Legacy GTB BG5 Japan model,engine code 23?

Hello Everyone.
I am new to this forum or any forum regarding my car. I have a GTB B G5 japanese model Legacy wagon and I have been having problems as of late with misfiring,black exhaust smoke. I have had the car in the shop 4 times in the past 5 weeks to no avail. The plaugs and the ignition coils have been changed, the timing belt is new(ish) 65.000 km on it. The primary and secondary turbo(s) are fine,air filter is new and so are all the other parts for that matter.
I finally had the mechanic at the shop (not a Subaru dealer..$$$$$) take a look at the engine computer today. The engine icon on the dashboard blinked twice slowly and then 3 times quickly (engine code 23)
My question to you is this...

On the internet for the legacy engine codes,code 23 refers to the 'oxygen sensor'. Is this true of all Subaru engines. Are the codes the same for all two stage twin turbos?

Thank you for your help,
Richard Beare.
Japan

This would be the infamous MAF sensor problem faced by the GT-B and other models (version 5 upwards).

The 5 pin MAF can become cranky after some mileage under its belt. You will accelerate its demise by using pod filters or other after market filters which do not do a good enough job as the original air filter i.e minute dust particles will sandblast the poor fella.

Symptoms : poor idling (this is erratic - sometimes good idling, sometimes bad & u never can tell when), black smoke.

Code 23 is indeed the 02 sensor.

Due to the faulty MAF sensor, the car will sometimes burn rich sometimes burn lean depending on whatever wrong readings it is giving to the ECU.

Code 23 in this instance is usually triggered when you go WOT and the MAF sensor is making the car burn richer than it should. This would heat up the 02 sensor more than it should and the check engine light would be triggered. If you ease off - the CEL will quickly disappear but it will be recorded in the trouble code history.

The solution is simply to change the MAF sensor. Works like a charm. Some garages who dont know better will change the 02 sensor and other stuffs but bottomline - the MAF sensor is the culprit based on the symptoms u described.

I just finished fixing my '96 TT GT that had this error code. A new air flow meter did wonders. It's right there on top of the engine so I bought it second hand and installed it myself. It's not too hard... beats paying someone $75/hr to do it for you.

Although I'm a bit curious as to why the part is an STI part instead of Subaru. I didn't think STI did legacies back in those days?

(By the time I got the car it had factory looking STI stickers on the sides and back... not sure what to think about them, are they for real?).