If the gauge goes up, I turn on heater to full. If that takes care of it, I leave the heater on full. If the gauge continues to go up with the heater on, I turn the fan on full. That will ALWAYS then bring the gauge down to its normal level within 30-60 seconds. When it’s very cold, I don’t then mess with the heater or fan – just keep them both on. If it’s 40ish, I will keep checking by turning of the heater or fan … the gauge fluctuates about 1/4".
I’m too scared continue to let the gauge climb. When the faulty thermostat was installed, it would almost get to the orange area – but that was before I figured out the heat/blower stuff. It’s normal range is about 1/8" below the marker next to the icon – when it’s rising, it will go about 1/4" above that – I don’t let it go above that.