You've all probably heard of or experienced the dreaded dashboard display showing a big F. We've seen it loads of times and normally it is something to do with the transmission. But not always, as we found out today.

Let me explain the symptoms as to what was happening;

RRS 2.7 (MY08) came in for a turbo replacement due to boost problems and RP putting the car in to limp mode. Changed the turbo and all was good again. Nice 25m test drive and car parked up. The customer comes to collect the car a week later on the wettest day in ages. The drive off and 30mins later say that the car has lost all power and the dash is saying transmission fault with the big F displayed. Our first thoughts were water ingress in the usual places (it was absolutely bucketing down). I managed to drive the car back to the workshop, just. It was gutless and hardly had enough power to pull away, like trying to pull away in the wrong gear and won't rev up. Sure enough, IID showed the gearbox in 3rd gear at idle.

TCM out and checked to find it was dry. Checked all the plugs and connectors, all fine. Problem still persists.

Time to dig out the big gun and plug in the Autologic to see what was going on.

Usual batch of DTCs and a few unusual ones. Engine was showing engine torque out of range error and fuel rail pressure to high. Hmmm...wasn't showing those codes when we completed the turbo job and besides, this is the engine, not the gearbox.

Back to the gearbox and cleared the fault codes. Start the car and no dash warning for 5 seconds, gearbox shows as being in first gear as you would expect, then BING and it's back. Now showing as being in 3rd gear again and the engine won't rev up past 2500rpm.

Getting a bit fed up here thinking I'm going to have to take the box out and strip to see what is going on. So to cheer myself up I'll have a look at these engine codes again. Autologic on and showing 350 bar pressure on the fuel rail at idle. Well I know can't be right for the code showing it is over pressure. Change the pressure sensor and noting has changed. Plug external pressure tester to the rail and shows the same low pressure.

Clear the codes again and this time start the car and hold the revs high before the F pops up on the dash. 3,4,5,6,7,8,9 to 20 seconds and still no F. Drop down to idle and BING up comes the F within 3 seconds! High pressure and engine torque errors again.

OK, lets try this idea. Ignition on, clears all the codes, start the engine and hold the revs up at 2500prm. No F displayed, gearbox in 1st gear (selector in natural) and no DTCs. Rail pressure is now roughly 600 bar which is a low idle pressure. I'm starting to suspect a faulty fuel regulator on the HP pump, but what has this to do with the transmission fault?

All set to change the HP pump to one off the shelf. This is not a 10min job and I sat there putting all the pieces together before cracking on with this job. I'll just check the pressure from the lift pump in the tank first as that is a 10min job. So I crack the fuel pipe from the tank to the filter and plug a pressure gauge on it. OK, so that's all good pressure there then. Up to the engine bay now and start the pump swap now. Out of curiosity I decide to check the pressure on the feed pipe to the pump. Low!

Time is marching on now and I'm supposed to be in a meeting. So I get one of the lads to change the fuel filter. When I get back, I'm told the F on the dash is still there as are the faults. Well I wasn't expecting anything different as I thought I was sorting out the fuel pressure problem, not the transmission fault.

So I'm sat in the car, start it up and hold the revs high. Autologic is now showing over 1200 bar rail pressure. Let the revs drop to idle and pressure is around 600 bar. So all good here now. Hang on, it's been sat at idle for over a minute and no transmission fault message or F on the dash. Quick check and gearbox showing as being in 1st gear.

Now I'm confused. So I put the old fuel filter back on and guess what; transmission faults again. Swap back to the new filter and off they go.

A call in to Autologic Tech and they confirm that a poor delivery on the LP side of the fuel system to the HP pump will default to limp mode. Well I already knew that, what i didn't know was that engine torque out of range error would result in the transmission going to limp mode by way of selecting 3rd gear.

So the lesson here is that DTCs can't be trusted (which I already knew) and that big fat F on the dash may not being a fault with your transmission at all. Joe

Interesting, thanks for sharing  2008 TDV8 Buckingham Blue- SOLD

Useful to know. A fuel filter change is easier than messing about with the gearbox. 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

Interesting indeed. Thanks for posting Joe  Kev

MY2014 L405 RR Vogue SE 4.4 SDV8 in Corris Grey
MY2010 L320 RRS HSE 3.0 TDV6 in Stornoway Grey

Yeah, nice write up, Joe! Also nice to know that there are Indies out there who practice the lost art of Troubleshooting and not just "replace what the computer says"  2014 BMW 530d Touring, 2006 BMW 650i, 2018 Mini Cooper S

Thanks for sharing that, As others have said, love the way you have worked through this problem. I'm busy looking at how far you are away from Gloucester!

50mins up the M5 if you're interested. Joe

....and a dodgy brake light switch will also cause this. No DTCs on brake switch but a whole load of other junk codes.