mode 6 data

If you want, perform a back pressure test

But in the end . . . it’s probably going to need a catalytic converter

Quite a few miles on this car. I will probably get some people disagreeing with me on this one, but I’m a firm believer that a catalytic converter won’t last forever, especially if you really rack up some miles, and maybe the engine’s also burning some oil . . . ?

And after it gets replaced and the code is cleared, it is imperative that the mechanic drives the car so that the catalytic converter readiness monitor runs to completion. And after that, he should again look at the mode 6 data to see if it passed with a comfortable margin


I just hook my scanner up and watch for this.


The test that Tester posts above is what your mechanic should be starting with. Ask them if they’ve done it. If not they may not have the necessary test equipment. In that case you’ll have to find a shop that does. P0420 gets set when the car’s computer does that same test and the relationship between the blue and red traces fails.