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Cadillac Deville 2001
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Discussion Starter #1
A couple of months ago, the MIL light on my 2001 Deville came on. My other vehicle is a 1977 Dodge van and I am the chief mechanic. So all this electronic stuff is new to me. The Cadillac has about 100K miles, so the fact that I had some kind of trouble didn’t alarm me.

I found out that I could get the codes pulled at AutoZone, and they came up with the P0134 and P0171 codes, both banks running lean. AutoZone’s opinion has always been pretty good for me, so I replaced the Mass Airflow Sensor immediately. The codes remained.

This was the first time I had worked on the car, and so I was interested in learning all I could. None of the little pneumatic parts nor vacuum tubes nor any of the other engine controls were known to me. I didn’t know their names. However, I do have the GM service manual published by Helm. This manual has three volumes and is about six inches thick, is not too well organized, but I began to learn things. Just for fun, I replaced the fuel filter and the spark plugs and in doing so, I found out that the fuel system really is under pressure when you turn the car off, and you’d also use the factory specified plugs, since the “rapid fire” plugs don’t work at all. So, I did do the spark plug replacement two times. The plugs I used were AC Delco 41-987, gapped to .040 inches.

One thing that is important to note, is that except for the rapid fire four pronged spark plug adventure, the car ran fine. I was just dealing with an illuminated light on the dashboard. But the next step was to look for a vacuum leak. I had gotten kind of familiar with the motor by now. After all, it is sideways. And I have a kind of sense about things based on how they look, and the engine was clean. All of the rubber parts were flexible, except for one little tube which I replaced with a generic piece from AutoZone. I could neither see nor hear any vacuum leak.

As a final test before I had to give up, I bought a can of carburetor cleaner and sprayed it around the throttle body and heads and intake manifold, but the car ran the same. The idle speed didn’t increase at all. Completely stumped, I went to Cadillac and paid them $75 to find out what was causing the P0134 and P0171 codes. With little trouble, their mechanic found the leak in the plenum. You can find out how to replace it here: . The leak came from a tear about one inch long underneath the plenum. I fixed it, and the light stayed off...for about a day and a half, that is.

That’s right, about a day and a half later, the MIL light came on again. This time, it was a P0420 code. At least I had fixed the vacuum leak, but then, this new code seems specifically reserved for the catalytic converter. According to the muffler folks, running too lean or too rich too long can destroy the CAT, and I guess that’s what happened.

The muffler shop replaced the factory CAT with a MAGNAFLO converter. A visual inspection showed a sagged exhaust pipe and discoloration from overheating on the old one. The new converter waited about a day and a half to re-trigger the P0420 code and cause the MIL light come back on again. So, the muffler shop replaced the first CAT with another variety (Magnaflow 94806) that was supposedly designed to make the MIL light go away on GM stuff. The “larger” CAT didn’t last 40 miles before the MIL light came on again.

During the course of the second installation, the mechanic at the muffler shop pulled the rear HO2S sensor and said that it was “really carboned up”. He wiped it with a paper towel, and put it back in. As you have read, this treatment lasted less than 40 miles.
Someone said once that for every Ph.D., there is an equal and opposite Ph.D. These are the authorities I have spoken with:
Before I replaced the CAT, I asked Cadillac if I had to use their factory part ($1300) to make the code go away, and they said “No, you can go to Midas if you want to. ‘After market’ works in this application”. I’ve asked that question up close and personal three different ways at the dealership. The dealership says that any quality converter should work. So does my (trusted) muffler shop, and so does Auto Zone, who has been pulling the codes for me. So does Midas.

I have the (poorly organized) service manual for my car. There is no mention at all of anything related to a P0420 code that would indicate a cause other than a converter failure. This matched the input from the dealer, and everyone else I’ve talked to, including this site.

Except that…

Once you’ve replaced the converter and still have the P0420 code, there is a kind of mysterious “squeak” that comes up from your previously expert Ph.D.s. It shows up in the dealer at an almost imperceptible level. Like a sort of tribal legend, it has been “heard of before”. My (trusted) muffler mechanic tried to inspect the HO2S sensor, remember? And a phone call to the specialist AutoZone uses got the same kind of squeak. “Both sensors”, he said. In other words, it’s the measurement system that’s at fault. The mechanic at the Midas shop actually said that the P0420 code was a sensor code to just further blur the information.

Why can’t we all just get along?

I replaced the rear sensor, the one just behind the new converter. That didn’t make the light go off. So I replaced the front bank one sensor too. These are factory sensors. Even paranoids can have real enemies, so I even used the Cadillac pop rivets to replace the three foot long shield that covers the harness to the rear sensor. But the cat came back, the very next day, the cat came back and it wouldn’t stay away.

At this point, I think I’d better synopsize all this.

A cleverly concealed one inch long rip in the bottom of the plenum caused me to run the car perhaps six hundred miles in a lean fuel/air mixture condition. This burned out the factory original catalytic converter. Cadillac, AutoZone, Midas and my (trusted) muffler shop have all agreed that an aftermarket CAT should work. All of these folks have replaced converters on 2001 Devilles before, so the replacement of the factory converter shouldn’t have caused all this trouble. Two brand new sensors haven’t been able to make that light go off either. And so, NOW…

My (trusted) muffler mechanic made a call to a friend of his. This friend used to work for Cadillac and now works for another GM dealership about fifty miles away. This person told the following story: “When I worked for Cadillac, and I had to deal with the ‘0420 code, if the converter was within warranty, I would reflash the PCM and the light would go out. If the converter was out of warranty, we would replace the converter and reflash the PCM to make the light go out. So it doesn’t make any difference what kind of converter you put in, you need to reflash the PCM.” He explained that he wanted $100 to do this and that he has to pay a subscription service each month to stay current.

Finally, to my question: does this make any sense? I can invent a story to match the details given, but is there a revision coded upgrade that takes place from time to time on PCMs or other computer modules on my car? I’m supposing that the original software only allowed for factory replacements, and the new revision allows for aftermarket converters, but that’s only a supposition.

I hope this story can help other people. But thanks in advance for any constructive comments. And while I'm thinking about it, are there better aftermarket converters that could be used?

Thanks again.
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