Simple Test Checks for Misfiring Spark Plug

Dear Car Talk:

I bought a 1996 Toyota Land Cruiser last year. A "code PO306" for "cylinder six misfire" was found by the local Toyota dealer. He swapped the plugs between cylinders five and six, and the misfire moved to cylinder five.

Should I have the plugs replaced? I am an old lady and know very little about cars. Thank you for your advice. -- Mary

Mary, I want to thank you for sending in the easiest question I've had in 10 years. Would you write in every week, please? You and your dealer did all the diagnostic work already.

I take it your Land Cruiser was running rough, so you took it to the dealer. They scanned your car, and the computer told them that cylinder six was misfiring. The next step was to figure out why.

So the dealer did a very smart and simple test. He removed the spark plug from cylinder six and put it in cylinder five. Lo and behold, the problem followed that spark plug. Now cylinder five misfired. So he knew it was the spark plug's fault.

That also allowed him to rule out a wiring issue, a compression issue and the possibility that someone left a screwdriver in your number six cylinder the last time you were in for service. All he had to do at that point was to replace the spark plug, charge you $850 for the diagnostics and send you on your way.

Now, it's possible that something is fouling that spark plug, but I'm assuming your mechanic checked for that and ruled it out. It's more likely that your spark plugs just haven't been replaced since 1996, and the ceramic insulation on one of them cracked. If that's the case, I would recommend replacing all of them at this point. Replacing all six will probably cost you in the neighborhood of $300 at the dealer. The plugs are about $25 each and, since there's an air hose and a few plastic covers in the way of the rearmost plugs, they'll charge you an hour's labor for the job.

But that ought to fix it, Mary. Enjoy your Land Crusher.


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