Help Needed – “Start Lock Fault” (diagnostic) and “Gearbox Fault” (dashboard) on Citroën C4 Picasso 2016

[Marwan Hashem]

Hello everyone,

I own a Citroën C4 Picasso (2016, 1.6thp) and I’m currently facing a strange intermittent electrical issue that I hope someone here might recognize.

A few weeks ago, the car had a serious fuel issue — the injectors were delivering too much fuel into the engine, flooding one of the cylinders. The ECU was initially replaced, but the real cause turned out to be faulty injectors. After repairing them, the engine ran perfectly again.

However, since reinstalling the original ECU, a new issue appeared:

The car sometimes starts normally, but other times it refuses to start.

When it refuses to start, the dashboard only shows “Gearbox Fault.”

When scanned with a diagnostic tool, it also shows “Start Lock Fault.”

When the engine does start, everything works perfectly — smooth gear shifts, no performance issues, and no warning lights.

To troubleshoot, we have already done the following:

Replaced every major electrical component related to the drivetrain (including TCU, transmission wiring, and BPG).

Checked and cleaned the fuse box.

Tested the battery (voltage and stability under load).

Tried a spare key to eliminate immobilizer issues.

Thoroughly checked the engine wiring harness for continuity, corrosion, and loose pins.

Despite all this, the issue persists — the car randomly refuses to start, showing “Gearbox Fault” on the dashboard and “Start Lock Fault” in diagnostics.

Has anyone experienced a similar issue or found a fix related to BSI–ECU synchronization or CAN communication between modules? Any guidance or ideas would be highly appreciated.

Thank you,
Marwan Hashem

[mickthemaverick]

Which gearbox have you got?

[Marwan Hashem]

the automatic aisin 6 speed gearbox

[mickthemaverick]

So you may be having trouble with the gearbox sensor which confirms to the ECU that the gearstick is in Park or Neutral. If it does not get that confirmation it will not allow starting. I have that problem on my Subaru but usually when there is no start I move the selector up and down and back into park and it starts then.

[Marwan Hashem]

i mean when i'm in ignition and it doesn't wanna start, it appears on the dash which gear im on, the gearbox fault only appears when starting the car and driving it but it drives normally. also checked if brake lights are working when starting and its functioning correctly.