My mom’s 2003 Alero has had a bad catalytic converter for the last few years… at least that’s what she was told when she took it to the shop. The engine light (P0420 threshold below efficiency) has been on over the last few years and I occasionally check it with a code reader to make sure no other …

Very often a bad cat code is actually caused by a bad o2 sensor. However I doubt that would cause your issue. What motor does she have in this car? Is it the 3.8 v6? Any way that gen if GM motor had a problem with intake manifolds, melting by the egr valve and causing an antifreeze leak. When mine …

That list comes from the fact that misfire code refers to a lack of combustion taking place rather than from a spark problem. Spark problems are one thing that can result in no combustion. The next thing to do is pull the plug back out and check compression since if there is some mechanical engine…

Sorry, but I have to ask, what made you think you could ignore the bad catalytic converter for several years? Before you start replacing things I would recommend you take it to a good shop and let them do the diagnosis. If that isn’t an option check all of the plugs again and do a compression test.…

I’ll do a compression check and report back. ‘‘Sorry, but I have to ask, what made you think you could ignore the bad catalytic converter for several years?’’ My mom decided not to replace it because we don’t have e-check here and I think they wanted something like 6 hundred dollars to replace it…

It’s a knock. And yes, it misfires at idle.

Since you just replaced the plugs recently, it could be simply a defective spark plug. I’d get another new one install it to see the results. A bad coil is also something to rule out once you replace the #6 plug. After those things are tryed you could be looking at something more complicated and …

I just tested but forgot to warm the engine up first. By the time I remembered that it was supposed to be warmed up first, I already had the plugs out, the fuel relay unplugged and the ignition system disabled so I just went ahead an tested anyways because the three plugs in the back are kinda hard …

#6 piston has a hole in it or a valve is sticking. How did the #6 plug look?

The plug looked fine. It’s only been in there a few weeks though. Whatever happened in that cylinder seems to have happened suddenly… How does a piston get a hole in it?

Misfire and engine knock

Maintenance/Repairs

Rod-Knox April 23, 2012, 3:22am 6

A cheap vacuum gauge will come with instructions and should enable you to quickly determine if the catalyst is obstructing the exhaust and possibly causing the knock and misfire. And is it a hard knock or a pinging? Also, is there a misfire at idle or only when accelerating?

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Misfire and engine knock

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