Citroen C4 2012 transmission fully slips on 2nd and 3rd gear

[grave frigate]

Hi,

It’s the AL4 gearbox on a 2012 Citroën C4 1.6 VTi 120. At least 2nd and 3rd gears won’t engage: 1st and reverse work normally, but when it shifts to 2nd/3rd (or I select them manually) the gearbox slips/free-revs and the car won’t move regardless of throttle. The fault started suddenly.

DTCs: P072F (controlled converter slip) and P0721 (vehicle speed data consistency). My assumption: P072F is a result of the slip; P0721 occurs because vehicle speed stays ~0 while 2nd/3rd is commanded and engine speed is high. The speed sensor seems OK otherwise (live data in 1st shows correct speed).

I compared “guide pressure” vs “gearbox oil pressure”. In the P0721 freeze frame, guide pressure is 21 bar but oil pressure only 11.5 bar. In P/N/R/1st, guide and oil pressures track each other, so the E1 circuit may be OK.

In D the box behaves oddly: it first tries 3rd (won’t engage), then 2nd (won’t engage), then keeps cycling and ends up “gear engaged: Refuge (Hydraulic 3rd)”. During this, guide pressure was ~13.3 bar while oil pressure was ~4.2 bar, but both fluctuate due to the cycling.

As far as I know the solenoids have never been replaced. Can failed solenoids cause a complete loss of 2nd/3rd engagement? (I can’t test 4th because manual mode won’t go that far.)

Fluid history: I did a partial oil change ~6 months ago; likely the first ever (no flush, so old fluid remains in the torque converter). Before the change it occasionally went limp with “pressure regulation fault”. After the change limp mode stopped and that code hasn’t returned. Current fluid looks clean, no burnt smell, no visible metal.

Any ideas where to focus next (solenoids vs valve body vs internal clutch/pressure leak)?

[shy escarp]

I don’t see any activity on this, and I have no documentation for this car. I will observe that leaky solenoids can result in a symptom of the car not going into the commanded gear; but usually the failure is that they will first take a long time to get into that gear. Since this happened suddenly and with two gears, I’d imagine that leaky or compromised solenoids are likely not the case. I’d also hypothesize that if this were an internal pressure leak, it would have issues with all the gears (I think). All of that said, you’re likely talking about pulling the valve body to deal with whatever this problem is, so unfortunately I think the answer to your question is, likely can’t diagnose it any further without disassembly.