Catalytic Converter problem?

I have a 2000 Bonneville with 93800 miles. Never any problems. The "Service Engine Soon" (MIL?) light recently came on. The first mechanic I took it too said he was unable to determine why and reset the code. He said if it recurred, I should take it to a dealer for additional diagnostics.

About a month went by and the light came back on. The next day, while on a 100-mile freeway trip, the car began missing and stalling on deceleration. Very rough/missing on idle. Hard to start. Dealerships closed on Sunday. Took to reputable chain operation. They found a loose vacuum connection and reconnected it. This cured the rough running, miss and stalls. Following this they ran computer diagnostics and said they recommend replacing catalytic converter. Their belief was the converter was clogged. They do not do exhaust work and suggested taking it to an exhaust shop. They said this was causing a pressure build up in the engine which could eventually cause the head "to blow."

So here I am. Are they right? Do I get a new converter or pay the dealership for a diagnosis?

The shop I took it to provided me the following diagnostic information:
Code P0440 evaporative emissions failure
TPS .6 - 4.19 sat
O2 = 895-895
EGR 32% at 2500
St Fuel Trim -2%
LT fuel trim -14%

Any advice would be appreciated.

The easiest and cheapest way to test this is to unbolt the converter from the engine and run the engine. It will be loud but if the converter is plugged the engine should come to life. Also you don't need to go to the dealer for the work any exhaust place should be able to check the converter. If you need to replace it don't go to the dealer there are plenty of converters on the market that won't damage your wallet. If you go the the dealer expect to pay through the nose for it. There is also another possibility and that is that the solenoid controlling the fuel evaporator is bad or the canister will need replaced. This is cheaper than an converter and the parts can be obtained at most larger auto parts stores. Try this first before you replace the converter unless because it will be cheaper. If that doesn't cure the problem then you probably will need to change the converter. Good Luck!

The shop you took your car to are idiots. Same goes for the chain store. A lack of knowledge and a lack of thinking by the shop would have you wasting your money.

First of all a P0440 code is a Evap problem not a convertor problem, 2 different systems. You might as well try rotating your tires to cure your check engine light you may stand a better chance then replacing a 300 dollar convertor.

Your evap system consists of the gas tank, purge valve, vent valve, charcoal cannister, and some tubing.

Most times the P0440 code gets set is because of a loose gas cap.

Now a bad convertor would most likely give you a P0420 code. This code can be a false code due to the software calibration being too touchy. A PCM reprogram is out there to reapair this condition. Also since your car is a 2000 model your convertor is under warranty for 8 years or 80,000 miles. Replacement is FREE!

On the data you gave.
The O2 sensor should swing from around 100mv to 900 mv. Your LT trim is kinda high should be as close to zero as possible. Your readings show a rich condition and the PCm is subtracting fuel to achieve a 14.7 to 1 mixture.

Anopther thing about convertors, theyt don't go bad on their own something always caused the convertor to go bad. Just replacing the convertor is a band-aid repair and the problem most likely will come back.

Flatrater you always hit the nail on the head. We should get together and write a book on mis-diagnosis.

Flatrater you always hit the nail on the head. We should get together and write a book on mis-diagnosis.


Its not mis-diagnosis just a lack of having the information handy. Most people don't have the needed information to fix their cars. Given the right info it is easier to fix the problem. When you have more than the needed information and you take the wrong diagnosis path that is misdiagnosis, otherwise its a guess or what I like to call a W.A.G (wild ass guess).

I am no rocket scientist I just have the GM info that the owners or independent shops don't have.