Temperature gauge.. is the engine getting hot? Are the pipes getting hot? Could be a bad thermostat or thermocouple to begin with.
There's one on the thermostat housing on the top which you found, there should be second one on the heater pipe but I couldn't find it myself when changing thermostat.
There's one on the thermostat housing on the top which you found, there should be second one on the heater pipe but I couldn't find it myself when changing thermostat.