I don't know, but if you are getting heat out of the heater you can
use that as an emergency radiator. Run it full blast with the windows
open, unless you want to simulate Australian weather.

the stoplight sign is if you got WATER IN YOUR DIESEL so drain the water
out of your diesel by undoing your drain plug on your filter
houseing.the fan will not run at 90c its got to go a little bit higher
than that..

The STOP lamp is illuminated for a number of reasons one is when you have no
cooling.

This site shows an explanation of how the dual fan system works for cooling
http://www.peugeotlogic.com/workshop/wshtml/electric/ac306/coolfan2/fanop1.h

I have a 1998 406 1.9TD with air con and had the same problem. This uses a
bitrol system to control the 2 fans. The system is switched on by the brown
thermostat of the three on the engine. This then switches  a changeover
relay and 2 other relays to either give slow speed both fans in series or
fast both fans in parallel. These relays are located under the badge on the
grill.

Sounds as though you have one of your cooling fans not working.
At about 80C they should come on in series both working at slow speed. Also
if air con
is switched on.
At 90 C they should both come on at high speed. These are controlled by 3
fan relays sited behind the rad grill under a cover, check here first for
dirty contacts or failed relay. The next possibility is a us
fan motor, faulty wiring or faulty earth.  There are two 40 an fuses in the
engine bay that feed the fans one for each.

Car temperature gauges are notoriously bad at providing the correct
temperature reading. You can only be sure of the gauge reading if you check
the water with a second temperature gauge.
The thermostat opens at 83 C.
The fans are controlled from an electrical system
Both fans operate in series at 96 C (slow)
Both fans operate in parallel at 118 C (Fast)
To check the fans do both work, with the engine switched on remove the brown
plug on the thermostat housing. After a few seconds both fans should come on
at high speed.
If your engine cools rapidly after reaching 90+ I would not worry.
Water in a radiator is pressurised to lower its boiling point. If you have a
leak in the cooling system it will affect the cooling system efficiency and
lower the boiling point of the coolant.

I forgot to say that the control unit switches the earth side of the relays
to operate no the battery positive as you may expect.
Mike

The stop light indicates a range of faults normally lighting at the same
time as another warning.  I've never seen an exhaustive list though.

Ours (petrol 306) seems to come on almost as soon as the engine warms up
with no other lights, and the temp indicator is sensible for the rest of
a 200 miles journey.  The fan runs on even after a short journey (e.g. 4
feet today!) even in cold weather, so presumably a dodgy temp
sensor/connection to temp sensor (I suspect the latter in our case).
Doesn't always mean a serious fault, anyway and often cooling related.

Chris