[SJS]

I have a 2003 2.7 boxster, which has just failed the NCT emissions test (roadworthiness test in Ireland)
The test here uses a device which samples and measures the exhaust fumes. The car failed on lambda out of range (high) with a reading of 1.10 (had it checked twice and confirmed). The technician who did the last check said that the lambda value was very unstable and would be in range momentarily/sporadically and then jump back and forth to a high out of range value.

I've done an ODB check and it shows a pending code P0136 (one of the rear oxygen sensors)> my understanding is that the rear sensors have no control over the mixture and therefore this is unlikely to be the source of the problem.?

In addition the car is making a screeching noise at start up and the car pulls a vacuum on the filler cap (but no white smoke) I believe this is likely to be a faultily AOS

1>Am I correct in the assumption that the code P0136 is unlikely to be related or the cause?
2> Can a faulty AOS cause a high lambda reading?
3> Any other potential causes for a high lambda reading

Thanks in advance for any help on this.

Stewart