MOT fail on lamda emmisions

[GarbhAllt]

My 2002 Boxster S has failed the MOT on emmisions.  CO and HC are almost suspiciously low but Lamda is high causing the fail.  It had two new Lamda sensors post cat last year and a new Variocam solenoid plus timing tweak this year but the Lamda reading remains the same at about 1.05.  Any advice?

Edited August 29, 2019 by GarbhAllt

[GarbhAllt]

I should also mention the CEL light is out and there are no errors stored.  I also have Durametric but no idea how I can use it to troubleshoot this problem.

[JFP in PA]

11 hours ago, GarbhAllt said:

My 2002 Boxster S has failed the MOT on emmisions.  CO and HC are almost suspiciously low but Lamda is high causing the fail.  It had two new Lamda sensors post cat last year and a new Variocam solenoid plus timing tweak this year but the Lamda reading remains the same at about 1.05.  Any advice?

 

I would look at the O2 sensor voltages before and after the main cats, and would also check the fuel trims.  A lamda of 1.0 would have the engine running at an air/fuel ratio of 14.7:1; if the lamda is above 1.0, the engine is running lean, if below 1.0 the engine is running rich.  You have a lean condition, which could be as simple as a vacuum leak.