Flywheel Or Cat Problem Or Just Starting Problems?

Hi fellow Ford owners,

Our car, an S-Max 2L petrol (2009), seems to have developed a fault but
the garage’s diagnostics are far from clear.

About 2 months ago the engine warning light came on and the local Ford
garage traced it back to the temperature sensor in the CAT. They could find no
fault in the CAT, so one possibility was that the temperature sensor itself was
faulty. They cleared the alarm and advised us to see how we got on. If the fault
came back soon then they said they would suspect the CAT itself. The car has
been driving fine and now 2 months later the engine warning light re-appeared.

This time I took it to a completely different independent garage and they confirmed
the warning again came from the temperature sensor in the CAT. They did
emission tests and checked the exhaust system and all was confirmed to be OK
with the CAT. However they noted that the car was difficult to start, unless the
clutch was pressed in when starting the car. We may have got used to it but yes
one usually has to rev it once or twice after starting the engine or otherwise
the rev may drop. And in the first few minutes of driving the rev can go up
& down a bit, sometimes causing the engine to race a bit, until the engine
has warmed up when everything seems to be OK.

This, they believe is due to the Dual Mass Flywheel not working OK.
They hypothesis is that if the flywheel doesn’t work as well as it should this
can cause unburned fuel to end up in the CAT, then when this ignites this caused
the CAT to report a high temperature.

I am no car engineer and with the car otherwise functioning OK it makes
it hard to justify spending the quoted £1300 repair for clutch & DMF
replacement. They say there is a possibility that not doing the repair could
impact the CAT and eventually damage it.

Has anybody else come across this problem, first of all of the car not always
starting well, the engine RPM being up & down for a while and getting faults
on the CAT temperature sensor? I see no other issues with the flywheel, no rattling
or grinding for example.

Many thanks,