2000 Other Citroen Models Berlingo

BERLINGO Multispace 'X' Reg. 1868cc diesel

I'm having some problems with my Berlingo. I have been unable to find any technical explanation of how the engine control unit works or sight of any circuit diagram. I'm hoping that someone will be able to give me a clue where to look from the following evidence.

There are 3 modes:
1. Engine starts, very clattery, rev counter reads 0, temperature gauge reads 0, there are no warning lights illuminated

2. Engine starts, sounds OK, rev counter reads normally, temperature gauge reads beyond full scale, STOP indicator flashing, sometimes yellow 'engine'(?) Symbol illuminated.

3. All appears to be normal, quiet engine, all gauges reading normal, no warning lights

Vehicle is in mode 1 or 2 most of the time. Mode 3 is very rare.
Normally engine may start in either mode 1 or 2, can't predict which.

The performance seems to be the same in each mode.

It has occurred that it 'switches' from mode 2 to mode 1, mid journey. I have looked thoroughly for any loose joints, wiggled cables re-made connections, to no avail.