OK, so that makes things easier to diagnose as you won't have the fan delay device.
The way it works is as follows.
Ignoring the aircon for a minute, the radiator fans are activated by the engine ECU detecting the water temperature by way of the sensor near the thermostat. At the first level, one relay ( by the fusebox ) is enabled and the permanent live to the fan(s) is grounded via the resistor near the fan. If the temperature continues to rise, the ECU activates the second relay and now the fan is grounded straight to earth without the resistor so achieving the high speed.
As you say in the first post, you need to ascertain that the resistor is still working. As you have two fans, you need to check BOTH resistors and both need to be working properly. The fans are wired in parallel. So the supply splits and goes to both fans. The other side of both fans go to both resistors. So if one resistor fails and stops conducting, both fans will try and ground via the one working resistor. Guess what happens then
If you have a multimeter, check the resistance of each resistor. It should be 0.18 Ohms ( +/- 10% ). Alternatively, if you have FiatECUScan ( has to be the paid for version ), you can use that to activate the fans at the two speeds to check everything is working.
If the fans pass the test OK, you can then ignore the engine cooling side of it.
So, even if the engine temperature is OK, the fans may still need to operate to cool the aircon gas in the condenser ( the radiator in front of the the engine radiator ).
To do this, there is a pressure switch. Actually, there are two pressure switches. One of them can be ignored as it won't allow the aircon to be on if the pressure is too low. As your aircon is going on, this is presumably working OK.
The other pressure switch has two functions. One part should switch off the aircon if the pressure in the system exceeds 28 to 30 bar. That 28-30bar is measured on the high pressure circuit. If the guy measured 2500psi! ( 172bar ), that indicates something very wrong. Either that maximum pressure switch isn't working, or the aircon engineer made a huge mistake. That needs to be resolved.
The second function of the pressure switch is to control the fans. When the pressure in the system rises to 15bar, a signal is sent to the car's ECU and the ECU then switches on the fans at the first speed ( through the fan resistors ). If the pressure continues to rise, at 20bar, the ECU switches on the fans at high speed. When the pressure falls, the high speed fan goes off when the pressure falls to 16bar and the low speed fan goes off at 11bar.
So, if the fans and resistors check out, you need to look at this pressure switch. Both from the point of view of the maximum pressure cut out apparently not working and also as a possible cause for the fans not cutting in.
The pressure switch ( AR part number 60625482 ) was used on a number of cars e.g. Fiat Bravo/Brava ( c. 1998-2001 ), 156, 147 etc and phase 2/3 GTV/Spiders. You can probably get the part new at a fraction of the cost of going to an AR dealer by getting the Valeo part ( part number 509476 ). Don't use the switch from the early phase with the M2.10.3 ECU. It is different.