I had similar problems getting my mk1 F through - in 2011 I replaced the exhaust, cat, down pipe and probe with new parts and refurbished a exhaust manifold, took the car along and it passed. I did all this with the car still on her wheels but with the O/S/R jacked up so the suspension was at full extension. I was happy with everything except the tightness of the 4 bolts on the manifold to downpipe joint so I asked the garage to nip these up.
12 months and 3.5k miles later she failed again on emissions despite them retesting that after an italian tune up and the dash gauge showing the oil at 80'.
I changed the coolant temp sensor, spark plugs and removed the airfilter and it was still out so I got MJS to have a look, he swapped the probe for one I hadn't fitted and found that the previous garage hadn't tightened the manifold/down pipe bolts so the gasket had burnt through allowing a leak.
She sailed through the test with spot on readings.
With your car I'd try the coolant temp first as this fails "cold" and allows the ECU to think the engine is cold and ups the fueling. It has been a problem on MGR cars for a long time with the T series suffering as well. Don't be fooled by the reading on the temp gauge on the dash, it has a different sensor.
If yours is a mk2 you can use a OBD2 reader to see what tempreture the ECU is seeing and compare that to the dash gauge, at running temp. just under half way the dash temp gauge sensor is seeing betewwen 88 and 110'. If it is a mk1 you'll have to use a T4 machine, alternativly treat the ECU temp sensor as a service item and change it anyway.
Home to black Alfa Romeo 159 3.2 V6 Q4 ,green MGF VVC and red MG Maestro T16.
MG - the friendly marque.
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