Check engine light on- Shop claims catalytic converter needs replacing. (vehicle, auto)

[Wartrace]

2003 checy silverado 4.8l 6cyl engine. It has 181,000 miles on it (the majority of it has been highway miles)

I had an issue with the vehicle at around 120,000 miles- the factory failed to install one of the bolts to the exhaust manifold. It started to leak but as soon as I heard it I put one in, the leak stopped.

I had my tires replaced and had them run a diagnostic on the check engine light. The service manager said my catalytic converter had to be replaced; he claims it is "clogged".

Reading up on it I am thinking he just wants to sell me a 600 dollar car repair. Granted, the Chevy has two catalytic converters (one for each side of the engine) but I do not experience any performance issues. You would think that if the converter was stopped up the engine would be "loping" or not idling properly due to it being unable to pass the exhaust.

I can't help but think the check engine light is on for some other reason. I am thinking it may be related to the exhaust system problem I had earlier.

Questions;
1) If the catalytic converter was stopped up wouldn't I notice it in the trucks performance?
2) Any ideas/items I should check other than the converter? I took it to Advanced auto and they said the code indicated trouble in "the left engine bank" which happens to be the same side the exhaust leak was on.

[Coldjensens]

We had both converters go out. (2002 Chevy Express) One fell apart completely inside. There was nothing in there. The other one clogged up and the car would not go over 40 MPH. Unless you do anyhitng but replace them, the car will not work entirely correctly. There are O2 sensors on each side of each converter. The read the before and after exhaust and trigger your check engine light is something is not right. You car's computer also adjusts the fuel mix based ont eh exhaust readings at the O2 sensor.

Thus while it is possible (but illegal I think) to bypass the converters, you check engine light will remain on, and you computer will be misled and continually provide the wrong fuel mix.

However on a car that old, spending $1500 to $2000 on new converters, O2 snesors, etc, does not make a lot of sense.

$600 seems cheap. On our car it was over $2000 for the two converters. I would guess that they both have the same engine. (5.7).

[Merc63]

The cat can be starting to be going bad and not doing it's job without there being a noticeable loss in performance. You could have an O2 sensor going bad, but I'd have to know the actual code being thrown.

[Wartrace]

That's what is puzzling me- there doesn't seem to be any loss of performance and my mileage is the same as it was when it was new. You would think that if the converter was clogged I would lose performance.

It could be just a bad O2 sensor BUT I would have thought the advance auto test would have indicated that. I know I wasted my money at the tire place having them read the codes because those places are notorious for selling service you don't really need.

I honestly wouldn't realize their is a problem if it weren't for the light on. I am going back to town today and will ask them to read the code for me. I will post it later today.

[00molavi]

Are you burning more oil than usual? If so then it is possible to burn the cat. For now I will get the code numbers and research online. In the meantime reset the engine light and see if it comes back again.

[POS VETT]

I'd replace the downstream O2 sensor(s) first.

[hoffdano]

Wouldn't a failing O2 sensor throw its own code?

A failing cat might not cause a noticable performance problem except during a period when the "clog" actually matters. Under normal driving conditions a reduction in exhaust flow might have no real effect.

[AAPManager]

let me guess...the code shown is prolly the dreaded P0420....catalyst efficiency below threshold bank 1.....thats ones always tough to diagnose. could be cat, could be O2 sensors. Have a mechanic check the voltage of the o2 sensors. if good your cat is probably the issue. If not good, then its one of the 02 sensors

[Sonata36]

I can't tell you how many times shops have told me that a cat was bad when it never was the case...they see P0420 and simply stop investigating there. :-(

[NJGOAT]

What were the actual codes that were thrown? Code P0420 is the OBDII code for the cat converter and would be directly thrown itself indicating the cat had failed or you had a bad O2 after-cat monitor. It sounds like the AutoZone tech was saying that there was a problem in the "left bank" which to me would most likely mean a lean or rich condition that is throwing a code from the front pre-cat O2 sensor, which has nothing to do with the catalytic converter. Multiple codes can be thrown for multiple issues and can all be related, so you need to know which ones were present to diagnose.

If the issue is really tied to the left bank having a lean or rich condition, there could be several things going on, including the leak that you referred to that may have come back and now triggered a CEL. This website might help a little with diagnosing what might cause a lean or rich condition and yes, the O2 sensor itself could have simply failed.

Oxygen Sensor Codes - A Complete Guide On Diagnosing Automotive Oxygen Sensors

If the cat itself is bad, it doesn't mean that it will impact the performance of the truck at all. The catalyst could simply no longer be working, but still intact and the only way you would know that is from the CEL being triggered with a P0420 code. Sometimes when they fail, they collapse and clog up the exhaust. When this happens, it is quite noticable.