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Check engine light

Dear Tom & Ray.

I sent a message in a few weeks ago and probably did not finish the email and it perhaps got lost.

Here goes again. I own a pristine 1996 Ford Crown Victoria equipped with a 4.6 liter engine as I am sure you know. It has 115 k miles. Usually gets great gas mileage. Usually about 21-23 in town and very close to 28 on a trip. I installed an OEM dual exhaust system eight years ago. It is equipped with a K&M air filter that is serviced on the recommended basis.

Recently the ?check engine? light came on. I brought it to my Ford dealer who performed the appropriate computer checking and the reading was as seen next line. Catalyst Efficiency Failures P0420.

The light was turned off. I was informed that the message could mean that the converter/s might be clogged and might have to be replaced, or an o2 sensor or sensors could be bad. It did not seem that engine response was suffering.

This particular car is very responsive and extremely fast. A few days later the light came on again. I was on I-291 and buried the gas pedal for several seconds. The light went off. A few days later the light came back on and stayed on for about 10 days. I did not drive the car for 6 days and when I finally drove it the light was off. I am willing to spend whatever it takes to get it back to normal but not before I know exactly what the problem is. OEM cats will cost close to $800.00 ea and after market a fraction. Or it could be as simple o2 sensors.

Please help. Regards, Xenophon A. Beake

PS. I viewed your photo album today. Very interesting. I have been a fan of your program since I was a small child. I have seen you on 60 Minutes and the Tonight Show. I love and respect your knowledge and sense of humor. Please check out my website at and you will find my interest in cars.

Xenophon A. Beake

2205 Boston Road Unit Q163

Wilbraham, MA 01095-1175

Check this site. I am afraid we are building a camel . That’s a horse designed by committee

This car is 12 years old. If the O2 sensors have not been replaced, they may be suspect. But, the cats may be too. I’d start with replacing the the secondary O2 sensors, and see if the problem clears up. if not, then the cats. With both replaced, the problem should be fixed.

The scan tool that the mechanic(s) used (or, should have used) does more than just read trouble codes; but, that’s (apparently) all they used it for. Too bad! The scan tool can be used to look at the response pattern of the oxygen sensor(s). This pattern will determine how the oxygen sensors are performing. There doesn’t have to be, “It might be this. It might be that”! There are other tests for the catalytic converter which they (of course) didn’t use.
Take your car to an independent shop for a mechanic who knows how (and, will) use his scan tool to its potential.

Good advice Busted. The entire OEM exhaust system was installed eight years ago and has only 70k miles but not the o2 sensors. Now I have a good Idea where to start.
I doubt that cats will not last more than 70k miles.

Thanks for your impute.Xenophon

Thanks for the advise. I do trust the Ford dealership where the test was made but perhaps the scan tool is only as good as the operators ability to run it. I think age is the culprit.


While I doubt if it was the cause of your CEL I would suggest that you be very very careful about oiling that filter. If not done just right, it can damage some expensive sensors. I might add that a stock car have little or thing to gain using a K&N filter as the stock air flow is quite sufficient for its needs. Today’s cars (including 1996) are far different than the 50’s when there was limited air flow and exhaust on cars.

Thanks Joe.
The K&N filter was installed long ago and is cleaned on the scheduled mileage. Although the car has 115k miles the OEM dual exhaust system has only about 70k miles. I do like to assume but with all the suggestions thus far, I doubt the problem are the converters. So I am faced to check the o2 sensor/s. One way to eliminate the possibility that the K&N filter is the culprit is to remove it if the light comes back on and replace it with an OEM filter. I am an old gear head and even though this new technology may be challenging to some it is not for me. This issue is just a challenge. I use to work on dragster engines and built a few Muscle Car street engines. I recently built a High Performance Muscle computer, thanks to understanding of automobile engines.
PS. I welcome any more suggestions and /or ideas on this issue.

First off, I am curious. What caused you to replace the entire stainless exhaust system after only 4 years? It seems unlikely that it wore out. If you went from single to dual it is an easy upgrade to just bolt on the extra parts in addition to the original single pipes/cats/muffler/resonator. You should have been able to go to duals with about 60% new parts.

I agree that you need to get this fixed. The ?96 is about worthless on the market, but should have at least as many good miles left in it as you have on it. It is probably a good vehicle for hauling around photography stuff if that is what you use it for. Does this one have the optional, leveling, rear air suspension?

I suggest one addition to the K&N removal experiment. Clean the MAF sensor wire with some electronics cleaner (spray) or just plain alcohol being careful of the wire. Spray makes it less likely that you will break it. That is where the K&N causes problems if it is going to. The wire gets coated with oil.

If you have not found yet, surf on over there. There is quite a bit of chaff, but there are some people very knowledgeable about the Panther platform cars and the 4.6 l modular motors. Careful though, they could have a guy like you hot-rodding Ole Vicky in no time. If you do need sensors or cats they will have suggestions about which aftermarket ones work and are a good price.

First is to change the O2 upstream sensors.
If you continue to drive with the check engine, you will replace both O2 and CAT.

Well Bead don?t ever assume because you know what that spells. I purchased the 96 in July of 2000 with only 27k miles. Most of the 8 cylinder Fords I have owned and driven are equipped with dual exhaust. When I purchased this pre-owned car it was equipped with the OEM single system so I made a conscious decision to go full Ford OEM duals.
As for my pristine 96 Crown Victoria being worthless on the market you don?t what condition I keep my vehicles in and have for many years. MY OLD VICTORIA IS IN SHOW ROOM CONDITION.
I don?t haul my photography stuff around only in the Crown Victoria. Whenever I need HAULI?N I use our extended length pristine Ford Earostar.
I am a hot rodder and have been for many years. FYI. The light is now off. You know the old clich? . ?If it ain?t broke don?t fix it? well it ain?t broke Appreciate your comments.

Thanks Roger. As soon as the light comes back on the o2 sensors will be replced first. You realize the problem could be a simple as a loose connection on a/the sensors.

I doubt that cats go bad with only 70k miles though.

Why change the oxygen sensor if the check engine light comes on, again? The oxygen sensor can be tested, in situ, with a scan tool! Which oxygen sensors?

As an old gear-head I am an individual who does not give up easily. I visited my Ford dealer. The system was scanned and again the reading stated the problem is in fact the right cat.
A spark plug defouler was installed and the light turned off. It will be interesting to see if the light will ever turn on again. Thanks for all for your posts regardless of the opinion.