Check engine light/ knock sensor - Subaru Forester Owners Forum
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post #1 of 28 (permalink) Old 07-19-2008, 08:40 PM Thread Starter
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Check engine light/ knock sensor

I own a 2002 L with 5 speed. A few months ago, the check engine light came on, so I pulled to the side, threw up the hood, and gasped a sigh of relief to find the engine was still there. Whew, that was a close one, I thought, but the light remained on, even though I had "checked the engine". Just kidding! Seriously, my check engine light has been randomly on and off for the past 2 months, twice in the past two days. At first I thought it was a loose gas cap, and got a new locking one, (I wanted one anyway), but that didn't solve things.

I took it to Advance Auto, and used their OBDII reader and got the code P0328, Knock sensor circuit, high input, no faults detected. My CD-Rom repair manual actually calls this a P0325, Knock sensor circuit high input. So I get out my multi tester, and run the tests, and it indicates a poor connection between the knock sensor, and the ECM. I cleaned the contacts, yet the "problem" continues.

Let me add this...the car runs, awsome! Starts as soon as I turn the key, and stays running, smoothly. Pulls smoothly, and strongly through all rpms, from 0 to highway speeds. Has plenty of power, and will pull 4th gear at only 20 mph without bucking, stalling or protest of any kind, and gets 24 to 26 mpg, about 20% better that the window sticker said it would get when I bought the car new.

Does anyone have any suggestions, other that going to the dealer, and spending huge $$s on a car that runs great, or pulling the check engine bulb, and practicing my overhand fast ball into the nearest trash can?

Added info...I live in CO, where "regular" fuel is 85 octane. I used this when the car was new, and it pinged like crazy. I read the service note about using 87, no matter what the altitude, and I did, until early this year, when gas prices spiked. Switched back to 85 to save some $, and the car doesn't ping now, (Maybe it's finally broken in at 55000 miles).
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post #2 of 28 (permalink) Old 07-19-2008, 10:23 PM
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You may have to actually replace the KS. It's surly known to happen. I'm surprised the O2 sensor didn't flash because that is a very popular issue. It may not ping at all, but the circuitry may have an issue.

I've always wondered if a good DIY grounding mod would fix some of these more minor electrical issues. I'd just maybe source a few sensors, and keep an eye on the CEL.

USMC - One Shot, One Kill. You can run, but you'll die tired.
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post #3 of 28 (permalink) Old 07-20-2008, 08:46 PM
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what did your test state you check.the measurement is suppose to be from knock sensor harness to ecm.then sensor to block.it is always the senor or its ground to block.but you can have the first wiring fault ive seen.

No. Step Check Yes No
1 CHECK HARNESS BETWEEN KNOCK
SENSOR AND ECM CONNECTOR.
Measure the resistance of harness between
ECM connector and chassis ground.
Connector & terminal
(B136) No. 4 — Chassis ground:
Is the resistance less than
400 kΩ?
Go to step 2. Go to step 3.

CHECK KNOCK SENSOR.
1) Disconnect the connector from knock sensor.
2) Measure the resistance between knock
sensor connector terminal and engine ground.
Terminal
No. 2 — Engine ground:
Is the resistance less than
400 kΩ?
Replace the
knock sensor.
<Ref. to FU-31,
Knock Sensor.>
Repair ground
short circuit in
harness between
knock sensor connector
and ECM
connector.
NOTE:
The harness
between both
connectors is
shielded. Repair
short circuit of
harness together
with shield.

CHECK INPUT SIGNAL FOR ECM.
1) Connect the connectors to ECM and knock
sensor.
2) Turn the ignition switch to ON.
3) Measure the voltage between ECM and
chassis ground.
Connector & terminal
(B136) No. 4 (+) — Chassis ground (−):
Is the voltage more than 2
V?
Even if MIL lights
up, the circuit has
returned to a normal
condition at
this time.
(However, the
possibility of poor
contact still
remains.)
NOTE:
In this case,
repair the following:
 Poor contact in
knock sensor connector
 Poor contact in
ECM connector
 Poor contact in
coupling connector
Repair poor contact
in ECM connector.
EN-179
DIAGNOSTIC PROCEDURE WITH DIAGNOSTIC TROUBLE CODE (DTC)
Engine (Diagnostics)
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post #4 of 28 (permalink) Old 07-21-2008, 07:51 PM Thread Starter
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Yes, Tech...this is the same page I was working from, and the same tests with the same conclusion, and I attempted to clean the 3 contacts. And I basked in the warm glow of a check engine light all day, while the car kicked butt, like usual. So do I try to repair the wiring, change the sensor, or just put a piece of black tape over the light and ignore it?
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post #5 of 28 (permalink) Old 07-21-2008, 08:20 PM
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i would like to know what your results are for each test.you havent stated would you got.and did you double check the pinout on ecu.what is the ohm reading from sensor harness connector unplugged and ecu unplugged
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post #6 of 28 (permalink) Old 07-22-2008, 09:48 PM Thread Starter
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Ok, I'm glad I wrote them down...The harness connector unpluged read 649k ohms. The ECM unplugged read 472k ohms. With these readings, my diagnostic chart said to plug the harness back in, turn on the key and measure the voltage between the ECM and chassis ground...I got 2.5v, it was looking for more that 2v, and said to check for poor contact in knock sensor connector, ECM connector, and coupling B21. I cleaned those contact points, and the light was off for a day, on for a day, and off and on again the next day, and now its on. Ran into a guy today, who used to work at the local Subaru dealer service dept., and he said he used to change out knock sensors on these cars all the time...it was the most commonly replaced sensor on the modern Subarus...is this true?
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post #7 of 28 (permalink) Old 08-06-2008, 04:02 PM Thread Starter
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Here's some additional diagnostics I came up with...

So, the results I was getting from following the diagnosic chart indicated a poor contact. If you get the same results, do the following...

1. Disconnect coupling B21, and see if you get a reading frmm the ECM side of the plug. If you do, then the circuit is making contact to that point.

2. Disconnect the knock sensor, plug B21 back in, and see if you get a reading from the ECM side of the plug. If you do, then the circuit is ok to that point.

3. Check the knock sensor itself. Check it hot and cold, when the check engine light is on and off. No reading, or one that jumps, (1600k ohms, 750 k ohms, 1175k ohms, 450k ohms, ect.) and won't stay steady indicates the KS is bad...I verified this to a tech at the local Subaru dealer.

The conclusion I came up with was the bad connection was inside the KS.

After handing over $116 to the Subaru parts dept., and installing the new KS in the car, things are back to normal, the car still runs great, with no check engine light.
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post #8 of 28 (permalink) Old 08-06-2008, 04:19 PM
 
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Step 1) Remove knock sensor.
Step 2) Check for cracks.
Step 3) Replace cracked knock sensor.

09 SWP Outback 2.5XT 5MT 263whp, 337 lb-ft
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post #9 of 28 (permalink) Old 09-04-2008, 04:53 PM
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I had an appt for inspection/emissions tests today. When I started the motor my engine check light came on. I had an unusually long run where it stayed off. Take it to my former employer. Got the PO328 code. Turned it off. hopefully will stay off and I can put off replacing the sensor. The car ran fine also and couldn't notice any problem as far as that goes.

I know I must not be alone but I hate that eng. check light. I have told so many people to just ignore theirs, taken them out of the speedos of some crown vics. they are notorious for the ABS light coming on. I know the tape solution sometimes is the best thing but the idea still never sits right with me. I want it off, the way it's supposed to be.

great resource all of ya are. did my search. found info. my cars running great and lights off so lifes good. tomorrows another day.

eb
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post #10 of 28 (permalink) Old 01-16-2009, 04:31 PM
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It sounds like your manuals are a lot better than mine

Where did you get your CD manuals? I have the Chilton and Haynes paper manuals which are essentially the same with the same pics and everything. They aren't all that helpful - certainly no troubleshooting of my knock sensor like you guys are referring to.

Also, any suggestions for the best online source for a knock sensor? The closest dealer is 100 miles away and his prices are crazy.
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post #11 of 28 (permalink) Old 05-07-2010, 03:35 PM
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Question knock sensor or spark plugs?

This thread has been great so far. I too would like to have a manual that gave the details of trouble shooting a knock sensor paper or CD I hope someone answers the quesion aobut where to get one from the previous post.

My 2001 Forester (80,000 km) has been running a little bit rough for a while. Not bad, but it was always really smooth. Now the CEL light is producing the P0328 code (Knock sensor 1 circuit High Input) that comes and goes. Replacing the knock sensor is pretty easy but a new one in Calgary is close to $200. I don't know when the spark plugs were last replaced, but it hasn't been done since I bought the car 30,000 km ago. Spark plugs are cheap, but sounds like it's not a fun job.

Any advice on whether to replace the plugs and see if that solves the problems, or go for a new knock sensor?
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post #12 of 28 (permalink) Old 05-07-2010, 04:37 PM
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[QUOTE=Skunkhammer;526720]Where did you get your CD manuals? I have the Chilton and Haynes paper manuals which are essentially the same with the same pics and everything. They aren't all that helpful -


I found my cd manual on ebay a few years back...it is pretty helpful. Yep,the paper ones are crap for most things.

I had to replace my knock sensor about a year ago. I hear they are a common failure. They are super easy to change and not to much money so I say just change it and be done with it.
Good luck

Kenny
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post #13 of 28 (permalink) Old 05-07-2010, 04:38 PM
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Just replace the knock sensor. They tend to crack/melt and otherwise get screwed up. Also make sure its mounted correctly.

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post #14 of 28 (permalink) Old 05-09-2010, 09:27 PM
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Smile

I removed the knock sensor and cleaned up a small bit of what was likely corrosion powder and reinstalled it with a thin coating of silicone grease on the contact face. There was no sign of cracking or distortion / melting. Then I cleared the CEL code. I can't believe how smooth it runs now - just like it should. I know it's too early to tell if the problem will be back or if the CEL reappears, but so far so good.
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post #15 of 28 (permalink) Old 05-10-2010, 07:03 AM
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Quote:
Originally Posted by cooteb View Post
I removed the knock sensor and cleaned up a small bit of what was likely corrosion powder and reinstalled it with a thin coating of silicone grease on the contact face. There was no sign of cracking or distortion / melting. Then I cleared the CEL code. I can't believe how smooth it runs now - just like it should. I know it's too early to tell if the problem will be back or if the CEL reappears, but so far so good.
Make sure you orient the wire correctly, I believe its supposed to be at something like a 60% angle to the centerline of the car.

04xt/4eat, 16g, STI bits and bobs
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