O2 sensors, engine light

[GTB25T]

Please help with advice.
Sorry if this has been discussed before but I couldn't find in the search.
I'm throwing a fault code when engine light is on and cruise light flashes and stuck in S mode.

2007-08 Lib GT-spec B Auto.

Clear the code and it comes back randomly, sometimes just at traffic lights idling or driving.

It points to the Pre-Cat bank 1 O2 sensor to lean.

Injectors done, fuel filter done, MAF sensor done, plugs ok. Low km car.

The O2 sensor currently is 22641-AA420 Denso.

I only have access to working used ones to try 22690-AA520 and 22690 AA850.

Will it be safe to try them?

These are from other EJ255 turbo engines like WRX Sti and Forester GT same year range. why do Subaru have so many variants of O2 sensors where other vehicles try to keep some commonality between vehicles

[bigBADbenny]

You can lookup the code on your odometer.

viewtopic.php?f=6&t=34980

Once you have the code, you can look up the diagnostic procedures in the FSM aka field service manual.

The diagnostic procedures recommend fault testing over parts swapping.

You didn’t mention the code, I’m not going to guess.

The preferred terminology is front a/f sensor, rear o2 sensor.

So DTC/cel code names generally refer to the affected system and not the part needing replacement. As such the system usually requires some inspection.

Eg, yes it might be the sensor, but depending on the code it could be the harness plugs, the loom, fuse, relay, fuel supply, ignition or exhaust or inlet pressure issues triggering the fault.

[bigBADbenny]

Whilst you can just replace the affected sensor, if the issue is eg loom or afr related, it may not fix the cel and could even harm the new sensor.

So it’s always best to diagnose the fault mode, prior.

Again, I’d need the actual code to comment more specifically.

[bigBADbenny]

As for replacement sensors other than oem subaru, you can find the Denso oem equivalent part number on the denso site, order from Cooldrive in Vic or Global EFI in Qld…

I’ve seen a Goss branded oem equivalent sensor being used and working fine, which does go against the grain of oem subaru branded sensors only dogma.

Either way oem part number equivalent sensor is the key.

If you’re testing for faults that caused the cel, you may need a multimeter and for too rich lean issues, try these diy inlet & exhaust pressure tests to help your car run properly.

viewtopic.php?f=6&t=36513&p=448984#p448984

Do get back with the actual codes as well.

[GTB25T]

Many thanks for the reply.
It was diagnosed by a subaru specialist of over 30 years (not a dealer).
He saw the code on his diagnostic equipment and then went further into detail where it said bank 1 pre-cat too lean.

However, I was concerned the O2 sensor was picking up the issue and was not the root cause. He gave me a print out of the old obsolete fault codes in memory and the current. I shall look for the details and get back to you.

I suspected the fault came up after the upper engine clean, then the on car injector clean. Reset and engine light went away.

Since then a less experienced mech installed a new in tank filter and the O ring did not seat correctly. I had starting issues and hesitation. Engine light on again and same code for the O2 sensor.

After re-work, all starting and driving smooth again. Reset and now been 200km or more with no engine light. I have not yet replaced the O2 sensor.

I was advised O2 sensors should be treated like spark plugs and replaced after about 100,000km for best fuel economy, etc.

[Ric]

My 3RB is a similar vintage, and I had to replace one of the O2 sensors about a year ago.
Random "Check engine" errors problems went from a couple of times a year to a couple of times a day before I replaced it.

[GTB25T]

Thanks for info.
I have the detail on codes to add now:
P0171 System Too Lean Bank 1, however this seems to have gone away after replacement of fuel filter and injector clean. I did clean the MAF sensor and the pre-cat A/F sensor but still suspicious of the expensive O2 sensor (Denso....AA420).

There was a code possibly in history also for P2093, Camshaft Position Actuator control circuit high bank 2. The graphs showed driver side cam advancing 13 deg but passenger side on 0 not advancing so I shall check the Earth connection and the Cam solenoid now I have read up on it.

It does have a minor stumble at idle and in hot traffic did stall only once. Otherwise is pulling very hard on boost and S+ shifts very well.

It is an amazing car...I would buy another just to keep it.
Still love my Forester 2015 diesel, especially for country trips but this GT-B is another level of driving experience.

[bigBADbenny]

Ok that’s great progress, but are you out of the woods yet?

The cam sensor diagnosis is reasonably simple, I’ve been through the process and found a harness wire that didn’t pass the FSM resistance test from the sensor plugs to the ecu plug.
I used the FSM tests for P0365…

The sensors are fairly reliable, the main issues are the oring seal failing (inlet tract leak via pcv system), sludged sensor face (cam signal dropouts) and sensor wiring faults.

However getting to the cam position sensor on bank 2 is a challenge on an H4DOTC at least, here’s how I did it:
viewtopic.php?f=10&t=36503


The avcs vvt angle issue can occur with or without triggering a cel/DTC… what’s more important is to identify the laggy side, as bank1 ocv solenoid
is fed by the same union screw that feeds the turbo.

If the cause of the lag is that filter being blocked by soot or varnish, turbo failure may ensue.

Here’s the avcs faq, with tests and considerations.
viewtopic.php?f=6&t=36504&sid=5a41ca936c034af7f0a20ff2fe5ed7c8#p448939

[GTB25T]

Thanks for the links and info Benny. Some excellent reading!

I'm not sure if this ECU and this car learn and substitute values but seems to run better.

I'm yet to have the left side VVT advance checked again.

The only observation I have is unless driven in S# there is some turbo lag.

I'm used to a 2L turbo diesel boxxer so this 2.5T feels the opposite in torque.

I have driven a tuned by MSE GT-B and the torque is instant at low rpm.

[bigBADbenny]

Yes the ecu will relearn.

You can monitor tuning parameters using a vagcom with usb otg adapter, btssm app on an android device.

This is the secret to knowing the state of tune and very few suprises regarding CEL/DTC codes.