I'm pretty sure that even if the AFM is dodgy it won't come up with a fault unless it is completely fucked. Just from memory taking mine bucket to a Suby dealer to plug in their uber fantastic diagnosis machine, only to tell me that it either reads the AFM fault as good or shagged.

On another note, shouldn't you get the car tuned after replacing the AFM? I think they reset the base throttle position and crap like that if they replace the AFM, or unplugging the battery for an hour to let it reset or something along those lines. Not a good idea if you've got the "amazing" brant crap, but if you've got something else it's ok.

Just personal experience, nothing more.

now finish the job by reseting your ecu:

Ensure engine is at normal operating temperature, turn engine off
Locate the two ECU connectors, located under the steering column and consist of a black plastic male and female connector, and a green male female connector
With the ignition OFF connect black to black and green to green.
Turn on ignition, do not start the engine, depress the accelerator pedal to full throttle, then return it to the half-throttle position and hold for a few seconds, and then release
Engine Check lamp turns on
Start engine and then drive for at least one minute, keeping road speed above 11 Km/h.
ECU is now re-set.
At this point the check engine light should start to flash the all clear signal (steady 1/2 second interval flashes). If the check engine light does not flash, or indicates some other sequence, there is a fault present in the system, and should be professionally checked for necessary repairs.
Stop the car and turn off the engine.
Disconnect the black and green connectors.
Job complete

I had meant to reset the ecu but didn't get around to it, I'll do it in the morning. Didn't think it would make that much difference but in hindsight it makes sense that everything should get reset.

Cheers astralex and Strubaru!

Just to let you guys know (and anyone reading this thread later).

I took my car to RMS today so they could have a look and sort out this issue once and for all. They found that it's almost certainly the O2 sensor. The turbo back exhaust on my car may have been meant for the GD as the sensor is too far down the dump pipe. So it needs to be moved up and much closer to the turbo. It's quite likely the sensor itself is shagged anyway which is bad news cause they're around $500 from Suby (More to add to my current woes with my stuffed brant alarm and slipping clutch... ARGH!!!).

It's at least good to know what the problem is while I save up to sort it out. My fuel economy may suffer but the engine's safe.

you can get a generic 3 wire Bosch O2 sensor for around $110. Just crimp it on and your done. No fucking around with Subaru genuine ripoffs.

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RS Liberty - Because WRX's are only good for parts...

I was hoping there would be a cheaper option.

Have you or anyone else swapped the OEM sensor for one of these with no probs or issues?

I have. Im using an ACA Universal 3 wire 02 Sensor in my Legacy. No problems.

__________________
RS Liberty - Because WRX's are only good for parts...

Cool, I'll see how I go after getting the sensor moved further up the pipe. If it turns out to be cactus anyway I'll grab one of these sensors. Been forced to buy enough genuine stuff lately (front bumper, brant keypad, etc.)

Cheers,

try removing the top mount and checking that the coolant hoses to the throttle body have'nt been bypassed, whilst you have the tmic off disconnect the 2 electrical plugs that go to the throttle body and spray contact cleaner in them and allow to dry before reconnecting, doing this worked wonders on my 99 and now idles alot smother, thought it may be a worth a try for you, the round plug at the top of the t/b can get wet from water coming in the scoop and could get dirty/corrosion in the contacts over time

hey mister two, do you happen to know the bosch part number for a generic 3 wire to suit a 99 gt forester? (its the same subi part number as a 99 wrx which is 22690aa320)

cheers mate