Brick

My engine light is always on. Garage said its the lambda, oxygen sensor issue and replaced them. Didn't solve the problem and they didn't charge me to be fair to them. They said they wouldn't do any more as they didn't understand what the problem could be and I'd have to go to Subaru! Sod that. I've done as instructed on here and I get the constant flashing of the light and can hear the car doing all the clicking and whirring as it tests bits. BUT when I disconnect the black and green connections my light is on permanently. I connected the blacks to get an error code but just get a permanently on light. What could this be? ps my car is a standard 2002 (51) Forester Sport, the one with the different grill. The car runs fine, no probs at all but that garage refused to do my MOT with that light on. A different garage got the same fault but being mad Welsh hillbillies I couldn't make head nor tail of their broken English explanations, they basically didn't have a clue but didn't charge me. There aren't many garages up here that like to touch electrics or Subaru.