1997 P0420 code

[doc_k]

Hi everyone.

I just got a CEL on and read the code P0420 code. What is the most likely cause of this on a 1997 Astro?

[chevymaher]

Dunno what is the description for that code?

[The Drummer]

http://engine-codes.com/p0420_gmc.html

[doc_k]

P0420 = catalytic converter low effieceny

[chevymaher]

Cat getting some age and less efficient.
Could be a weak O2 sensor.
Spitting injectors. make it run rich and fouls them.
Tune up and run her hard every once in a while. Keep her cleaned out.

[kennysmoke]

PO420 = previous owner was pot head....

[Sailing_Faith]

PO420 = previous owner was pot head....

You should get a p187 code for that one....


... Cause yOu are killin me!

[doc_k]

How do I determine it's a bad cat?

[thegreenmachine]

How do I determine it's a bad cat?

If it climbs on the counter and licks the milk from your cereal bowl.

[sadglad]

It could be as simple and cheap as a vacuum leak. See here. viewtopic.php?f=24&t=50657
New cat is expensive. Check the cheap and easy stuff first.

[chevymaher]

How do I determine it's a bad cat?

Check everything. O2's make sure it is tuned correctly. Good cap rotor wires. Vacuum leaks was a good tip. Once it is running right run it on the freeway a half hour. If it isn't the cat or it is just getting weak. It will clear the code and it wont come back for a long time.
Once you are sure it has good ignition. Injectors are clean and working correctly. If it then stays off as long as you are running on the freeway but keeps popping up in traffic. Then we leaning toward the cat being shot.

Mom's cat is dead as a doornail. But before she tested it I would put injector cleaner in it. And just wail the tar out of it a hour or so. She would go test and it passed. Then after a couple weeks it came back. The code I mean.
We went through that for years before she moved back home. Now it don't matter they don't test here.

[redfury]

If you have a IR temp gun, you can look at the cat temps to see if it's heating up properly. A rear O2 sensor that isn't reporting correctly will also trip the code...you can see that on a scan tool that can read live data. Unlike the front O2 sensors, it should hover around .450mv instead of fluctuating above and below like the upstream sensors do.

[doc_k]

The only tests I made so far are with an actron CP9145 code scanner which gave me the P0240. I can look at data and all o2 sensors fluctuate. The actron doesn't graph, but the O2/2 sensor which I believe is the downstream one fluctuates from .045 to .855 v. It's hard to read because it's constantly jumping. The van also used to make this rattleling noise. My brother said it was from the cat (heard it while under van, he said honeycomb was cracked). It's not rattleling anymore. Where do I measure temp on the cat? Van needs emissions testing soon!!

[redfury]

Rattling cat means your honeycomb has broken down unless it was a loose heat shield. You're looking at a new cat. The rear O2 should be steady, not acting the same as the upstream sensors. The fact that it is means nothing is happening in the cat to convert the gasses. I don't see any other solutions.

[doc_k]

Can an air/vacuum leak cause the downstream o2 sensor to fluctuate, or is it a certainty that the cat is bad? Is there another way to determine/coorberate a bad cat?

[Meterpig]

How do I determine it's a bad cat?

If it climbs on the counter and licks the milk from your cereal bowl.

[WoodButcher]

Rattling cat means your honeycomb has broken down unless it was a loose heat shield. You're looking at a new cat. The rear O2 should be steady, not acting the same as the upstream sensors. The fact that it is means nothing is happening in the cat to convert the gasses. I don't see any other solutions.

ditto that^^^

[doc_k]

Where are the o2 sensors, especially the last downstream one? I see two on either side of x over pipe after flange but none on cat or after.

[WoodButcher]

I haven't been under my '97 in a while but IIRC it was on the cat itself. Look on Rock auto for the part and you should find a picture.

Expensive GD pipe it is.

[doc_k]

There is nothing on pipe or cat. There is no wire harness or connector. How can this be read 0420 without a post(downstream) sensor? HELP!