A problem with the probe sensors in the coolant tank IS a sensitivity to the coolant fluid's electrical resistance (Ohms) and if not just right, your car will throw P2181 codes. As has been stated, don't use an excess of antifreeze (as suggested, 60% water and 40% antifreeze is a good mix). Also don't use "purified" water, use only the stuff that comes out of the water hose and hopefully there's enough mineral content in your water to maintain an operating fluid resistance level.
As suggested, check your coolant mix ... you might have too much coolant ... my 2003 Audi TT had too much antifreeze at 35K miles. If someone added "purified" water to your coolant mix, then you're really screwed.
If your handy with a digital voltmeter, measure the before and after resistance of the coolant tank solution at the tanks connector ... with the cable connector removed. Hopefully note a lower resistance.
And if your P2181 codes occur at specific street locations, that's because a nearby radio transmitting device is affecting the coolant tank sensing circuitry. Radio signals are introducing a signal voltage on the coolant probe cable going to the ECU. So the solution to the problem is (1) use more water with minerals, (2) use less antifreeze, (3) use a grounded, braided shield over the sensing wire.
Application of silicone insulating grease on the coolant tank's probe connector pins would help with this "dry" circuit. "Dry" circuit refers to a circuit with very little electrical current, causing connector contacts to become more easily contaminated. Therefore, not assuring a good, electrical contact for "dry" circuit conditions.
I have been plagued with this error on and off for the past 6 months and had the water pump changed (with timing belt), and the sensor. When I go back in time I did go to mr Lube and have them change the fluid. I remember they put in the green stuff and flushed out the high mileage red stuff. I told the dealer and they said that's fine. It was shortly after this fluid change I recall that the problem with my CEL and coolant gauge dropping to C, yet it still blew hot air even at -30C outside.
I was just about to pay and have the tstat changed but I think I'm better off to change the conductivity of the coolant. Do you know approx the value I should read off the sensor? Also what kind of tolerance in that value I might have?
First check ECT sensor resistance, if its within spec let car cold soak then take on test drive. Use your scan tool to monitor temperature and check to see if the car gets up to 175ish degrees within a few minutes. Average speed must be over 6mph to trip the light so keep it up over that. If car is to slow to reach operating temperature, 2181 will set. 99% of the time its been the thermostat.
Resistance should be between
Min Ohms Max Ohms Temp F
5000 6500 32
1500 2000 86
500 650 140
275 375 176
200 300 194
Trouble code 18613 (P2181) indicates the cooling system is not reaching operating temperature quickly enough.
1. With the engine at 68 degrees F. under normal driving conditions 175 degrees F or more should be indicated after about 4 to 5 minutes of driving.
2. If the engine is not reaching operating temperature quickly enough, a faulty thermostat is usually the cause of the slow warm up condition.
3. Check for poor or corroded connections at the Coolant Temperature Sensor (CTS) as well.
i have just replaced the sensor with the green one and when i bought the car i had to replace the radiator fan. and i also replaced the coolant res. not because i thought it would have been a problem but it broke.
i have a VF stage 2 super charger on the car i don't think that should have any problem. i find the problem only comes with cold starts or if you don't let it idle for at least 5 mins in the morning. i know its inconvenient.but you should do it every day any way.
I just wanted to update this thread to say that I was wrong!!
It wasn't the coolant. I tried swapping out the green and replacing with G12 and the Code came back. Also the sensor was replace but the code came back after a day.
So on the weekend I bit the bullet and replaced the thermostat. What a PITA job on my A3 but in the end everything is working fine now and temp sits perfectly in the middle of the gauge.
There are lots of tips I could give someone that wants to do this. So if any of you want to change this part be aware it's not fast or easy. If you don't like getting your hands dirty and don't have patients then just take it to the dealer haha.