have you gone into the diagnositic section of the monitor and looked at the voltage reading? then compare it to a functioning one if possible or call your dealer and ask what correct reading should be

 

I don’t like the temp gun. I have never been able to read the same as a temp probe in water as shooting metal on the outside of the engine with a fan blowing at you. 20-30 series combines have had issues with thermostats. 9120 and 9230s may not be an issue with 7120s. Check your engine fan also . If you can turn your fan either direction and slip the belt then belts are loose or worn

 

A infrared temp gun will give slightly different readings from different colored surfaces due to the way the paint color radiates heat.... but it's quite minimal.

 

Be certain screens are cleaning, radiator is clean. If you have washed it out when dirty it can be very hard to clean after that. I have had to remove radiators and coolers to pressure wash or send in for a good cleaning after a person washed with a garden hose. Turns the dust into hard sticky chunks that get very hard to remove. Use a light and look through it best you can. Usually one person shining light through one side and look through the other if possible. Make sure the fan is spinning and belt not slipping. Make sure fans fins are in good condition. Make sure radiator is sealed all around and not pulling around the coolers instead of through them. Make sure thermostat is working properly. May just change it out. Maybe do a good system flush. Systems do not often cake like in the past anymore. If it had gotten oil or fuel in the system it may cause a slight cooling problem. If it had poor coolant added it can cake in the system insulating it from proper cooling.

A good check is temp differential from bottom of radiator to the top when system is hot. Use your temp gun to check it. I forget what a good temp difference should be. Engine Motor Cooling | Heat Load Exchange Temperature This may help to give a basic idea. FYI, to much temp difference is also not good. It can mean low coolant flow or other problems.