2009 Forester Code P2097, bad fuel economy, replaced both O2 Sensors, what now?

[dmcclosk531]

Hello All,

 

I'm new to the forum, and to Subaru ownership. I picked up a 2009 Forester SOHC, 5 speed, now with 65,000 miles. When new to me the car regularly got 27-28 MPG on the highway going 70mph.

 

Over the last month or two i noticed it slipping to 25-24-23, then one day the CEL came on, traction control light came on, and cruise control shut off and started flashing.

 

Got the code checked and cleared with the following results " P2097 Post catalyst fuel trim system too rich bank 1 confirmed" drove the car for approximately 100 miles, with fuel economy around 20mpg highway and then the light came back on, same code. So, i replaced the post catalyst O2 sensor. Drove another 100 miles or so, light came back on. This time i replaced the pre-cat upstream O2 sensor. (also changed the air filter while i was in there)

 

I haven't driven the 100 or so miles it seems to take the CEL to come back on, but judging by the fuel economy, around 20mpg highway, its only a matter of time and i have not fixed the underlying problem.

 

Any ideas what other systems or sensors might be causing this issue?

The car other wise runs, drives, idles, and starts normally. Power seems normal. perhaps a little bit of stutter when starting from a stop, but i might be imagining it. it's very slight if anything at all. might just be my clutch action....

 

Thanks for any help or ideas. I'll update again when/if CEL comes back on.

[idosubaru]

Hopefully someone a little more versed in this steps in as this sounds fairly abnormal for any Subaru particularly this low mileage.

 

I wonder if one of the fuel injectors may be leaking too much fuel into the cylinders.

 

Is it ever hard to start?

 

Fuel injectors aren't hard at all to replace so don't let it scare you. It is very rare for them to fail so even a low mileage used one is a reasonable fit particularly for one this new. Sometimes new prices are a bit absurd for parts that rarely fail.

 

They can also be cleaned/tested/rebuilt by RC Engineering or Witchhunter

 

But again hopefully someone can offer some specific testing/symptoms/suggestions to narrow it down for sure.

[dmcclosk531]

I ran a bottle of fuel injector cleaner through, didn't see any change. Haven't looked into the injectors any further than that.

[dmcclosk531]

Always seems to start easily

[ShawnW]

How old are the spark plugs?

[dmcclosk531]

Not sure, bought the car used and haven't changed them

[GeneralDisorder]

You should change the plugs - air filter too if you haven't.

 

After that if things don't improve then you need to hook into the live data and do a sanity check on what's coming from things like the coolant temp sensor, MAP sensor, and MAF/IAT sensors. The mixture is too rich and the ECU's fuel trim routines think everything is ok because one of its sensors is out of calibration - that would be my guess.

 

GD

[russerts]

I just encountered this problem last night on my 2012 Forester (24k miles). CEL, Traction Control light, and flashing Cruise light.  I stepped on the throttle to pass a truck at 50 mph with four passengers on board and got this as I accelerated - right after it downshifted.  I have not checked the codes yet but this certainly seems like a specific, though odd, set of indications. I don't know whether there is any effect on mileage because I was only 50 miles from home.

Edited June 29, 2013 by russerts

[Beepo]

I recently joined the message board for this same condition. I have a 2006 Outback Wagon, 127k, 2.5; within the first 150 miles after driving away from dealer, CEL and flashing cruise light. Codes P2097 and P0131. Very disappointed with dealer because I bought this car partly because of the impeccable maintenance log and I couldn't help but surmise that someone had reset the engine codes right before I test drove it. Anyway....I tried injector cleaner but no change. Last check of the codes, I have added P0026 & P0028. I have hesitated on replacing the O2 sensor because I have read many accounts on-line where replacement didn't change the situation. I have now put 800 miles on car and ready to surrender. Any thoughts or insight? Thanks.

(Forgot to mention that it is getting 28 to 30 mpg ...very happy about that )

Edited June 24, 2016 by Beepo

[Fairtax4me]

Did you buy it from a Subaru dealer? You got a warranty on the car? 60 days, 2,000 miles? Take it back to the dealer.

Should have taken it back right away when the light came on.

[Beepo]

Sadly, I bought it from a Subaru "offshoot" company ...they use the same business office and fiscal managers, but this allows them to sell "as is" and I guess I'm going to pay for it. I was ready to drive it back and give 'em heck, but I live 80 miles and couldn't get off work. When I sent the salesman an email telling of the scenario, he offered some "quick fixes" but no offer to remedy directly ( and I would have been willing to meet half way on the cost). Instead .....I'm on the dealership's service email list, and at this point I'm not feeling warm and fuzzy with them.

[mikaleda]

I would suggest monitoring the front o2 sensor with a scanner that has live data and see if its stuck in a closed loop.

That is a surefire way to tell if the 02 is bad or not.

IIRC the front 02 is the only oxygen sensor that is used for fuel trims.

 

There is also the possibility of ECTS and or MAP sensor malfunctions.

 

I would also suggest checking all of your intake tubing for leaks, that can throw of air fuel meetering as well

[Marathonman]

Hopefully someone with computer knowledge sees this. My 2010 Forester with 5 spd is doing the exact same thing with P2097. I have 108,000 miles now. I did timing belt, water pump, plugs and wires and replaced air filter and pcv about 5,000 miles ago. I am scanning my computer now as suggested previously. MAF is 0.36. MAP is 8.3. Coolant is 199. ST trim -3.1 and LT trimmis 28.1. Cat con temp is 413. O2 voltage bank sensor 0.955. O2 sensor bank 1 sensor 1 is 0.086 is that enough info to know what’s going on? Please help and thank you!

[GeneralDisorder]

Hopefully someone with computer knowledge sees this. My 2010 Forester with 5 spd is doing the exact same thing with P2097. I have 108,000 miles now. I did timing belt, water pump, plugs and wires and replaced air filter and pcv about 5,000 miles ago. I am scanning my computer now as suggested previously. MAF is 0.36. MAP is 8.3. Coolant is 199. ST trim -3.1 and LT trimmis 28.1. Cat con temp is 413. O2 voltage bank sensor 0.955. O2 sensor bank 1 sensor 1 is 0.086 is that enough info to know what’s going on? Please help and thank you!

 

 

Long Term fuel trim is WAY high. It's adding fuel like crazy. This means the computer thinks the mixture is lean - either due to an air leak past the MAF, bad MAF, or bad data from the primary O2 sensor. 

 

With those newer models we do see a fairly high rate of MAF sensor issues. And they aren't throwing MAF codes either - just running horribly and trimming fuel under load.

 

You need to do some Volumetric Efficiency calculations with your datalog information to give you a sanity check on the MAF readings. 

 

Top suspects would be the MAF and upstream (primary) mixture control O2 sensor. 

 

GD

[3Pin]

I read this on another subaru board.  You have done some of this already.

 

could be a bad rear o2 sensor, basically that sensor deals with emissions. it's not saying that the catalyst efficiency is below normal which is common, it's saying there's raw fuel there.

so, guesses ... leaking inj, improperly adjusted exhaust valve, failing rear o2 sensor.

try some injector cleaner [in the tank], make sure the rear o2 sensor connector is secure. this may sound goofy but make sure the gas cap is on correctly and tight, this isn't a typical loose gas cap code but you never know. generally evap codes take a long time to clear with no recurring problems [anywhere from 50- 300 miles] 

[golucky66]

could be a bad rear o2 sensor

 

Rear O2 only has slight correction on fuel trims. +/- like 3% total trims. So the downstream O2 alone can not cause fuel trims to be altered that much (28.1 LT) that high of a number points to what GD is talking about. Air is getting into the engine that isn't accounted for. 

 

Either from a bad MAF sensor not reading the proper volume of air

Bad/ biasist upstream O2 (Air fuel ratio sensor on your car)

Or from an air leak from somewhere after the MAF.

 

In theory injectors not spraying enough can cause fuel trims to go up. But typically the injectors don't get clogged up in this fashion.

If you're concerned about fuel injectors, it would be wise to check fuel pressure. As low fuel pressure would cause the ECM to command more fuel.

 

But, again. It's likely one of the few GD posted. 

[GeneralDisorder]

Right - there are other "exotic" causes but only go there after doing the VE calculations and watching O2 sensor data and you are SURE it's not one of these. 

 

I have not seen that generation of top feed injector have any issues, and the fuel filter is in the tank so it's unlikely to be an injector issue in my experience. Subaru's don't typically have low pressure either - the pumps usually just die completely if they are going to have an issue. Again failure rate on that model year of injectors and pumps is basically zero. I have not heard or seen of such a thing. 

 

GD

[somick]

My 2010 Forester manual calls for a fuel filter replacement at 60000 miles.  Have you replaced yours?

 

GD mentioned a bad fuel pump.  It could be insufficient fuel pressure.

 

Good luck,

Sam

[golucky66]

On these newer cars, the fuel filter is in the tank. Part of the pump.

 

No idea why Subaru would recommend replacing the "filter" as all it is is a mesh screen (fabric kinda) to prevent the pump from sucking up chunks.

[Marathonman]

Awesome feedback from all you! You all explained everything so easy to understand too. Thank you! I have a lot more things to look at this weekend due to you help. I’ll report back my findings.

[ccrinc]

If you check out Google, you will see thousands of posts for this same problem, most of them starting in 2009.

My 09 Forester has the same thing on a reman'd. engine with less than 3,000 miles on it.  I did replace the exhaust manifold and the upstream O2 sensor, but it needed those anyway.  If you have a code reader, you can clear/turn the code off.  It will stay off until you drive the car fairly hard, like 2nd gear uphill for about a mile.  Nobody (especially  not the dealers) really know what's going on.  The engine runs great, the mileage is about 24mpg (pretty standard) and it's great except that I think the exhaust smells too strong. 

Note that during the rebuild it got new plugs, wires, etc. and the injectors are good.  Meh. Every Subaru "generation" seems to have its own idiosyncrasy.  It's part of their charm.

 

Emily

[GeneralDisorder]

It will stay off until you drive the car fairly hard, like 2nd gear uphill for about a mile.  Nobody (especially  not the dealers) really know what's going on.  The engine runs great, the mileage is about 24mpg (pretty standard) and it's great except that I think the exhaust smells too strong. 

 

Emily

 

In our experience this trim issue under load is a problem with the MAF. Have seen this and corrected it with a new Denso MAF on multiple occasions. 

 

GD

[Fairtax4me]

Hopefully someone with computer knowledge sees this. My 2010 Forester with 5 spd is doing the exact same thing with P2097. I have 108,000 miles now. I did timing belt, water pump, plugs and wires and replaced air filter and pcv about 5,000 miles ago. I am scanning my computer now as suggested previously. MAF is 0.36. MAP is 8.3. Coolant is 199. ST trim -3.1 and LT trimmis 28.1. Cat con temp is 413. O2 voltage bank sensor 0.955. O2 sensor bank 1 sensor 1 is 0.086 is that enough info to know what’s going on? Please help and thank you!

Really only two things that will throw the LT off that far, MAF or O2 sensor.

MAF failures seem to be common on the "newer" models. It's also the easiest part to replace. I would start there. New Denso MAF for that car is about $90 from rockauto.

[Marathonman]

I truly appreciate all the feedback. Everything under the hood looks good. I'm ordering a MAF sensor from Rock Auto. I'll report back once I get it changed.

[Marathonman]

Happy Nee Year! So I installed my new Denso MAF. Was good for a bout 5-10 miles. Fuel trim was low and MPG went up and exhaust didn't smell foul. Then it happened again. The fuel trim went high again, MPG dropped and exhaust smelled bad again. I'm afraid the dealer might be my next move. More guessing on $100 sensors will add up quick. Any other suggestions? Is the primary O2 sensor still reasonable to replace? Someone mentioned that was the next most likely issue. Then to the dealer if it doesn't help? Thanks again.