yes, i would definately replace those plugs. they fire with a screwdriver, but maybe not so good under compression. for the love of GOD, fix that coolant leak and get some antifreeze/coolant in there lmao.
The ecu only uses the temp sensor to determine how much fuel the car needs. ie.... when the car is cold it needs to give more fuel, thats why you get a high idle at startup. There is a different sensor next to that for your guage.When the car reaches operating temperature it reads the oxygen sensor to see the difference from the air inside the exhaust and outside the exhaust so it knows whether to enrich or lean. That is called closed loop operation. You will actually know when it is checking when your thermostat opens, the cool coolant hits the sensor and signals to your computer to switch on the electric fan.
Your fault code is telling you that your coolant is low. it will not read when it is dry so it knows your out of coolant.