CarTalk.com Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

Check engine light

Check engine light is back on after each of the following: replacing catalytic converter, replacing both oxygen sensors. Problem is NOT a loose gas cap. Diagnostic message is “lean exhaust”. Can ANYONE tell me what is the problem? Can’t get an inspection sticker with check engine light on in good old Massachusetts!

What codes caused you to have the catalytic and sensors replaced? Did you do the job or a mechanic? Was the light off after you had the parts replaced and come back on or did it stay on after service?

Exactly what is(are) the fault code(s)?

What year is the car?
How many miles does it have on it?
Any other symptoms?

PS: there are hundreds of codes, and none of them say “lean exhaust”. We need the actual number. It’ll probably be something like a P0170.

So many incompetent people working on cars these days. It seems mechanics have become a bunch of parts changers. You need someone to look at the mass air flow sensor. You are probably getting a P0171 generic code and the person you have working on it probably doesn’t even own a multi meeter, because thats all you need to check the things that where replaced to see if they needed to be, and they didnt.

my mechanic told me the catalytic converter needed to be replaced - and then he did it, when the light came back on, he said it must be the oxygen sensor, after he replaced it and the light came back on, he sent me to another mechanic with more sophisticated diagostic equipment, he said it was the other oxygen sensor. In each case, the light came RIGHT back on after the repair, except the last time, it stayed off for a couple of months and then on and off for a couple of weeks,then stayed on. I don’t know what the actual codes were.

Go to a parts store that offers a free scan tool scan. Post the number here. The off and on behavior is certainly odd. The primary parts that control the lean/ rich mix are the O2 sensors for the fuel and the mass flow sensor in the intake for air. Without the original codes and year of the car I can’t say whether they did the right thing for you but its a bit late to worry about it since it did fix it for a while.

I do not have the actual fault code. It is a 2000 Corolla with 157,000 miles on it. It sometimes hesitates when accelerating, almost to the point of not moving and I’m pulling over to the side of the road thinking I’m breaking down.

Okay, the original mechanic replaced the upstream oxygen sensor and when that didn’t solve the problem, the guy he sent me to - his paperwork says the code was “P0171 Lean Exhaust Bank 1” replaced the “Reman air flow sensor XTP450010” and the air filter. that’s the repair that worked for about a month or two and then the light came back on and off then stayed on. So is that the mass air flow sensor that he replaced? He told me it was the oxygen sensor. And he charged me for 2 hours labor at $75 an hour just to diagnose the problem. I know he tried cleaning it before he replaced it. So now what? This is a 2000 Corolla with 157,000 miles on it and I have to keep it for at least another year - and I have to solve the problem long enough to get an inspection sticker. Thanks for your help.

Many auto part stores will read the codes for free. Get the numbers. That CEL is like the kid in class waving their hand to answer the question. If you want a right answer, ask them (get the codes) and don’t ignore them and don’t ask someone else who might not know as much.

Maybe, the engine really IS running lean! Why do people, including many mechanics, suppose that the engine computer is mistaken?!

Having an air leak anywhere passed the MAF intake sensor or before the O2 exhaust sensor will cause a lean exhaust code and may be the real problem. Since the trouble cleared for a while after working on the MAF sensor perhaps the trouble (air leak) is near that area. Spraying some WD-40 around suspected trouble spots on the intake side while the engine is running may show up the trouble as the idle should change if the fluid is sucked in.