Please help, Changed Water Thermostat now getting Engine Management Warning and Fan On

I recently changed the water thermostat on our B6 1.9TDi to cure low temperature problem. This seemed to cure the problem but noticed the coolant level had dropped from Max to just below Min in the space of a week. Topped up the system yesterday, and tightened up the two bolts on the flange of the plastic housing which attaches to the engine. The thinking being that the thermostat seal wasn't quite water-tight. This seems to have cured the leak however this morning a short run of 8 miles the temp gauge is stuck on 50 and as I arrived back home the engine management light came on and the fan was going. So it's like the thermostat isn't even operating (closing) at all now. I can't see how nipping up the mounting bolts has caused this. Obviously a massive concern, any ideas?

Low reading might meant the CTS is goosed

turn the heater onto full blast full heat. it should get scorchy hot. does it, or do you get just warmish air.

it could well be the new thermostat is faulty

as suggested, let engine tick over for 10 minutes and put heating to the hottest it can go, air con off & put the heater fan on full blast for 10 minutes. this gets rids of any air locks in system.

Also feel top radiator hose to ensure hot water is going into the radiator !

Thanks, I will check that as soon as I can. The thermostat was working perfectly until yesterday when I tightened the bolts slightly, and the temp reading was 85 degrees which I believe is normal. Seems a coincidence. Also the sensor was replaced in the last year too (but was probably unnecessary).

Hi chaps, just braved the miserable weather outside. I ran the engine for about 10 mins, heater on full and warm air was blowing through - not what I would call hot but maybe needed to run longer as was starting from cold. However I did feel the hoses - radiator top hose felt warm, hose to bottom of coolant reservoir was warm and I could feel warmth in the rubber hose which connects up towards the thermostat housing. Warm but not hot but then should they be hot within 10 mins?

The Engine Management light is still on. What do you think?

Also, is there a socket where I can plug an ODB2 code reader in?

The stat came from????? obd socket under the steering above the accelerator.

I don't think the 1.9 gets hot quickly..... more so in this weather.Seems like the stat is stuck open.EML could be the CTS but needs scanning.Was it a genuine one?

You need to put the new thermostat in a pan of slowly boiling water until it opens, then check water temperature if you can

Or if you have the old thermostat, boil the water with new and old thermostat to see if they open at the same time.

Does anyone know if there is more than one thermostat in this car ?

Sometimes there is separate thermostats for the turbo intercooler and for the heater matrix.

85 degrees is too low for this car, 90 is what it's supposed to run at. You have to buy genuine thermostats and coolant temperature sensors, all the cheap/copy/ebay stuff is a load of old pony. Tighten torque is about 10 nm IIRC that's moderately tight with a screwdriver.