Dear Car Talk:

I drive a 2005 Toyota 4Runner SR5 with 85,000 miles on it. Other than routine repairs, the vehicle has been trouble-free for all the years I've owned it. About two months ago, the check-engine light came on. I took it to my regular shop to have it checked out and was told that the service code indicated a problem with the ''knock sensor.'' Not being a mechanic, I thought: ''How bad could that be?'' Wrong! Because of the sensor's location on the engine, I learned that it would involve considerable labor and cost roughly $2,200 to replace. I was stunned and decided to wait on the repair until I could do some research. I found that having to replace the sensor is rare, but is indeed a costly repair. But here's the thing: The warning light comes on intermittently, shutting off for several days before reappearing. The engine appears to be running smoothly, no rough starting, idling or knocking. So before spending a chunk of change, possibly needlessly, I thought I would get your input. Are there other possible causes for this problem, and can engine damage result from not replacing the sensor? — William

Wow, you hit the bad-news jackpot, William. The knock sensor is a complete pain in the tailgate to replace. You have to remove the air plenum, the intake manifold, the timing belt and lots of other stuff to get at it.

The fact that Toyota buried it like that tells me that they never expected it to fail. And perhaps it hasn't failed. It easily could be a broken or frayed wire leading to the knock sensor that's causing the warning light to go on and off.

Once your mechanic removes the plenum, which is easy, he should see a wiring harness that leads to that sensor. Who knows? You might find a rodent nest in there and a chewed-up wire or two.

You really do need a working knock sensor. The sensor continuously gauges the timing of the explosions in your cylinders, and it adjusts the spark timing if the explosions start happening too early (that's called knocking, or pre-ignition, and it's damaging to your engine — and in extreme cases, it can even burn a hole in a piston).

Here's my advice: Find yourself a mechanic who is willing to take it a step at a time. Have him start by removing the plenum and checking the wiring first. You can be standing next to him with a stack of $20s and dole one out to him every 15 minutes as he works.

If you're lucky, you'll be able to stop doling well short of $2,200. I sure hope so, William.

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