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Registered
2005 Forester XT
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7 Posts
Discussion Starter #1
Hi all, I have a 1999 Forester S and I've noticed it drives really "rough". It hesitates when I give it gas and would shake when idling. I haven't checked the code but after reading I think it is either the knock sensor or o2 sensors. I was thinking I should just replace the o2 sensors first. Does anyone have any suggestions as to where I could get good quality o2 sensors and could the problem be caused by anything else?

I live in Toronto, Canada local or online stores is fine.

Thanks
 

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Premium Member
2015 Highlander AWD XLE 6AT
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4,255 Posts
You might first try checking with a local auto parts store (i.e., something like Auto Zone or Advance here in the States) to see if they'll read the CEL code for you at no charge.

If you do need a new O2 sensor (or two), Bosch offers a broad range of OEM and aftermarket models. I borrowed a 22mm O2 sensor socket like the one on the right in the pic to R&R the two sensors on my 2000 pickup truck.



HTH,
Jim / crewzer
 

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Registered
02 Forester L (sold) Manual
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1,220 Posts
You need the code(s) first before you go flushing money down the parts replacement toilet. Have you pulled the spark plugs and had a look at them? How old is the wire set? Coil packs are known to fail and will cause odd drivability problems. Without a code to point you in the right direction you're just shooting in the dark.

The rear O2 sensor does nothing for the drivability, it merely reads the exhaust coming out of the cat and gives it a yes/no approval for being clean. The front one is expensive but should be replaced by an OEM style one and not a universal fit as too many people have complained of problems with them. The O2 sensor is very likely seized into the exhaust pipe and may be a real problem to get out, and you can't do it without the special socket shown in the post above.

So, get the code(s) and report back. :icon_cool:
 

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The Modfather
2019 Impreza 5dr Sport - Manual
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8,054 Posts
I agree, although I'm always big on replacing the front O2 sensor after say 80K miles, get the codes. One thing you can check that is very simple and won't cost anything are the connections at the coil pack. The coil pack is on the top of the motor, the little square thing that all the spark plug wires (cables) come out of. It's not uncommon for those to get corroded and make for a rough running vehicle. Pull a cable out, one at a time and if the end isn't shiny, buff it up with some emery cloth or similar. If they are the original wires, I say get new ones. OEM wires are the best and not very expensive.

Either way good luck and let us know what you find.
 

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Registered
2005 Forester XT
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7 Posts
Discussion Starter #5
The spark plugs and wires have recently been replaced as they were corroded. I will try and get the codes read and keep everyond updated.
Thank you for the replies
 

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The Modfather
2019 Impreza 5dr Sport - Manual
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8,054 Posts
How long ago? It might be worth checking them again. I was surprised how fast they got corroded again.
 

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Registered
2005 Forester XT
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7 Posts
Discussion Starter #8
got the code read, its the knock sensor. also got around to look at the front o2 sensor it looks rusted and like its about time to be changed too.

gonna change once the sensors arrive will update again
 

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Registered
1999 Forester
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12 Posts
You need the code(s) first before you go flushing money down the parts replacement toilet. Have you pulled the spark plugs and had a look at them? How old is the wire set? Coil packs are known to fail and will cause odd drivability problems. Without a code to point you in the right direction you're just shooting in the dark.

The rear O2 sensor does nothing for the drivability, it merely reads the exhaust coming out of the cat and gives it a yes/no approval for being clean. The front one is expensive but should be replaced by an OEM style one and not a universal fit as too many people have complained of problems with them. The O2 sensor is very likely seized into the exhaust pipe and may be a real problem to get out, and you can't do it without the special socket shown in the post above.

So, get the code(s) and report back. :icon_cool:
I just ordered two generic ones for $46

Should I cancel the order and get a more expensive one?
 

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Registered
2007 forester x
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2 Posts
I have a P0171 code on a 07 Forester. Service engine soon light is on as is my cruise control light. Car has 87 000 on it. Any ides?
 

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Registered
2006 Forester XT
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657 Posts
I have a P0171 code on a 07 Forester. Service engine soon light is on as is my cruise control light. Car has 87 000 on it. Any ides?
That code means the engine has adjusted the fuel trims as rich as it can but the mix is still lean....

First thing to check is intake leaks - any air getting in the system will cause this code. Done any work lately? Left the dipstick out or the oil cap off?

Next step is the front O2 sensor.
 
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