I have a 1996 Explorer with 148,000 miles on it. I recently performed a dynostic check because the check engine light lit up. The code told me the aft oxygen sensor was causing it. I was told the catalytic converted was failing. Is this typical and if left unresolved....what is happening to the engine or pollution control system? It seems to be running fine but may be laboring a little. Is this because the air flow through the converter is becoming restricted? I am facing a $450.00, labor and parts. The truck is in great shape and I am the only owner and plan on keeping it until it becomes too expensive to operate safely. Thoughts anyone.......?

Mike
Definitely replace the catalytic converter if that is the problem.They not only restrict exhaust flow for poor engine performance but can get so hot that it could cause a fire..



Roy

Smallblock:

Thanks a bunch.....will do.

The converter can fail two ways. It can become restricted and cause engine running problems and overheating, or it can just become inactive or actually hollow itself out. In the second case, the engine won't be affected at all.

But it will cause a code either way. The oxygen sensor in front of the convereter is used as a fuel system trimmer, always making adjustment for rich and lean in what is known as closed loop operation. When it goes rich, it drives it lean, when it goes lean it drives it rich. This cycling not only keeps the engine fuel mix inline, it makes sure the catalytic converter has the correct compounds to do it's job.

They won't convert NOX when the mixture is lean. To convert NOX there needs to be CO and almost no O2.
To convert CO and HC there needs to be O2. The cycling helps supply what is needed to convert all compounds.

The O2 sensor after the converter is there to ensure the cat did it's job. When lean, it stores oxygen, which is released and used when rich. The result is a very steady level of oxygen after the converter, instead of the cycling level infront of the converter. This compairson is how the computer knows the cat is working, and not getting lazy.

So either the cat is failing, or the sensor is failing. It's a good idea to know which before doing repairs. You may need both.

If the signal from the front sensor matches the rear sensor, the cat is bad. If the front is normal but the rear is unresponsive, the sensor is bad.

When the rear is steady but responsive, and the front sensor is cycling, all is normal.

Thanks for the explanation...Mr. Styer.