I also have a side exit exhaust. Insufficient back pressure was offered to me last week as the reason the tester had 5 attempts before recording a pass.
He also got the engine stinking hot in the process. He said all of a sudden the lambda reading plunged from the failure zone to well within the pass zone.
All of which indicates to me that there is either something amiss with the stability of the exhaust gasses, the testing equipment or the overall procedure.
Jobs before next MOT: A temporary tailpipe reducer and some exhaust wrap.
