Join the Car Talk Community!

Discussion Rules

Welcome to the Car Talk Community!

Want to ask a question or join the discussion? Great! Join now.

Sign In Register

Misfire and engine knock

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 codes have been added. Up until today - as far as I know - none have been. Now today when she was driving it the engine light started flashing and running rough and making a knocking sound.
She brought it straight home and I hooked my scan tool up and it gave the same cat code as I just mentioned, plus a P0306 misfire code. So I pulled the plug wire on number six cylinder and hooked up a spark tester to it - and it showed a decent spark. And to verify the spark being normal, I tested at another plug wire and the spark looked about the I don't think anything is wrong with number 6 plug wire or coil or anything like that but am not sure. Also, the spark plugs were replaced a couple of weeks ago because the ones in the car were worn down about .010''. I can't imagine this having anything to do with the misfire because I did them myself and made sure they were gaped properly and that all the wires were put back on the correct plug/cylinder and plus it ran fine after the plugs were installed - but thought I would mention it anyways. And I was very careful not to damage the platinum on the plug when gaping them.

Did a little bit of research and found this: A code P0306 may mean that one or more of the following has happened:

Faulty spark plug or wire
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector
Burned exhaust valve
Faulty catalytic converter(s)
Running out of fuel
Poor compression
Defective computer

So where should I start? Any ideas?



  • 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 went it caused a missfire in number 6 as well, but mine happened on start up sometimes. That would be my first guess, second would be a compression check.
  • 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 issue there's not much point in chasing anything else. You'll really need to check all 6 cylinders since what you need to know is how they compare to each other as well as how high/low the compression is.

  • 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. At least you won't have replaced the cat an O2 sensors only to find the engine needs a rebuild.
  • 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.

    gsragtop, it's got the 3.4L V6
  • 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?
  • It's a knock.

    And yes, it misfires at idle.

  • edited April 2012
    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 expensive. I'd think about a compression check on the #6 cylinder next.
  • 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 to put in and take out. I only tested cylinders 2,4, and 6... will do the other 3 if it is necessary for a proper diagnosis.



    6:120,90,15 (three seperate tests) then added three squirts of oil and got 130. Then let the pressure out of the gauge and got 25.

    Piston rings?
  • edited April 2012
    #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?

This discussion has been closed.