Leaderboard

An air leak in the exhaust (after any sensors) could be pulling in fresh air and diluting the tailpipe measurement, A leaky vac hose in the engine bay, like the brake servo one might be giving false air. Doubt it would be ECU. Might be able to watch lambda sensor activity on VCDS to see if that appears to be switching sensibly rather than at one or other limit constantly? Catalytic converters only do their job fully and only survive longterm when the mixture is maintained near stoiciometry, which is why tailpipe lambda must measure within tight limits for MOT. CO2 might be lower with leaner mixture, but NOx, HCs and particulates will probably go up instead.

Have you checked that the connector has clean, undamaged contacts, and that you have continuity of each of the 3 wires to Engine ECU? Is it the original ECU or one that came with the engine?

What's the engine code? Or VIN if you don't know what I mean by engine code.