I've got a 2002 Monte Carlo SS, love it, but have recently run into a problem. The symptoms are:

1) P0420 code from the check engine light. I erased it to see if it would come back, and it did.

2) Low, terrible, fuel mileage. I put in 7 gallons exactly last time I filled it up, and got a measly 23 miles out of that. Now the car has 130,000+ miles on it, and I used it almost exclusively in stop and go city stuff, but that's horrendous.

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Research tells me it's either the Catalytic Converter or Rear 02 sensor. Is there anyway to know for sure?

And if it's the rear 02, I'm assuming that's a relatively easy thing to replace?

 

The cats clogging up are very common. I would have an exhaust shop do a pressure test, but my money would be on that. Especially with the poor gas mileage.

 

Is there any quick way to check if that CC is dying? Smack with a hammer, listen for rattling? I don't get any odd sounds from it while running, or shifting problems, which seem to be symptoms other people have run into.

 

If people are getting those symptoms, they are side issues caused by the main problem.

When the cat went in my ex-wife's Avalanche (at 30K miles! Yay, NAFTA!), there were no noises. There was no warning. The CEL came on, the engine stopped, she instantly lost power on I-95 with my kids in the back seat. That was it.

 

If it's very poorly clogged, if you hop out of your car after a spirited driving session at night, you might see it faintly glowing red because it is way too hot.

Sometimes if the insides have snapped and are clogging that way, a simple tap of a hammer should allow you to hear the rattle.

You can disconnect the downpipe at the manifold and go for a drive and see if the car acts quicker (but it will be extremely loud)

 

I second a muffler show checking or replace the cat.

 

All good advise above.
A couple other methods to test for a bad cat if you have the tools are:

1. Using an infrared thermometer.
With the engine warmed up to operating temperature, aim the lazor of the thermometer at the exhaust pipe at the inlet side of the cat. and note the temperature. Then aim the lazor at the exhaust pipe at the outlet end of the cat. On a good, efficient cat. the outlet temperature will be much hotter at the outlet. If the temperature is the same or colder it's bad.

2. Using a scan tool capable of reading live data.
Go to O2 sensor data/voltage measurements.
With the engine at full operating temperature, compare the volt readings on the #1 and #2 O2 sensors. The #1 sensor volt reading should vary from about .1V to .9V and be rapidly changing. The #2 sensor volt reading should be around .5V and slowly changing, but stay close to .5V. If the change is rapid and has a wide fluctuation similar to the #1 sensor the cat. is bad.

 

Thanks for the answers, guys.

Anyone here done/had a replacement job done for the Catalytic Converter? I'm wondering what I should expect when it comes to the price.

 

Easy if you replace with a "direct fit " cat. Just a matter of removing the #2 O2 sensor, unbolt the connections to the downpipe and resonator, unhook the big rubber support o-rings at the outlet end and remove the old cat. Be sure to soak the nuts the night before with a spray lube like PB Blaster or Aerokroil. Be prepared to extract and replace stud bolts if they break. Replace gaskets if necessary (some aftermarket gaskets are junk). Reverse process to install and follow manufacturers break-in procedure. You can get a new cat. from RockAuto.com for about $169 (non-CARB for non-California emission cars).

 

Dealership: ~$1000 (OEM cats are very pricey)

Purchase a direct bolt in from magnaflow or what-not, and you are looking for the cost of the unit and your own time, or maybe $50 from an exhaust shop to do it for you.