I just purchased a 98 Volvo V70awd with 2. 4T engine. Engine code P0116 and P0118 were coming up with check engine light. Replaced thermostat and topped up coolant, reset code. Now same code is back and the engine termperature gauge reads in the red. I am going to replace the coolant temp sensor but is there some trick to topping off the coolant to avoid air pockets? Why the high temp on the gauge.