CEL- knock sensor malfunction or actual knocking?

[ttt]

Hi everyone, recently the check engine light has been illuminated ~90% of the time when I start my 1994 Legacy Wagon. The only time it won't come on is *occasionally* the first time I start it in the morning (seemingly when it's been a cool night, although sometimes on cool mornings it will still come on, so it's not perfectly consistent).

I took it to a shop when it started happening and they said it was a bad knock sensor.

I'm wondering how they knew the error code was caused by a bad knock sensor, and not actual knocking in the engine?

Basically, how can you tell if your car is knocking? The car makes a repetitive sound that could be considered "knocking" I guess, but also somewhat just sounds like something that could be normal.

Anyway, I want to be sure we know the sensor is bad before I pay to have it replaced, or if there could be a different underlying problem.

[Rooster2]

A bad knock sensor won't cause your engine to make knocking noises. What it will do, is give a bad signal to the engine computer, so that the motor runs in a lower performance "default mode." In default, your engine will have less power, and your gas millage will go down a few mpg. Your car will continue to run in this situation, and won't leave you stranded. It is just better to get it replaced.

 

Replacing the knock sensor yourself is not a difficult job, but kind of difficult to get a socket onto it. Plenty written on this subject can be found using the "search" function on this forum.

 

Let us know how you come out on this problem.

[GeneralDisorder]

The ECU can tell that the sensor is electrically bad. If you have a code then the sensor needs replaced or the wiring is faulty.

 

GD

[ttt]

Ok, a little more research on google told me this. So the code is indicating that the sensor itself is bad, not that it's sensing something bad?

[Recian]

the sensor makes a ground circuit to the ECM. If the sensor goes bad it'll provide an open circuit internally even tho it will sometimes look ok and cause the code. You probably wont notice any drivability issues but you'll notice an improvement after it's replaced. For the price difference i recommend going with OEM this time. For me it was only a $40 difference roughly.

[ttt]

Sorry this is so hand-holdy, but I'm just getting started!

But is this the part you would recommend buying Recian: http://tinyurl.com/bmybyzm

[Recian]

yep that's it. I got mine from subaru for about $94. That was OTC cost not employee discount or anything like that.

[ttt]

great, thanks.