PLEASE HELP!! 96' lumina catalyst trouble code..cant figure it out!!!

I have a 1996 lumina that has been nothing but trouble since i bought it...recently my check engine light came on and i took it to the local autozone to have it checked on their obd II tester....it showed CATALYST trouble code..Before i have the catalytic converter replaced(for 250$)i'd like to be pretty sure thats the problem... i think that the EGR valve or the O2 sensor could set off this code as well...what im really hopng for is someone that has had this problem before and found a resolution....PLEASE HELP!!!

o and i forgot to say that it runs well at low speeds but when i give it gas it sputters and cuts out...

Sounds like you've got a few different things going on.....
Please post the exact code(s).

i dont kno the exact code but if it will help...i can go back and get it...will post when i do......

....it showed CATALYST trouble code..


o and i forgot to say that it runs well at low speeds but when i give it gas it sputters and cuts out...


It seems odd that the only code you would have would be a P0420 (catalyst efficiency), and not a P03xx (misfire) as well.

If you can get the exact code(s)...it would sure help

OK got the code PO420...thats it, no missfire codes...

OK got the code PO420...thats it, no missfire codes...
Google p0420, lots of tech articles on the cause and diagnostics for this code, it can be a complex problem to pin down. It is not always a cat issue.

Spend $25 for a diagnostic vacuum guage. Hook it to a manifold vacuum source, warm the motor up real good, then hold it at 2500 rpm steady and watch the gauge...if the reading falls off the longer you hold it, the cat is partially blocked..

If you can run the motor up fine in park but not in gear, I'd be wondering about a fuel delivery problems, which causes incorrect mixture and subsequent exhaust gas/cat efficiency problems...like you said, the cat code could be symptomatic, rather than the problem...

i tested the vaccum and there was no change.....so does this mean its not a cat prob??? if not, what are the other most likely sources for the problem?

By the way...motor revs fine in park, but when off the throttle, idle is rough, almost like a miss...but no missfire code comes up... could it be backfiring???

Id also like to know how to check the egr valve to make sure its working properly...and i found out today that there are three separate 02 sensors in my exaust 2 befor the cat and one after...if it is a sensor problem would it b the ones befor or the one after the cat????? PLEASE HELP

[QUOTE=jeffcoslacker]Spend $25 for a diagnostic vacuum guage. Hook it to a manifold vacuum source, warm the motor up real good, then hold it at 2500 rpm steady and watch the gauge...if the reading falls off the longer you hold it, the cat is partially blocked..


I bought the vacuum gauge hooked it up and there was no drop while in park with the motor reved to around 2500 steady (no tach)..i put the car in drive and held one foot on the brake and revved the motor again and had a slight drop (10 psi)..i found that i can hold my hand covering the exaust pipe with barely any pressure blowing out(i can actually plugg it off with no air excaping just with my hand) Catalytic Converter??

For now, discount the converter theory. It's evident that you probably don't have a data scanner directly at your disposal. That would make diagnosis a lot easier, since you could check both the main HO2S and the rear (cat efficiency check) O² for output signal. You could also sample injector pulses on each bank for comparison, and as a hint to the real fuel mixture.

This PO420 error condition can be caused by many things. One way to generate this error code is with a failing converter. The PCM monitors the two main HO2S and compares their average signal to the rear O². If there is not a higher EGO at the rear sensor, the PCM will presume a problem. If the converter has been contaminated by oil, coolant, incompatible fuel, or has simply lost all its catalyzing rhodium and palladium, it may no longer be converting exhaust gasses efficiently.

Another possible problem includes a failing oxygen sensor. The typical failure mode of a zirconia oxygen sensor is a weakening output signal. A lower voltage signal normally indicates to the PCM a lean exhaust (more oxygen), so the PCM compensates by adding fuel until the sensor is satisfied. If the signal is still within the allowable limits, but weakening, the resultant extra fuel on one bank of cylinders will cause an overall rich condition in the exhaust, and that will be sensed by the rear O² (behind the converter). This difference in signals will trigger the PO420. As long as the PCM doesn't detect a difference in the HO2S on each bank above a threshold level, it won't trigger a front O² code. It relies very heavily on the O² signals being accurate. If both front HO2S are weakening, the situation could get extremely rich, and as long as there is not a serious difference between the two bank HO2S signals, or injector pulse widths do net exceeed a threshold warning level. the PCM will not necessarily interpret that as an unusual mixture condition. However, the rear O² will see the overall mixture as rich, and trigger the PO420 DTC.

In other words, if one or both of the front HO2S are weak, and the rear is working properly, the PCM could interpret that as a poorly performing catalytic converter.

Similarly, any air leakage into the exhaust manifolds or pipes ahead of the front two HO2S can dilute the exhaust gasses and create a false lean condition, creating the same phenomenon. Even an EGR valve leak can contribute, although the leak will usually be only recycled exhaust gasses and therefore should not all significant oxygen to skew the sensor readings.

Again, oil, coolant, incompatible fuels, and incompatible gasket sealer compounds can render an oxygen sensor useless, as well as a catalytic converter.

You could replace all three oxygen sensors on a hunch, but the front two are about $80 each, and the rear sensor is about another $80. That's a lot of wasted money and effort if the problem is really the converter or even an exhaust leak.

It may well be worth the expense to have a GOOD diagnostic check performed. For the price of one possibly unnecessary oxygen sensor, you should expect an answer from a skilled technician.

[quote=jeffcoslacker]Spend $25 for a diagnostic vacuum guage. Hook it to a manifold vacuum source, warm the motor up real good, then hold it at 2500 rpm steady and watch the gauge...if the reading falls off the longer you hold it, the cat is partially blocked..


I bought the vacuum gauge hooked it up and there was no drop while in park with the motor reved to around 2500 steady (no tach)..i put the car in drive and held one foot on the brake and revved the motor again and had a slight drop (10 psi)..i found that i can hold my hand covering the exaust pipe with barely any pressure blowing out(i can actually plugg it off with no air excaping just with my hand) Catalytic Converter??

It's getting kinda hairy here without more info, but if it was me, I'd be looking into a fuel starvation situation, as revving freely in neutral or park, while stumbling in gear (especially without misfire indicated) is kinda indicative of a lean stumble...which could also code for the cat, if there's a large amount of excess HC in the exhaust stream do to the lean under load condition....the EGR should not cause this, rather when they malfunction they act as an enormous vacuum leak, and want to stall or run rough when the throttle is released...but that's not to say it's 100% ruled out...but the EGR should be nearly wide open when you trounce the gas, so regardless of if it's working or not, should not affect throttle response/acceleration..only idle stability if stuck open...

Best way to check this is to rent a Fuel Pressure guage from AZ, and if it has enough line on it, you could tape it to the windshield or cowl so you can watch it as you drive...see if the fuel pressure is within spec when it stumbles or breaks up, or falls on its face, or whatever...then if you come up empty there, you might look at throttle position sensor and Vehicle Speed sensor readings...

I agree with Blue, you should have the car scanned by a shop. If you don't want to spend the money at a shop, if your car has over 100K, and has never had the oxygen sensors replaced, that is where I'd start. BTW, your car only has 2 oxygen sensors, one in the rear exhaust manifold and one past the converter. Cheap scan tools will usually trip a "bank 1, sensor 1" and/or "bank 2, sensor 1" code even when only one pre-cat sensor exists.