Citroen C4GP Auto 2016 Immobilised by Gearbox Fault Warning

[Bob100]

I would dearly appreciate help with this please.
Have had an ongoing year-long but intermittent problem with my 2016 C4 Grand Picasso Automatic and I'm trying to cover all elements here, so apologies for the length of this post. The problem involves a dashboard warning which shows 'Gearbox failure - have repaired' It occurs mostly when the gear selector stalk is moved from Reverse to Drive after reversing (from a driveway for example). When this occurs, no forward or reverse gears nor "Park" or "Neutral" can be selected as the system does not seem to detect or acknowledge such movement of the gear selector stalk. (Occasionally the warning message will alternate between this one and 'Anti-Rollback fault - have repaired' ) Turning off the engine in an attempt to clear the issue results in an immobile car as the warnings prevent restarting. Leaving the car for an hour or so usually (temporarily) resolves the issue ...until the next time!! The problem began a year ago and was initially thought to be linked to a significant battery leak. After this leak was discovered, a clean up was undertaken, new battery fitted and all was fine for about 6 weeks, when the problem reappeared. After further cleaning, all was fine for a further month, with some but few instances during the following few months. A parasitic drain appeared (unrelated??) with multiple fault warnings, which was resolved by an auto electrician who replaced the Battery Management Protection Unit and the Negative Battery Terminal Sensor. The gear selector stalk unit has also been replaced. The vehicle was fully re-coded and all the faults resolved until the following day when the 'Gearbox Failure' and it's associated issues explained above reappeared, although the parasitic drain seems fully rectified.
On the occasions when the above gearbox fault warnings do not appear, the car drives faultlessly.
Any assistance will be much appreciated, thank you.

[mickthemaverick]

My initial reaction on reading your post was possibly a dodgy selector switch or wiring to it. As you have had that replaced, presumably the wiring was checked and proved ok, in which case my next move would be to check the connectors on the gearbox and the main earth straps to engine and gear box. Often a bad earth is 'weather dependant' and hence can go high resistance if very dry and work better when the air is moist.

[Bob100]

Many Thanks mickthemaverick. I'm not sure the wiring to the selector was checked thoroughly so it could still be a good place to start . Interestingly, there does appear to be some kind of 'weather dependence' feature but oddly it seems to be the reverse of your observation in that this fault often manifests itself more when atmospheric air conditions are on the damp side. I mentioned all this to the auto-electrician previously and I suggested a thorough clean up of all connections/earthing etc in the area below the battery tray (since that's what the battery fluid would have leaked on to) He didn't seem convinced at the time but with your input I'm even more sold on the idea.

[RichardW]

You need to get it on a diag computer when it's playing up so that you can see what signals are being sent by the selector / steering wheel unit and received by the gearbox.

[Bob100]

Thanks RichardW. I don't have that kit myself but my autoelec guy does. Problem is that the car doesn't always manifest itself at the right time .. i.e. when he's 'on site' ... to get an accurate reading, as the issue can clear/reset itself before he arrives, so he's not seeing the problem in real time
It's a case of trying to synchronise the two but that's the nature of intermittent faults I guess.

[Bob100]

Brief update. Same problem materialised again and more out of frustration than anything else I 'jiggled' the selector stalk and then went slowly through the gears, which then were selected successfully. This is the first time 'jiggling' has worked! No further problems arose when I tested it out by driving forwards and in reverse several times. It could have been coincidence but logic says to me that the (replacement) selector is at fault so it will be replaced with another when it arrives but it still seems unlikely to me that both old and new would fail with the same issue ...unless anyone has any other ideas as to why this might be the case?

[Bob100]

Well friends, I had the earthing strap/connections cleaned up and a brand new selector fitted as planned and it was absolutely fabulous! Drove perfectly....for a month, almost to the day, then..you guessed, everything was back to square one! The "gearbox /parking brake/anti-rollback faulty" lights flashed and I sank to the depths of despair once more! On speaking to the auto-electrician, he maintained his theory that the clean up was just a red herring and it was because the battery was disconnected during the clean-up procedure the system was re-set and it was this, not the cleaning that led to the temporary respite. I'm sceptical about this as I maintain that the problems all started after a major leak of the previous battery which I believe has led to corrosion forming on various connections below It. Additionally, the current battery has been disconnected on numerous other occasions during this saga, with no long term improvement (for no more than a couple of starts anyway, if that). Each time a connector clean has been performed the car has worked fine for about a month, which seems to suggest that one or more of the wires/connectors are still being affected by contamination re-forming over time...but I do accept that this could also be a total coincidence.
He also suspects the possibility of a BCM issue but the intermittent nature of the fault makes this difficult to diagnose with certainty.
I asked if it was worth getting yet another new battery as the problems seem (to me anyway) materialise more when the battery has taken a bashing from short trips, heater on, wipers on etc. ....but that idea wasn't supported the auto-elec guy.
So I'm at my wits end, yet again, so any ideas will be very much appreciated. Thank you for your assistance.