If the engine really is not heating up, the thermostat could be stuck open. Or you could be low on coolant and the gauge is not reading correctly.Or if the engine is warming up, the sensor for the gauge could be faulty.
injectors fire like spark plugs, u need to have a diode tester lite,unplug an injector plug the diode into injector plug, then turn engine over and that will let u know if they are firing