Adam, as the vehicle is fitted with LPG it no longer needs a cat for its MOT, it may be tested without and would pass if below 3.5% co and 1200 ppm hc. Visually the tester may require that it appears to be in place and you may wish to remap the ECU so it ignores the post cat lambda.

I suspect the vehicle has further issues when running on LPG that may have caused both these faults, we have carried out several conversions to these and they are not straight forward.

Regards Eddie Zyla.
CRD Performance Bolton.