I have a 2015 Focus 1.5 tdci which has a weird problem. When I first start the car it works normally and the revs stay at 800 rpm. Once the engine heats up the car works normally when driving, but when I stop the revs stay around 950 rpm and the engine runs a bit rough.

If I turn on the AC the revs drop to 800 and the car works normally.

If I turn off the headlights or if I stop braking only after the revs drop under 1000 the car again drops to 800rpm, but if I touch the gas pedal the revs go back up to 950.

Before I started to notice these things I took the car to a service for an oil change at which they told me that they had to do some work on PCM because of a recall. I started noticing these weird things around a week or two after the service, but I can't say surely that the car was working normally before it.

After I noticed these things I took it back to the service at which they told me that the car isn't throwing up any codes, they don't see anything wrong and that it could be DPF regen.

I was doubtful that it was regen because I drive it on the highway and I see when the regen starts and stops on the fuel consumption scree. Also the start/stop works normally when this is happening and the fans aren't working loudly like they do during regen.

I came back a few weeks later with the same problem after I drove the car on the highway every day for those few weeks, but the mechanics told me the same thing.

Could anyone offer any help?

If the revs are high at idle, it's trying to regen the DPF. Let it finish.

If you are using crappy diesel and doing short stop/start journeys it will try to regen more often