Hi all,
Is there a way to test a temperature sensor on the 55 s without removing it? also needs to be done at the roadside, so just basic tools + voltmeter.
If so, how?

Which temperature sensor?

if you mean coolant temp sender, i cannot think of a way to do it unless you have an infra-red temp sensor.

But with the car cold you could whip out the sensor get a friend to stick their finger over the hole to stop coolant coming out and then just test it the normal way in a pan of water and a multimeter (you need to measure ohms so a simple voltmeter is not enough). Then after go release your friend and stick it back in - you may lose a few drops of coolant but you can just top it up.

With a ohm meter should be able to do it in place. If you can get at the wiring loom probably easiest disconnect at the ECU. You then need to work out which pins corresponds to the sensor and then measure, first with the engine cold. I think the resistance should be larger over 1K ohms. Then with the engine warmed up the resistance should be in the region of 100 ohms. Also if the sensor is bad showing low resistance when cold then the car will be difficult to start when cold. Disconnecting the sensor at the sensor side will allow the engine to start good if the sensor is the culprit. High resistance when hot will cause over fuelling condition. Also the sensor (if a two wire)may be physically damaged which may show correct resistance at the wires but one of the wires may be grounded. This will cause low resistance and hence the effect will be hard to start the engine when cold.

You do not need to remove it from the car to check operation.

Leave sensor in place. Remove wiring connector and hold pins of multimeter on two sensor terminals to measure resistance (presuming cts).

Either that or remove sensor completely but be prepared to leave a puddle of coolant behind you.

i really thought you'd need some way of measuring the actual coolant temp to test it properly. Yes you clearly can check the resistance changes as it should but could it not be the wrong resistance for the temperature?? or do they just not break that way?

On the -- rather large -- assumption that the OP means the engine coolant temperature sensor (and not the air temp sensor or the rad fan switch, etc.) then it should give 2400 - 2600 ohms at 20 degrees C.

yeah i get that - i just meant whenever i have tested it i have taken it out and used a pan of water and an actual thermometer to check the real temperature against what the resistance should be at 20* and 90* (i think from memory they are the 2 temps shown in the haynes against resistance).

Perhaps we should just wait for the OP to confirm which sensor he means

The figures do tend to vary a bit.

I think the OP has gone to sleep, though.

Thanks all, Yeah it's the coolant temperature sensor i want to test, in the engine block i think

no, its in the inlet mani, blue plug.

why need it checked?

staying in open loop and cold start fueling?
this being the ecu temp feedback and NOT the sender which u can usualy just touch the wire to earth ( no resistance) to see needle hit high temp rapidly

im new to fiat but found the free smartphone app " torque lite"comunicates on the iso protocol.
you need an elm327 interface though for this to work ( £15)

im referring to a mk2

just see what temp the acu sees and it should be in the ball park
e.g cooling fan kicking in around 95ish degrees and kicking out at 89 ish degrees

that should tell you if its accurate enough

This is a Mk1, not OBD2 compliant.

The car was over fueling when cold intermittently. Not yet found the problem but cleaning the wires and connectors in the engine bay appears to have solved the problem... Earths were all appeared all fine, no marks, no dirt. The coolant sensor was covered in dirt so it might have just been giving a poor signal maybe?