2001 Chevy Cavalier engine light on code P0420

RICHIE25RICH

MEMBER
2001 CHEVROLET CAVALIER
4 CYL
2WD
MANUAL
74,000 MILES

P0420 is Catalyst Efficiency below threshold, bank 1
When the engine liight comes on, I erase the code with my code reader.
Sometimes I can drive over a hundred miles before the code comes on again, sometimes just 20 miles, if something was really wrong, wouln't the engine light be on all the time? Should I replace the heated oxygen sensor after the catalytic converter?

I replaced the manifold oxygen sensor with a Bosche,
I replaced the Catalyticconverter with a national auto OEM fit and it walked right through state inspection without any problems.

I cannot find any leaks in the exhaust system.
I pulled one of the spark plugs and it was clean.
I put a test light between the spark plug and the wire, the light was really strong, lit up the engine compartment! But it was not steady all the time.

To try and solve the probllem, I went to a higher ocatane gas and put Prestone fuel injector and fuel pump cleaner in the fuel tank and ran it down to 1/4 tank. The car always had good pickup, no hesitation.
When they went to ethanol in the gas, about 3 years ago, my mpg dropped to 26mpg, before I was getting 34 mpg.

So after running a tank of 89 octane from Sunoco, and the Prestone fuel injector cleaner, the darn engine light still came on. Most of the time it comes on after a pothole, sometimes not!

This is driving me crazy, because there are no leaks in the exhaust system, I have a strong spark, good pickup, and the wires appear to be plugged in tight on the oxygen sensors.
I hope somebody can help!
Thanks, Rich

5 Replies

DANAUTHIER

MECHANIC
156 POSTS

Hi Rich,

You mentioned ethanol fuel, is your engine rated for flex fuel? The higher percentage of alcohol may be contributing to your problem if the engine is not designed for flex fuel.

Your P0420 is a catalitic converter below efficianty code, but cat conv do not commit suicide, they are actually designed to last the life of the vehicle. If engine performace is maintained properly. At below 80k it may have been covered under warranty, if the fault was the converter. Since you have changed the conv and you sitll have the code, chances are that was not your problem.

My guess is that you have a drivability problem that is keeping your engine from operating at the optimal 14.7 to 1 ratio, that is the most common suspect for a P0420, especially with a vehicle in the 75k range. You stated the one plug you pulled looked clean. Have you changed the plugs and wires and if you did, what did you change them with?

Let me know and we will do our best to help you out.

Thanks,

Dan

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Saturday, March 6th, 2010 AT 12:12 AM

RICHIE25RICH

MEMBER
5 POSTS

Hi, thanks for the response, the cat converter was replaced because the clamp on the muffler in the back of the car rotted off and broke, which I was unaware of, and going over speed bumps, caused a split along the seam of the cat converter from the huge leverage of the weight of the exhaust system pulling down on the rear of the cat converter, That is why I replaced it. There were no engine lights on at the time. When I installed the cat converter unit onto the manifold, it was not lined up properly and it leaked at that point, which the inspection station did not pick up, and it still passed. About two weeks went by and I had 600 miles on the new converter at that time, and the engine light went on. I did not fix it immediately becuase it was so bitter cold and lots of snow and I work in my gravel driveway! I completely took off the whole cat converter and put it on the right way with a new gasket on the manifold connection, and even rethreaded the wholes in the manifold so I could get true torque values. I replaced the unheated two wire oxygen sensor in the manifold with a Bosche. I went 170 miles before the light went on. At that point I kept erasing the code, hoping it would stay away! Then I put Prestone fuel injector in and, wow what a difference in pickup! But the light still comes on intermitanlyt, today I was going to go over to NAPA and buy a heated oxygen sensor that goes on after the cat converter in the rear, that is probably the original one.
I have always done all my own work. I held my hand over all the connections, and could not detect any air flow leaks with the engine running.

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Saturday, March 6th, 2010 AT 9:06 AM

DANAUTHIER

MECHANIC
156 POSTS

The rear oxygen sensor could be the culprit. If it is not reading properly it could give you the in-efficency code without a rich or lean code. Do you have any way to view live data, from the O2 sensor? The front should change, the value should jump up and down. The rear sensor should remain relatively the same. When the pipe broke and the exhaust came down it could have damaged the wiring to the rear O2 sensor as well, be sure to check it on both sides of the connector.

Let me know what you find, I am very interested in the answer to the puzzle.

Thanks,

Dan

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Saturday, March 6th, 2010 AT 3:28 PM

RICHIE25RICH

MEMBER
5 POSTS

Hi, I had replaced the spark plugs with genuine GM plugs that were 10 bucks each!
Oil consumption is low so the oxygen sensor was not gummed up when I replaced the one in the manifold.
My code reader just gives me the code number, then I look it up in the supplied book.
I do have a FLUKE brand volt ohm meter, how could I hook it up to the oxygen sensor to check it?
I had replaced all the wires and spark plugs about 3 years ago.
Tonight I had driven 117 miles without the engine light going on. But then at 117.1 miles it just went on again!
I have a sensor on order from Napa, so I was just going to drive around a little to see if the light will go off on its own.
The shop manual says that if I get three engine cycles that are good. The light will go off on its own.
I assume that means starting and shutting off the engine. Or it could mean a long drive where everything get s up to a certain temperature to be considered a full cycle.
I will most likely put that new rear oxygen sensor on this Saturday.

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Tuesday, March 9th, 2010 AT 10:23 PM

DANAUTHIER

MECHANIC
156 POSTS

You could back probe the at the connector for the O2 sensor. The tan/white and purple/white wires are the ones to the PCM, the black wire is a chassis ground, and the brown wire is battery voltage. The down side to this is, if you are not careful, your could damage the connector or even the PCM. Do you know anyone who has a scan tool that shows live data?

There is a tool made by "Launch Tech Company", called a "CReader V" that reads codes, allows you to see live data, and performs some tests, including forcing an Oxygen sensor test. It is less than $200, I know it is available from MAC Tools, (they even have it made with the MAC logo on it now, I bought mine from my local MAC dealer when they first came out. No MAC logo, but identical to the one my buddy bought that has the MAC logo on it ) but there may be other outlets selling it as well.

It is a handy tool for anyone who wants to do their own work, but doesn't want to invest two to three thousand dollars on a pro-grade tool. It is limited to OBD II, but that is what most of us are working with anyway. It also only does powertrain control, no ABS or Body Control. It does come with (last I knew) lifetime updates, you download them from the company's web site. I use mine regularly, it is great for quick checks and easily monitoring various sensors and at the $159 that I paid for it I don't mind letting friends borrow that one.

Anyway, back to your car. Then engine cycle that you mentioned is not just turning the engine on and off. The engine has to run long enough for it to go into closed loop operation and there are other (driving) conditions that must be met before the PCM performs an Oxygen sensor test. I do not recall the exact amout of time, driving distance, or speeds that your car requires to prompt an O2 test, but this is why there is no much variety in the distances you drive before the malfunction indicator light comes on.

The only other question I can think to ask at this time is, was the catalitic converter that you replaced the OE one with a direct fit replacement part? If the distance between the converter material and the oxygen sensor is change by as little as one inch it could change the way the exhaust gases go past the sensor and could disturb the readings of the sensor.

I hope this information helps, let me know.

Thanks,

Dan