Question : P0420 Catalyst System Efficiency Below Threshold

[Keebler]

I just bought a 1997 Oldsmobile Cutlass Supreme with 126,000 miles on it. I bought it looking pretty good but below the price I would normally expect.

When I first bought it it had the "Secuiry" light on and the "Low Coolant" light on. Within the first 24 hours I got a Service Engine Light with P0401 Reduced EGR Flow. Over the weekend I fixed the security light and disassembled the throttle body to clean a blocked EGR passage and cleared the P0401 code.

I also removed and carefully cleaned the Low Coolant Sensor and expected it to be fixed. Strangely enough, it flashed a steady pattern for a good long while and it slowly got faster before coming back on steady.

Tonight I got the P0420 Catalyst System Efficiency code. I might be kind of paranoid but I'm wondering if I'm seeing the signs of Lower Intake Manifold leak.

The oil has always looked fine and there's no clear sign of coolant loss at this point.

How much coolant needs to go through the exhaust to ruin the Catalytic Converter? Would I notice it?

How much oil needs to get in to the coolant to make the Low Coolant Sensor malfunction?

Should I wait and see what happens or should I go ahead and assume the worst?

Can mere mortals replace the Lower Intake Manifold Gasket or is professional help the only reasonable solution?

Thanks...

[RedogOnyx]

Never heard of coolant in the exhaust ruining the cat.

Sometimes the coolant will leak out the side instead of an internal leak, but with no coolant loss, you should be fine.

You can do the LIMG yourself if you have a manual and know your way around a tool box, but if you don't feel comfortable, let a pro do it

[Keebler]

Never heard of coolant in the exhaust ruining the cat.

Sometimes the coolant will leak out the side instead of an internal leak, but with no coolant loss, you should be fine.

You can do the LIMG yourself if you have a manual and know your way around a tool box, but if you don't feel comfortable, let a pro do it

I do have the complete set of GM Service Manuals on DVD and I thought that's where they mentioned antifreeze as a source of contamination. I'll have to take a second look and make sure I understood what I read.

I've only driven the car like 150 miles so far so a slow antifreeze leak wouldn't show up yet.

If I had a better appreciation for the difficulty of replacing the LIMG, I'd consider doing it myself as a precaution.

[pocket-rocket]

A friend of mine bought a Cavalier with an Iron Duke that had a bad head gasket in it. The gasket was bad enough that besides the normal milkshake, it steamed like the wand at the steam cleaners shop. We fixed the gasket and changed the oil right there before driving it home and I bet the thing steamed for about 20 miles since the cat was full of coolant. He drove that car for a good bit longer before selling it and never had a problem with the exhaust. Sure, I bet antifreeze is not good for a cat, but I don't think a lot is going to totally ruin it since normal combustion creates some moisture too. Does the car have an exhaust leak before the cat? Reason I ask is I've worked on a Honda that had the flex pipe break and so the ECM thought the cat wasn't working because the rear O2 sensor wasn't reading what it should due to the exhaust escaping from before the cat through the hole in the flex.

[91LQ1Lumina]

Well I know water is not going to hurt the cat, as the last step in the converter is to transform unburnt fuel into H2O and CO2. I would like to know what the glycol (or what ever it is that is in radiator fluid now days) mite do to the cat?

http://www.youtube.com/watch?v=rmtFp-SV0tY