1996 Cougar 4.6L 109,000 miles

CEL came on, scanned it at autozone, PO420 bank 1. The scan tool also said 1 of 4, but no other codes came up. Do i have a bad catylatic converter?
What is this telling me?
O2 sensors?
Bank 1 is drivers or passenger side?

Before the CEL came on the mileage was not as good as usual. All of the oxygen sensors are original, as is the complete exhaust system.

Plugs, wires, PCV, air and fuel filters are newer and good.

 

Bank 1 is the passenger side. It is saying that the passenger side cats are not cleaning the exhaust properly. It could just be a bad sensor though.

 

Would replacing both O2 sensors, upstream and downstream, in bank 1 be a good start before condeming the catylatic convertor? All 4 of the O2 sensors are original.

If the cat is dead, will it affect engine performance or gas mileage if it is not replaced? I know if the cat is restricted or plugged up it could, but if the catylist isn't functioning anymore because it has lost the ability to clean the exhaust, would higher emissions be the only negative result?

Would a dead cat with both O2 sensors being new hurt gas mileage?

 

Hood,
As I understand it (and I hope someone will correct me if I am wrong) there are 2 sensors on each side.
The pre-cat sensor monitors O2 content of the exhaust gas and uses that data to pass along to the ECM as a factor in determining fuel delivery (fuel air mixture) so as to maintain the best mixture/burn of fuel/air.
The down stream sensor monitors the exhaust gas post-cat to determine the efficiency of the catalytic converter. I do not think the data from that sensor is used in any way expect for alerting you that the catalyst efficiency is not proper.
I believe the sensor is usually the culprit and not the catalytic converter.
Based on my experience (and reading) the O2 sensor is a much cheaper "first stab" at resolving the problem.

If the catalyioc converter is clogged, it will have a negative effect on performance. You just have a clogged exhaust pipe.
I'd imagine - but have no first hand experience - that if the catalytics converter just "goes bad" (not sure that happens) then you would only have an emissions problem.

I'd try replacing the sensor after the catalytic converter and see what happens.

If you disconnect the battery for 15 mins or so, then reconnect it, that will clear the code and you can see how long it takes to come back.
I believe the system will allow 3 or 4 "failures" before it will flag the problem and set a code. I think this is done to allow for occasional problems that might be due to very short drives, or similar situations that might mimic a catalyst failure.

Looking for corrections if any of this is incorrect.

Thanks.

 

P0420 ( and P0430) is not due to the sensors, either upstream or downstream.

The sensors are tested for in tolerance operation before running the cat effficiency monitor test.

If you were to monitor the output voltage of the upstream and downstream O2 sensors simultaneously (my PC-based OBD-II software does this), a dead cat is obvious as both sensors will switch almost in tandem. A good cat will have a slowly switching downstream sensor compared to the switching rate of the upstream sensor. It is this ratio of upstream vs downstream switching that is used to test for efficiency. When efficiency drops below a certain rate, the ratio of switching drops below the preset threshold and the cat efficiency code sets.

Options to address:

Ignore the CEL. Black tape helps.

Replace the cat.

Install MIL eliminators to fool the computer into thinking the cat is working.

Which option you choose will depend on your situation, ie, emissions testing, tree-hugger, etc.

Steve

 

Steve,
Thanks for setting the record straight.

That is some good info.

hood- let us know what you do and how it turns out.

 

Steve,

If i have no codes present for the O2 sensors, i must accept the fact that the cat is dead because of the "420" code. But what killed the cat? Do you think 109,000 miles is premature for a catylatic converter?

My cougar has a "Y" pipe that has all 3 cats together. So, i'm looking at mucho bucks for the "Y" pipe. A local shop quoted me $640 for the "Y" pipe with 3 cats. He said it had a 5yr/50,000 mile warranty. The old O2 sensors are reused unless you buy 4 new ones. 4x$60= $240 ( 4 O2 sensors). Hmmm....240+640=880 plus sales tax. Call it $900 and some change for the PO420 code.

I was under the car tonight for about an hour trying to figure out how to unplug the O2 sensor. With the limited wire and space, the plug-end is almost unreachable. I guess i'm basically screwed. Where's the black electrical tape ????

I will go back to my cave and mull my decision over. When i figure out my plan of attack i will re-post my final results. Thanks to everyone for the input.

 

In the case of my wife's 98 Contour 2.5, it is my suspicion that a persistent problem with the upstream O2 sensor resulting in a rich exhaust mixture caused eventual pre-cat failure.

We bought the car used. It was a factory repurchase (lemon) that had been repeatedly serviced for recurring P1131 codes during its first 30K miles. After Ford's repurchase, it had the PCM, harness, and upstream sensor swapped to repair it prior to my purchase. I saw the complete OASIS record of the car before buying it.

By 70K miles, I had already been through another upstream O2 sensor, also a P1131, then misfiring (plugs and wires), then finally the P0420 came up.

Dead pre-cat. Plain as day on the scanning software.

It is my theory that the extended period of time that the engine ran with excessive unburned fuel in that bank's pre-cat eventually caused the catalyst to fail.

Happy ending though. One of my nephew's friends was a tech at a local dealership, ironically the same store that did the pre-purchase work, and swapped out both pre-cats under the 8/80 emissions warranty. Free repair.

FWIW, MIL eliminators run about $30 a set.

Steve

 

UPDATE:

Had all 4 O2 sensors replaced for $270....parts & labor. No more CEL and the car purrs like a kitten. I also replaced the fuel injector pressure regulator just for the hell of it. I look at it as cheap insurance.

It's been about 2 weeks and 500 miles or more and the CEL is not on. I believe the problem is fixed. The oxygen sensors were shot. Thanks to all for their input.

 

They really are supposed to be replaced every 60,000 miles, but most people (including myself) wait for them to fail

 

My 98 Crown Vic threw the 420/430 codes and I installed MIL eliminators. I then had it put on "the sniffer" and it was as clean as a new one. Now tell me it's not the o2 sensors......

 

It is NOT the O2 sensors.

If your engine is already burning cleanly, there's little work for the cats to do anyway.

Steve

 

That's what I asked for isn't it............. Thanks!

 

Your inital response indicated that the codes were caused by faulty sensors and were eliminated by the MIL eliminators.

My point is that the codes were not due to faulty O2 sensors, but by properly operating O2 sensors that were detecting a failure of the cats to function properly.

The MIL eliminators bypass the actual problem by fooling the PCM into receiving O2 sensor readings that mimic a functional cat.

In a clean running engine, there's very little for the cats to do so a sniffer test will still pass. With dead cats and a clean engine, the only thing that will fail is the on-board cat efficiency test.

Steve

 

I had my Mustang sniffed without the converters one day just to see how it compared to running the cats. I only ran the cats 1 day a year when I used to have it emission tested. It blew 720 with no cats and 001 with them on. They do make a big difference even on a perfect running engine.