I have a 2010 3500 6.7 and today the engine light came on. Then I noticed the engine temp was running very low. It was about 80 degrees outside and I was on the interstate with a speed of around 75 and the engine temp did not get over 155. Back roads on the way home speed around 50 the engine temp did not get above 140.

Any idea's?

 

Just a few...Either the temperature sending unit could be giving a wrong reading, or the gauge could be interpreting the signal it gets wrong...Either way you need to verify the actual temperature of the coolant with an independent thermometer...I always kept a cooking thermometer in my tool box for that purpose...I just stuck it in the fill neck of the radiator...An electronic pyrometer may be as accurate...If the two readings don't agree, it's time to check the sender with an ohmmeter, or the gauge with a variable resistor...There is also an outside chance that the thermostat could be stuck open not allowing the coolant to reach operating temperature...That's an easy check...With the engine cold, open the radiator cap, start the engine and if coolant starts flowing immediately, you need a new thermostat...I've only seen that on rare occasions though...that's as far as my limited thinking takes me......Ben