I went to autozone and used their scan tool, found that there was a p0325 code (knock sensor). This is the second time that has happened (didn't replace it back then, just cleared the code and turned off the CEL, that was a year ago).

 

When looking at the conditions that set the code, I noticed that the long term fuel trims were at negative 10.8%, meaning it was leaning out the fuel mix a lot.

 

Could a very lean mix somehow cause a knock sensor code? Just want to know before I go and buy a new knock sensor if I actually need one.

 

*edit* forgot to mention it's a 1999 Legacy with the 2.2 SOHC.

Just replace the knock sensor. The code only gets thrown if the sensor malfunctions, not if something is beyond it's capability.

Thanks, I know that the fuel trims are controlled by the O2 sensor, I just needed to know if a bad O2 could cause conditions that might fool the ECU into thinking the knock sensor went bad. Looks like I need to replace 2 sensors now.

On older OBD1 cars a bad KS would cause 'limp mode', but I'd think the car would run Rich not Lean, for safety.

 

If you're over 100K the O2's may be due - should be 2.

 

GL,

TD

Edited October 13, 201114 yr by wtdash

On older OBD1 cars a bad KS would cause 'limp mode', but I'd think the car would run Rich not Lean, for safety.

 

If you're over 100K the O2's may be due - should be 2.

 

GL,

TD

 

I think that a bad KS results in "limp mode" computer function, that retards engine timing to prevent detonation (Ping). Does the KS do this, or affect rich/lean mixture instead?

I think that a bad KS results in "limp mode" computer function, that retards engine timing to prevent detonation (Ping). Does the KS do this, or affect rich/lean mixture instead?

 

This is for OBD1, which may not match an OBD2 system.

 

From Legacy Centrall BBS - How to Read Trouble Codes From the ECU:

 

 

Code:22

Item: Knock sensor

Diagnosis: Abnormal voltage produced in knock sensor monitor circuit

Fail-Safe <limp> Operation: Sets in regular fuel map and retards ignition timing

 

 

I don't know what 'sets in regular fuel map' means, but it does affect fueling.

 

TD

Edited October 14, 201114 yr by wtdash

That's if it goes into limp mode. Just because the code is present doe not mean the ECU is in limp mode. There are several ways a knock sensor can fail and cause a code to set, but the same code sets for all of those failures.

 

Inspect/replace the knock sensor. Deal with the fuel trim issue if it's still around after resetting the ECU.

I replaced both sensors, the fuel trims are back to 0.0%.

 

That o2 sensor is in an awkward place, had to rent a crows foot tool to get it out since there was no room for the socket type tool.

I replaced both sensors, the fuel trims are back to 0.0%.

 

That o2 sensor is in an awkward place, had to rent a crows foot tool to get it out since there was no room for the socket type tool.

Did you replace O2 sensors (are there 2 as stated earlier?) and knock sensor or just O2 sensors?

I replaced the knock sensor and the pre-cat o2 sensor (I didn't see the other o2 sensor). Like I said earlier, the fuel trims are back to normal and it has a tiny bump in horsepower.

But today I could feel the engine lose power and see the RPMs drop when the new knock sensor retards the timing when I give it a little throttle while the engine is cold.

 

So now I've either uncovered a new problem with the engine, or I installed a bad sensor (it said "made in turkey" on the box), I'm going to take a closer look at the old sensor.

Well thanksgiving is coming...

 

One other thing, since it was fine then started loosing power, a partially clogged cat. Get yourself a vacum gauge. If it starts out high then slowly drops you have collapsed/clogged exhaust system, which means a cat.

I know it's not a clogged cat, they happen kinda gradually, this happened immedeatly after putting in the knock sensor.

I recently had a bad knock sensor right out of the box that was brand new. Same deal, car came in with a P0325, replaced knock sensor with a new one and the check engine light came back with the same code. Replaced it with another one and it was good to go.