You can read for faults by connecting the the green /black plugs together under the drivers side of the dash near the steering column. the check engine light will flash and give the code. Long flash for 10's and short flash for 1's.

As for the Maf, id change anyway. They are one of the prime causes of melted pistons on Subarus. Their easy to change on post 1998-1999 cars as the sensor is seperate from the housing. Its removed by unscrewing the two torx head screaws and it just pulls out. Around 80 quid from subaru i think. the earlier ones were all one and bolted to the airbox.

As said by others the front lambda is for the ecu/fueling, the rear is just to show failure of the cat. IMO if it was the lambda sensor, it would like be throwing loads of check engine lights as the ecu has various sensors and would show and 'out of range' signal. The MAF sensor on the other hand usually only throws a light when it well and truley knackered. But can cause engine damage long before that happens.

I used to change the MAF sensor every second service on my 2000 impreza.