Hello there , If you are having issues with emission issues when running on petrol , pop into your MOT place ( if you can ) and ask them nicely to perform a free reading test , just to show you exactly what is being recorded by the machine .
If you still appear to have problems , send your car for test RUNNING ON LPG & inform the tester it is running on LPG . There is a different criteria for LPG emission readings , I get this sometimes on petrol , fails & always pass when tested on LPG .
With regards to number of Lambda sensors , simply buy as many additional bosses as you need , drill & weld onto pipes to match original set up making sure there are NO leaks as this will affect lambda readings, so you can happily use a 200 cell Cat . Also take your car for a good thrash before presenting for test, we get many that fail emissions testing as they drive very short journeys / never get hot /bimble around at 9 MPH & the cat is unable to obtain working temp , becoming unable to function correctly . A "drive it like you stole it " journey should generate sufficient heat to allow the Cat to begin functioning
Good Luck
