Cat Efficiency Code P0420

I recently took my car for an emissions test, the car passed all aspects of the test eccept "Malfunction Indicator Light". I need to correct the light as according to the test my Cat Converter is functioning correctly but the code for the light says it is not. If I reset the light by disconnecting the battery and truning on the headlight switch, would this correct the problem? I have a 2001 Oldsmobile Alero if that makes a difference.

How did you test the converter? Most of the time a P0420 means the converter has failed. Have you checked out this article yet? http://www.obd-codes.com/p0420

Nope! if the code is there it's likely no good! It can work sometimes enough to keep the emission clean for a tailpipe test but the light is on and that means 95% of the time it's not working enough to satisfy the code setting criteria!

The light is on because the converter is bad. You can try to clear the code and drive it untill all the emission monitors set and hope the light does not come back on.

Well I've replaced my Cat 3 times now, and the damn light keeps coming back on... so NOW WHAT???

you know everyone on this site replaces converter after converter and most of the time they go to the exhaust shop and get the universal fit and this does not do the job... so I must ask where did yours come from?

The last Oxygen sensor does really nothing for the engine. The 1st one does. The second one, the one passed the Cat, tells the PCM if the Cat is functioning correctly. Check the wires and the connectors for it. When you replaced the Cat, did you bought one that spliced on the old tube or the whole tube assembly. If so, that Oxygen Sensor could be bad and/or damaged during the replacement of the old Cat (they are pretty closed to each other). Is an $80 part, they are not cheap ! You could find on in a salvage yard for less. Usually does not goes bad.

Bought three, all from my local parts store, which was nice enough to honor the warranty on the first 2. The first was the $80 Universal Maremont, the second was the $120 Magnaflow, but also a Universal. And the third was a Direct Fit Magnaflow, which just bolts on, but we did weld the spice after the Cat where it just slips together. The first 2 I did have the spliced in and welded. So after 3 New Cat's I am starting to wonder if the problem is some where else.

Appreciate it!!

I am more inclined on the 2nd sensor giving a wrong reading.

If you are able to monitor o2 voltages the one after the cat should be steady voltage if it moves should be very little. If it moves as much as the front o2 then the converter is bad

Maybe a long shot...

On this GMs the have Lower Intake Manifold (LIM) gasket failures. If you have a coolant leak and is going to the intake valves, the Glycol destroys the Cat, oil can do that too.

what engine do u have in the car, magnaflow converters arent very good to use, they r more for performance than emissions which causes failure also.

I am pretty much guessing a V6

I just ran this code after replacing about $600 worth of parts on my car, including the converter. It was a Magnaflow and it was welded on. I hear chunks of converter guts rattling around!! I know the upstream O2 voltage should fluctuate and the down should be steady but if both are too close it will pop the code. Driving a Saturn Vue(which is a brand I don't recommend) with 2.2 Ecotec (new & improved Quad4). God help us all.