UMT Posted August 4, 2011 Share Posted August 4, 2011 I've been having a on/off check engine light going on for about 2 weeks. Though it was just that pesky gas cap but decided to go to Advance Auto and have a code reader put on it today. Computer says bad knock sensor. What is a knock sensor and what does it do? What kind of problems can a bad one cause? Watched a good YouTube video on replacing one so that's no big deal. 99 Impreza Outback Sport - EJ223 Engine - 186000 miles and running strong! Thanks, UMT Quote Link to comment Share on other sites More sharing options...
danbennett2u Posted August 4, 2011 Share Posted August 4, 2011 Knock sensor "listens" for detonation, or pre-ignition.. basically the fuel/air erupting too early. It tells the computer if the timing needs to be adjusted if I am not mistaken. Quote Link to comment Share on other sites More sharing options...
Rpm90001 Posted August 4, 2011 Share Posted August 4, 2011 Very common to go bad. I would just replace it. I just bought one from advance auto no more CEL. I believe the code was po325. Borrowed from the net. The knock sensor responds to spark knock caused by Pre-detonation of the Air/Fuel mixture. As the flame front moves out from the spark plug ignition point, pressure waves in the chamber crash into the piston or cylinder walls resulting in a sound known as a knock or ping. This is caused by using a fuel with a low octane rating, overheating, or over advanced timing. Sometimes it can be caused by hot carbon deposits on the piston or cylinder head that raise compression. A knock sensor is comprised of Piezoelectric materials; Crystals that when impacted, generate a voltage (same idea as a BBQ ignitor). This voltage is monitored by the computer, and when an irregularity is detected, the computer corrects timing in VVT (variable valve timing) engines, or triggers a DTC Diagnostic Trouble Code) in older vehicles. Quote Link to comment Share on other sites More sharing options...
UMT Posted August 5, 2011 Author Share Posted August 5, 2011 Thanks for the replies. That YouTube video I watched showed the gal (yes, a gal) pointing out where it was located, how to remove and replace. When she got it off, she pointed out that it had a hairline crack in the plastic and that's what was causing the problem. So, I've decided I'm going to pull mine, give it a careful inspection and cleaning, put some contact cleaner on the electrical connections and watch it for awhile. Big question: Why such a huge difference in price? Advanced wanted 82.00 for the cheapest one all the way to almost $150! I looked online at autopartswarehouse.com and they start at $30.00 and go up to $150.00.... What's the big difference? Obviously, my car has 186000 on it so I'm leaning towards a cheaper one. Tell me what's up with this.......... Thanks, UMT Quote Link to comment Share on other sites More sharing options...
1-3-2-4 Posted August 5, 2011 Share Posted August 5, 2011 nothing much really.. I got one for my EJ25 for $26 on ebay.. also they fail with the crack because moisture can get in it. I took my old cracked one and put it on my oscilloscope and it still works when I tap on it but it was time for a new one. Quote Link to comment Share on other sites More sharing options...
nipper Posted August 5, 2011 Share Posted August 5, 2011 It is a piezeo electric sensor. Think a microphone on a old portable cassette recorder. This makes an AC signal and sends it to the ECU where the tiny men with the fast calculators figure out what to do next. They get hairline cracks in them then are worthless. They are under rated. A knock sensor hears knock before you do. It is important that this functions as it really protects the head gasket from little hot spots. Ping has a another cause and can be annoying but harmless. Ping is another post. Quote Link to comment Share on other sites More sharing options...
1-3-2-4 Posted August 5, 2011 Share Posted August 5, 2011 speaking about cracks and all make sure the replacement is put on the right way and not overtightened. Quote Link to comment Share on other sites More sharing options...
mpudz Posted August 5, 2011 Share Posted August 5, 2011 speaking about cracks and all make sure the replacement is put on the right way and not overtightened. Very true. When my knock sensor was cracked, I didnt get a CEL but the car ran like crap (low on power due to the ECU putting it in limp mode) and I was getting horrible mileage (around 11mpg). Quote Link to comment Share on other sites More sharing options...
1-3-2-4 Posted August 5, 2011 Share Posted August 5, 2011 Very true. When my knock sensor was cracked, I didnt get a CEL but the car ran like crap (low on power due to the ECU putting it in limp mode) and I was getting horrible mileage (around 11mpg). Funny my friend that has a 02 Forester his CEL kept going on sometimes and sometimes not.. I felt so bad he drove it all the way to my house only for the CEL to go off.. I scanned it anyways and no stored code.. Finally he did catch it and it was the knock sensor. I did mine early this year and mine never went into limp mode.. but I think my plug wires and oxygen sensor need to be changed.. sometimes I will get some heavy hesitation when leaving from stop lights, only happens when engine has warmed up. Quote Link to comment Share on other sites More sharing options...
Rpm90001 Posted August 5, 2011 Share Posted August 5, 2011 Not all bad sensors have cracks in them. If it is tossing a code, good chance it is bad. Go to the link below, it is a link to advance promo codes. There is a few for $20 of a $50 purchase. Or $40 off 100. You get the point.. Order the part on line, and pick up in store. Your $80 sensor now becomes $50. I buy all my parts this way. Never pay full price. If you buy say stuff for an oil change with the sensor, you will get $40 off $100 Just order on line, put in the code during check out, and click you want to pick up in store. They have it ready with in the hr... My sensor ended up being like $50 and has worked perfect. No more CEL http://procouponcode.com/advance-auto-parts-coupon-code Here is a link to some codes, you have to click on them to give you the code. Just puck the one that will save you the most money. Geo Quote Link to comment Share on other sites More sharing options...
1-3-2-4 Posted August 5, 2011 Share Posted August 5, 2011 call ahead to make sure they have the part in stock! I got burned like that when my lower radiator hose went.. I was stuck at 2 Am so I had my laptop with me and I wen to the Advance site and it said they had it in stock.. went that sunday morning as soon as they opened and after 10 min of looking they did not have it.. I figure I would warn you first.. I had to walk 2 miles down to Auto zone to get the hose... Quote Link to comment Share on other sites More sharing options...
nipper Posted August 6, 2011 Share Posted August 6, 2011 call ahead to make sure they have the part in stock! I got burned like that when my lower radiator hose went.. I was stuck at 2 Am so I had my laptop with me and I wen to the Advance site and it said they had it in stock.. went that sunday morning as soon as they opened and after 10 min of looking they did not have it.. I figure I would warn you first.. I had to walk 2 miles down to Auto zone to get the hose... Thats happened to me too. Quote Link to comment Share on other sites More sharing options...
UMT Posted August 11, 2011 Author Share Posted August 11, 2011 Well, decided to just get a new knock sensor when I found one on Ebay from Chandler Motors in Wisconsin. $20.00 DELIVERED. Got it in 3 days. Pulled the old one and could not see any cracks of any sort but where it sat on the engine block was a bit corroded. Probably could have just cleaned it up and used it again but decided to install the new one and kept the old one for a spare, should I ever need one, which is probably doubtful at this point. Total R&R time, about 20 minutes. Piece of cake. This car always ran well but it seems to run even a bit better with the new knock sensor! Makes sense I suppose, now that the 'little men with the very fast calculators' are getting better input! Ha. Anyway, thanks to all who posted for the information and advice. UMT Quote Link to comment Share on other sites More sharing options...
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