Hi,
I have a 2.2l (185) diesel Mazda3 Sport (BL) and there are a couple issues which I think are connected.

To preface, pretty much since I'd bought the car I have been having issues with the turbo underboosting which would send the car into limp mode (eml and dsc light permanently on). A restart would let the turbo come back on, and the lights would normally disappear after a day and a half or so. Provided it didn't go into limp mode in that time. Now I know that this is a separate issue as to what I'm going to describe but it explains why I hadn't paid attention to my check engine light. About a month ago, I had a fairly major issue with the car as after parking up for work in the morning, it suddenly wouldn't go into 1st, 2nd or reverse. I tried it both hot and cold but no joy. So I had to get the RAC out, and in order to be able to have a look at the gear linkages he took the airbox off which triggered a MAF warning and another error connected to that. The engineer however also noted, that there was either a crankshaft or camshaft position error in the logs, which obviously hadn't been caused by removing the airbox. The only issue is, I can't remember which sensor was throwing the error message.

I have also started noticing that when the RPM's climb to about 3000, the RPM needle would just drop to 0 and bring on the power steering light. This would usually happen in 3rd gear and above, more regularly at motorway speeds. It was annoying but I could live with it as I don't necessarily need an RPM gauge to know when to change gears. Soon after, the engine started making a squeaking noise which I believe means that one of the auxiliary belts are getting worn out. It was at this point that I decided to book the car in to have the belt replaced and also to have the RPM needle looked at. I have time off from work starting tomorrow, so the car is booked in for then.

Between the time I had booked the car in and now however, another symptom has appeared. This is getting a rough idle especially when warm, while the RPM needle is now on 0 most of the time. This started happening about a week before my car was due in for the inspection but as I needed the car to get to work I decided to wait until the appointment and keep using the car. I deducted that if I just keep the RPMs above idle it wouldn't stall, which has worked. It would sometimes stall though if I didn't do this. But as the week went on, it felt like the idle was getting rougher and rougher and on Thursday when I last used it there was a bunch of white smoke coming out of the exhaust when starting the car. It also had a bit of difficulty actually turning over and starting. I haven't used it over the weekend in the hopes that it would give me enough to limp it to the garage tomorrow morning.

I have been doing some research and I have read that a faulty crankshaft position sensor can send the wrong signal to the ECU regarding the RPM's which would explain the needle dropping to 0. If I'm correct, a faulty crankshaft position sensor can also cause valve timing/injection issues which would explain the misfiring and therefore the rough idle. Starting from the RPM needle, to the point where the car now has a very rough idle, I would like to think that this is consistent with a gradually degrading and failing crankshaft position sensor.

I would just like some opinions before my garage visit tomorrow, and whether you guys think that simply replacing the crankshaft/camshaft position sensor (based on the error code obviously), would fix my recent issues with the rough idle and RPM needle? Given the symptoms, I don't believe the issue is with the IAC as it wouldn't cause the RPM needle issue AFAIK. Is it possible for the actual crankshaft or camshaft to be faulty rather than just a sensor?

Any opinions would be appreciated