What does it mean when the Check Engine light comes on and goes off?

[Yousuf Khan]

I got a check engine light come on a couple of days back, and it was
there for less than a day before it went blank again. I have a car code
scanner, and it came back as P0325 (Knock Sensor problem). Now the fact
that the light went out again, does it mean that the knock sensor fixed
itself? I checked the code again, and it still shows the same code, but
that's probably just because it records the last problem.

I know that the Check Engine light sometimes automatically goes out
after a certain number of days, but I thought that would be after 3 days
or so.

Should I clear the code, and see if it recurs?

Yousuf Khan

 

[mulder]

I got a check engine light come on a couple of days back, and it was
there for less than a day before it went blank again. I have a car code
scanner, and it came back as P0325 (Knock Sensor problem). Now the fact
that the light went out again, does it mean that the knock sensor fixed
itself? I checked the code again, and it still shows the same code, but
that's probably just because it records the last problem.

I know that the Check Engine light sometimes automatically goes out
after a certain number of days, but I thought that would be after 3 days
or so.

Should I clear the code, and see if it recurs?

Yousuf Khan

It means the problem is being detected intermittently by the ECU. When
the condition isn't seen for a few driving cycles, the light will go out
but the code remains in memory for retrieval. If the problem is detected
again, the light will come back on.
Most likely the sensor itself is the problem, but it could also be a
wiring fault. The knock sensor is not an expensive part and is fairly
simple to replace.

 

[Yousuf Khan]

It means the problem is being detected intermittently by the ECU. When
the condition isn't seen for a few driving cycles, the light will go out
but the code remains in memory for retrieval. If the problem is detected
again, the light will come back on.
Most likely the sensor itself is the problem, but it could also be a
wiring fault. The knock sensor is not an expensive part and is fairly
simple to replace.

I kind of figured it isn't too hard to fix, as the dealer is only
quoting 45 minutes for labor on this. However, he's also quoting $160
for the part! I figure if I have to pay that much for the part, I might
as well save on the labor by doing it myself.

Is that as good a price as I'm going to get for that part? It's in
Canadian dollars, but the US and Canadian bucks are pretty much the same
these days.

Yousuf Khan

 

[1 Lucky Texan]

I kind of figured it isn't too hard to fix, as the dealer is only
quoting 45 minutes for labor on this. However, he's also quoting $160
for the part! I figure if I have to pay that much for the part, I might
as well save on the labor by doing it myself.

Is that as good a price as I'm going to get for that part? It's in
Canadian dollars, but the US and Canadian bucks are pretty much the same
these days.

        Yousuf Khan

seems high. Regardless, I always price stuff with Jamie at
www.subarugenuineparts.com and then decide. Sometimes you can get a
local dealer to match online parts. (this worked for my springs and
got close enough on my SS braided brake lines to spend the money
locally. Other stuff, I bought (engine undercover, Roki oil filters,
STI windshield wipers for my WRX, etc.) from them. There 'may' be a
Bosch or other aftermarket part - but folks have had bad luck with non-
OEM for other sensors, knck sensor might be OK aftermarket. YMMV

Carl
1 Lucky Texan

 

[S]

Hi Yousuf!

I got a check engine light come on a couple of days back, and it was
there for less than a day before it went blank again. I have a car code
scanner, and it came back as P0325 (Knock Sensor problem). Now the fact
that the light went out again, does it mean that the knock sensor fixed
itself? I checked the code again, and it still shows the same code, but
that's probably just because it records the last problem.

I know that the Check Engine light sometimes automatically goes out
after a certain number of days, but I thought that would be after 3 days
or so.

Should I clear the code, and see if it recurs?

Yousuf Khan

This is quite common.
And yea, clear the code. It will most likely re-occur, but as long as
the CEL is goin' away after a day or two, I wouldn't worry about it.
If it starts staying on for longer periods. you'll want to replace the
sensor; your gas mileage will suffer otherwise.
I'd suggest getting one at a salvage yard; probably $30 or so. There
is a way to test one with an ohmmeter. I don't recall the exact
procedure, but sou should be able to find it on-line, and the yard
guys should know in any event.
It's about a 10 minute job to swap one of these; takes a 12mm socket,
an ~4 inch extension, a ratchet, and an ice pick or similar to work
the connector.
Let me know if you need details.

ByeBye! S.

Steve Jernigan KG0MB
Laboratory Manager
Microelectronics Research
University of Colorado
(719) 262-3101

 

[Yousuf Khan]

seems high. Regardless, I always price stuff with Jamie at
www.subarugenuineparts.com and then decide. Sometimes you can get a
local dealer to match online parts. (this worked for my springs and
got close enough on my SS braided brake lines to spend the money
locally. Other stuff, I bought (engine undercover, Roki oil filters,
STI windshield wipers for my WRX, etc.) from them. There 'may' be a
Bosch or other aftermarket part - but folks have had bad luck with non-
OEM for other sensors, knck sensor might be OK aftermarket. YMMV

Unfortunately, those guys don't ship to Canada (I've asked already). So
it's not certain that the dealer is going to try to match the online
price, if they know that these guys won't be shipping into here.

Yousuf Khan

 

[Yousuf Khan]

This is quite common.
And yea, clear the code. It will most likely re-occur, but as long as
the CEL is goin' away after a day or two, I wouldn't worry about it.
If it starts staying on for longer periods. you'll want to replace the
sensor; your gas mileage will suffer otherwise.
I'd suggest getting one at a salvage yard; probably $30 or so. There
is a way to test one with an ohmmeter. I don't recall the exact
procedure, but sou should be able to find it on-line, and the yard
guys should know in any event.
It's about a 10 minute job to swap one of these; takes a 12mm socket,
an ~4 inch extension, a ratchet, and an ice pick or similar to work
the connector.
Let me know if you need details.

ByeBye! S.

Steve Jernigan KG0MB

Thanks Steve, I found an online instruction page with pictures showing
where the knock sensor is and what it looks like (at least for a Legacy,
might be the same for an Outback).

http://www.lovehorsepower.com/SubaruDocs/subaru_legacy_knock_sensor.htm

Yousuf Khan

 

[S]

Hi Yousuf!

Thanks Steve, I found an online instruction page with pictures showing
where the knock sensor is and what it looks like (at least for a Legacy,
might be the same for an Outback).

http://www.lovehorsepower.com/SubaruDocs/subaru_legacy_knock_sensor.htm

Yousuf Khan

De nada.

Legacy, Impreza, SOHC 2.5, DOHC 2,5 all pretty much the same knock
sensor arrangement; good ol' Subaru. Some models, you may have to pull
off more of the air intake plastic to get at it, that's about all. I'm
lazy, and usually just take off everything as a unit, all the way to
the throttle body; that way everything stays clean and more or less in
alignment.

When you put it back on, be sure to re-connect all of the PCV hoses,
and be careful that the boot at the throttle body doesn't get pinched
under.

This makes a good time to install a fresh air filter, and PCV valve if
you're so inclined.

Have fun!

ByeBye! S.
Steve Jernigan KG0MB
Laboratory Manager
Microelectronics Research
University of Colorado
(719) 262-3101