Emissions warning light on

[peterb11]

2007 Vec 2.2 direct estate, Z22YH engine, 64K miles.
Emission warning light came on (constant, not flashing) day before booked in for MOT at local garage - brilliant! So £40 Tech2 reading gave: “P0136 heated oxygen sensor (HO2S) bank 1 circuit”. But it then PASSED the MOT with clean as a whistle exhaust. CO and HC less than 25% of max permitted, and Lambda in middle of range at 0.999.
Garage said the warning light was probably triggered by some one time cough in the engine and it would reset (go out) after 20 starts if the system was now happy. But after well over 20 starts it’s staying on.
Any help appreciated. What could be causing this ‘false’ warning or how to put it out...? Not worth changing a Lambda sensor if they’re both doing a fine job, and it runs really well under full range of gentle and vigorous driving, no problems obvious at all.
Thanks, Pete

[peterb11]

Sorry to bump, but are there any ideas on this mystery...? I was hoping to get advice that I'd missing something obvious... ;-) I still have the light on telling me something is wrong even though passed MOT.

Thanks Pete