I hopped in my 90 Legacy Wagon L today and I noticed the check engine light was on. Not cool. I hate that light. lol. So I plug in the test connections and I get a 22 which I looked up on the internet to be a knock sensor.

 

So my question is, what are the causes of a knock? Aren't they something like timing, fuel mixture, valve adjustment? Um are any of these correct? Any others?

You're not getting this code because of engine knocking. You're getting this code because the ECU is getting abnormal readings from the knock sensor itself. I.E. The knock sensor may have gone bad or there is a problem with the wiring to the knock sensor.

Yep. This will cause the car to not run all that great because it pulls timing. I needed a new one awhile back and was getting some hesitation at anything more than light throttle off the line. My replacement was $48.18 from subarugenuineparts.com and it's easy enough to change if you have some ratchet extensions.

Wait, so how do you know it's not knocking? What makes you just assume it's fine and the sensor is bad? I can easily replace it but I don't want to replace it if it's actually tellings me something.

 

But on a later note, halfway to work the CEL turned off. I haven't checked the computer to see if it's still saying the knock sensor code. Maybe I'll buy a sensor, we'll see.

if it's knocking, the knock sensor code won't come up. The code means there is a problem with the sensor itself.

 

The knock sensor is basically a microphone that listens for a specific sound that is probably knock. If it hears it, it changes the timing to stop the knocking. Also, sometime in the 90s the knock sensor design was improved because the clips tended to break. If your sensor has a grey connector it should probably be replaced anyway (new sensors have a white connector).

 

My CEL didn't stay on but the car still hesitated. When I reset the ecu and cleared the codes, it popped back on for a second. Then I replaced the sensor and the car is back to normal.

Make sure that the connection to the knock sensor is ok first before doing any replacement. A bad connection will cause the code to actvate also.

I'm with Jamal.

Wait, so how do you know it's not knocking? What makes you just assume it's fine and the sensor is bad? I can easily replace it but I don't want to replace it if it's actually tellings me something.

 

But on a later note, halfway to work the CEL turned off. I haven't checked the computer to see if it's still saying the knock sensor code. Maybe I'll buy a sensor, we'll see.

 

OK engine 101. All cars knock (ping is actually a better description) because they run very lean for best emissions and performance. What the knock sensor does is "listen" to the pinging. The sensor itself is a piezo-qaurtz semsor that emits an AC signal to the ECU. The ECU adjusts the engine timing so the pinging is kept under control. All this is done before you can even hear it (sometimes engines just ping, annoying but harmless).

You really need to learn to trust the people on the board. Its real simple,the computer is teling you its get no signal from the knock sensor. the knock sensor under nornmal circumstannces is always puting out a signal, and the ecu determins if it needs to do something or not. These things dont get better they get worse. Physically inspect the sensor to make sure its not cracked. Check the connection to the sensor and make sure the sensor is not shorted to ground.

 

nipper

You need the sensor. Replace it and be happy.

Now that I know how the sensor works it makes perfect sense. It wasn't that I didn't trust them it was just that I didn't understand the situation. I thought it was a code for engine knock and not that the sensor had quit sending the signal. Thanks, I'll check it out as soon as I get a chance. Which may not be til tomorrow because I have school and I go straight from there to work and from work I go home at 12:30 at night and go to bed and do it all over again at 7:30 the next morning. Tight schedule.

Everybody... thanks for this thread! We have 4 Subies - Wifes' 2006 Outback Wagon, My 2002 Impreza, Daughter's 2001 Forester, and Son's 1998 Legacy wagon. I had knock sensor go bad on Forester .. daughter replaced @ $125. I noticed CEL on in Impreza - dealer advised replacement cost $265! Forget about it! Went to Genuine site ordered part and thanks to you guys will replace over w/e @ 1/3 cost? You guys are like farmers .. outstanding in the field! Just have to learn about clearing ECM codes...

Everybody... thanks for this thread! We have 4 Subies - Wifes' 2006 Outback Wagon, My 2002 Impreza, Daughter's 2001 Forester, and Son's 1998 Legacy wagon. I had knock sensor go bad on Forester .. daughter replaced @ $125. I noticed CEL on in Impreza - dealer advised replacement cost $265! Forget about it! Went to Genuine site ordered part and thanks to you guys will replace over w/e @ 1/3 cost? You guys are like farmers .. outstanding in the field! Just have to learn about clearing ECM codes...

 

and ocassionally we step in poo

 

nipper

 

glad it all worked out

Poo or No Poo, some/most Autozone stores will clear sensor related codes for you, if you tell them you just replaced the sensor. They're a bit more skittish about clearing things related to Cat Converters, etc...

 

But you can also take the $$ you saved by doing this yourself and invest in a relatively cheap hand-held OBD-II reader. They cost less than $100 these days, and they'll clear codes for you.

i thought you could disconnect the battery for a good while to clear codes??

 

Poo or No Poo, some/most Autozone stores will clear sensor related codes for you, if you tell them you just replaced the sensor. They're a bit more skittish about clearing things related to Cat Converters, etc...

 

But you can also take the $$ you saved by doing this yourself and invest in a relatively cheap hand-held OBD-II reader. They cost less than $100 these days, and they'll clear codes for you.

i thought you could disconnect the battery for a good while to clear codes??

You can. Thirty seconds for a soft reset (clear codes) overnight for a hard (clear the computer memory of your driving habits). Most leave it disconeected for an hour.

 

 

nipper

Ahh!! I forgot about the battery trick! Been spoiled -- I've had my code reader for about 2 yrs....

Now that I know how the sensor works it makes perfect sense. It wasn't that I didn't trust them it was just that I didn't understand the situation. I thought it was a code for engine knock and not that the sensor had quit sending the signal. Thanks, I'll check it out as soon as I get a chance. Which may not be til tomorrow because I have school and I go straight from there to work and from work I go home at 12:30 at night and go to bed and do it all over again at 7:30 the next morning. Tight schedule.

it pays in the end. you can get a faster subaru after school is completed. be well.

Okay this thread explained the knock sensor question to me also. (Too lazy to search...)