Hi Jim, every car 's difference in the Co to a point, a lot depends on the Mileage, age, what service & how often it has been serviced, my own car is 10yr old has not long been through the test & the readings were spot on, what I do before the test put in a bottle of Red X and go for approx a couple of mile run and get the car really warmed up, A lot of MOT stations tend to race the engine when the sensor is in the exhaust to get the Emission reading, they do not like doing this as, at times they have snapped the Cam Belt in turn costing them a lot of money under claims, my advice is to do what I have suggested by doing that your engine will be at it's normal running temp. The use of Red X help clean the Injectors etc & have never had any problem in its use.