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2000 Honda Accord check engine light mystery

I have a 2000 Honda Accord V6 sedan (under 100k miles). For two years now I have had a check engine light.

Originally I was told it was a CO sensor that was broke.
But now when I get the car inspected, the inspector says I have a catalytic error.

So off to the mechanic. They replace the catalytic converter, claiming it was just an empty shell.

Check engine light comes back on the next day after all that money spent. Back to the mechanic. He says now that the intakes into the catalytic system there are all clogged, so that needs to be scrubbed.

Many hundred dollars later, and the problem persists. Still a check engine light.

Has any else experienced a similar issue? Does anyone have a clue what could be causing this check engine light nightmare? I cannot get the car inspected so I have that added head ache.

Thank you much.

Take the car to Autozone and have the codes read. They will do this for free (unless you’re in California, in which case take it to the dealership or an independent mechanic who is not the mechanic you’ve been taking it to, and pay for a diagnostic). Make sure you get the actual codes (it will be a 5-digit alphanumeric code such as P0100), not the clerk’s interpretation of the codes, and post them here.

It might be a bad O2 sensor. The exact code would provide a clue.

Also Google “How to pass a smog test”, there are some good informative links there on this topic.

Have you gone to the dealer ? There are a few instances where the sometimes higher rates of the dealership are offset by actually solving the problem.

The code is P0420.
The ‘code definition’ reads: Catalyst System Efficiency Bellow Threshold (bank 1).

Car inspections here in Houston are not the kind you can just easy get past like they used to be due to our area-wide high pollution.
My next step, if I could not find a solution, was to just take it to a dealer. But I spent so much money already that things are getting aggravating.

I had this code on a Corolla, a new rear O2 sensor fixed it. Often this code will appear if there is a leak in the exhaust system before the cat.

When my car had that code failure, I did a lot of googling. And, a lot of mechanics said in most cases it is not the cat itself, but the sensors.

A review. There is no direct way to test the cat. So, there is a sensor at the input and one on the output. When the car is cold, the convertor cannot work. Once it is hot, the computer expects the input to have crud, and the output to be burned clean. Thus, there will be a difference between the two sensors.

If the cat is bad, there will be no difference. If the output sensor is bad, it may not show a difference. If the input sensor is bad, it may not show a difference.

It seems most mechanics throw the most expensive part at it. Then, when that doesn’t fix it, sell you the sensors.

I bought a scanner which supplies sensor data. I got an EQUUS 3140, more than you need, because I live in Mexico where there are beau coup old cars, and the 3140 will read OBDI on some makes. If you hook up and note the normal readings when the car is working, you will have a better idea if this fails later.

It could be the sensors, but on newer cars the software can usually tell if the sensor isn’t working. My guess with that code it is the cat that has failed. Or is failing. The cause of the failure could be something wrong with the engine of course. But it wouldn’t be that surprising that a cat would fail at around 100K.

I guess what I’d do is bring all the routine engine maintanence recommended by Honda up to date, and if that didn’t fix it, I’d replace the cat and probably both sensors (before and after the cat) at the same time. That should get you through the smog test with flying colors.

If you want to try to fix it on the cheap, just replace the sensors. There’s chance – slim perhaps – that will be good enough to pass the smog test.

Could be simpler: My brother had the check engine light go on in his car (Toyota Camry) and his pickup (Dodge). I suggested that after refueling he should tighten the cap till it clicked at least three times. Voila! No more check engine light problem.

The check engine light can indeed go on if the gas cap isn’t tightened properly. But the code reader would say the gas cap or something associated with the gas tank vapor capture system was the cause. The code for this car didn’t say it was the gas cap. It says it is the cat converter. There are sensors before and after the cat, and if the cat is working, the sensors should read different values. If they read the same, that means the cat isn’t working. Unlikely to have anything to do the the gas tank. So, while there’s no harm trying, I doubt tightening the gas cap will work for @whitehonda.

I had this same issue, still coding P0420, with a new cat. But it was aftermarket, and was too small. A larger 1 solved the problem. I have never had a bad o2 sensor set this code on a Honda, just a bad cat.