Ford S-MAX - Starting problem, temperature fault in CAT

Honest John Back Room » Technical matters
View Threaded
Mon 19 Jan 2015 15:51
Ford S-MAX - Starting problem, temperature fault in CAT - Geoidea

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 itself, 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 since then though now 2 months later the engine warning light re-appeared.

This time I took it to a completely different 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 RPM may drop. And in the first few minutes of driving the RPM 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 that this is the link to the temperature sensor failure in the CAT as 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 technical car expert and with the car otherwise functioning OK it makes it hard to justify spending the quoted £1300 repair for clutch & DMF replacement. The 2 nd garage told me that 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, and the car drives well and doesn’t make any usual noise.

Many thanks,

Geoidea

Reply | Report as offensive | Link
Mon 19 Jan 2015 18:22
Ford S-MAX - Starting problem, temperature fault in CAT - elekie&a/c doctor

This all looks very confusing.There is no temp sensor on the cat,although there are 2 lambda sensors,neither of which would cause bad starting.The dual mass flywheel is part of the clutch system,if it was faulty there would be a bad rattle from the gearbox,possibly difficult gearchanges and driveability issues,juddering/vibration.Really can't see how this affects starting.How old is the battery and has it been checked recently??

Reply | Report as offensive | Link
Mon 19 Jan 2015 20:08
Ford S-MAX - Starting problem, temperature fault in CAT - Geoidea

Thanks. The starting could indeed be related to battery though it isn't clear why it starts better / ok actually when the clutch is used. I will get the battery checked.

The engine error code is P0420 which relates to the oxygen sensor. You are right, it is not a temperature sensor. So far two errors over 2 months.

Any idea why the car has an irregular RPM just after starting? Maybe because we then rev it a little we end up with unburned fuel in the cat.

I am just guessing here. It looks like there could be more than one issue here.

Reply | Report as offensive | Link
Mon 19 Jan 2015 21:58
Ford S-MAX - Starting problem, temperature fault in CAT - elekie&a/c doctor

Error code P0420 indicates that the engine control system has identified a catalyst that is working inefficiently.This will not cause any starting or running issues and is also unlikely to be a problem on the emission test for the mot. Even the engine fault light illuminated is not an mot fail (at the moment).Starting with the clutch depressed reduces drag on engine,I always use this method.I would strongly suggest that the battery be checked,and if it is the original from new,get it replaced.

Reply | Report as offensive | Link

Looking for a garage you can trust? Honest John's Good Garage Guide has thousands of reader recommended garages, plus how-to guides and tips on servicing.

The Good Garage Guide