Problem is often that when you change gear, three heavy duty motors operate for a fraction of a second. This drops the battery voltage momentarily and upsets the engine ECU - and maybe then the gear change misses the very small time window. So everything has to be perfect - especially the battery.
If the problem continues then you may need to disconnect every plug and socket in the clutch and gear actuator circuits as the contacts can corrode inside.
These autos are fine - except that very occasionally in hot weather and heavy traffic with a lot of stop/start driving they will bring up a snowflake. The "cure" then is to wait up to 15 minutes and all will be well.
The paliative is to flick the gear into neutral at any long stop (like traffic lights) and the problem seems to go away.
Cause is possibly the gearbox ECU overheating (mounted on the side of the gearbox) under exceptional circumstances. With many thousands of these gearboxes on other Citroen and Peugeot cars, it is not unreasonable for one or two to give problems in abnormal conditions. Mine has never given any problem.
After removing the air silencer box and pipe to the air filter, Vivian Carelse has taken the ECU off the gearbox and moved it up to between the radiator and battery box. Wires are long enough without disconnecting it. It now needs a bracket to mount it on the battery box.
This can happen as a protective precaution if something overheats, eg in hot countries or in heavy traffic.
BEFORE you disconnect the battery you must let the systems go into stand-by or you will create more 'errors'. And after you reconnect the battery you need to allow the systems to initialize before opening the doors or doing anything further.
I accidentally simulated this snowflake situation one day after leaving the engine ticking over on the drive for a protracted period. I believe that the gearbox ECU is being heated by the gearbox - and the gearbox ECU senses overheating and trips out which suggests that the problem may be lack of airflow.
If the answer is airflow then if you have an undertray, take it off. But mine has an undertray and I have never had the snowflake out on the road so maybe adding an undertray has ducted the airflow - who knows?
In addition to flicking it into neutral at every prolonged stop, put the air conditioning ON. This should make the radiator fan come on at full speed and help blow air past the ECU when the car is stationary.
As far as I know, no-one has tried mounting the gearbox ECU in a cooler area.
When it happened to me, after three attempts I still had the snowflake and auto symbols flashing - though I could now engage 1st and reverse. These indications cleared after driving the car a very short distance (25 yards). For some reason this worked for me; possibly because the alternator cut in and raised the battery to 14.3 volts
Note: you may also find that the power asssisted steering has not enabled, which may take you by surprise first time. It comes back after a while, as do the other gears. It appears that there is a step by step recovery of normal running as each part of the system reports back to the BSI and it adjusts to the new system state.
Neil Gibson reminds us that what you might think as a transmission problem could be a symptom and not the cause.
Each electronic control unit feeds data via the BSI unit or body control module to the gearbox. A momentary glitch can have interesting results.
A recent problem with an ABS pump resulted in the sliding doors failing to open. Why should the ABS impact on the doors? The door control module won't allow operation above 3 mph. No speed reading. No door opening. The doors get blamed again!
The gearbox is the same. Virtually every power train sensor from throttle position to wheel speed can affect gear selection