Expert Automatic Transmission Defect

Whoa, that's rough! Slipping gearbox and it won't even start? Does it feel like it's stuck in gear when you try to start it? Have you tried checking the basic stuff like fuses or the battery voltage? Could be a low voltage issue messing with the control unit.

Battery voltage is fine. Fuses also checked, all good. When you say "stuck in gear", what exactly are you referring to?

Okay, voltage and fuses ruled out. By "stuck in gear" I mean does the gear shifter feel really tight or difficult to move out of park? Regardless, with the check engine light and slipping, the defective control unit sounds spot on. It's probably fried. Better get it to a workshop, that's beyond driveway fixes and specific for Peugeot automatics.

Thanks, that confirms my suspicion. Replaced the control unit as suggested; cost 1000€. Problem solved.