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Discussion Starter #1
Hi Everyone,
I have a 2012 Ram 1500 4X4 with the 5.7 hemi in it. The hemi was replaced 3000 miles ago. The motor was replaced with a remanufactured motor due to cylinder 3 cam lobe being destroyed by a failed lifter. This morning the truck did not come up to the normally operating temperature and a check engine light came on with a code of P0128 “thermostat rationality test”. I replaced the 203 degree failsafe thermostat with a new one and I’m having the same low temperature reading of 185-190 degrees as this morning. I live in the desert it’s 112 degrees outside right now. Normally this truck runs 210-215 operating temp. Also, this morning it run a little rough for 10-15 seconds after start up. Any Suggestions?
 

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Discussion Starter #3
Could be the temp sensor. I’ll put a new one in and see. Do you think it could be the fan clutch not disengaging? Thank you
 

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If you put in new thermostat, you would like to think its a good part. Sensor is my "best guess" also.
 

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Discussion Starter #5
I replaced the thermostat and coolant temperature sensor with OEM replacements and solved the problem Today. The failsafe thermostat had failed and was locked open preventing the motor to come up to normal operating temperature. I like the idea of a fail safe thermostat but having 2 fail within 4 weeks is not very reliable. I also found the locking clip of the electrical connector for the coolant temp sensor to be damaged and probably causing a loose connection.
Thank you everyone for your help.
 

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Hi Everyone,
I have a 2012 Ram 1500 4X4 with the 5.7 hemi in it. The hemi was replaced 3000 miles ago. The motor was replaced with a remanufactured motor due to cylinder 3 cam lobe being destroyed by a failed lifter. This morning the truck did not come up to the normally operating temperature and a check engine light came on with a code of P0128 “thermostat rationality test”. I replaced the 203 degree failsafe thermostat with a new one and I’m having the same low temperature reading of 185-190 degrees as this morning. I live in the desert it’s 112 degrees outside right now. Normally this truck runs 210-215 operating temp. Also, this morning it run a little rough for 10-15 seconds after start up. Any Suggestions?
When the thermostat was replaced , was the O Ring replaced? Many people have had this problem to find that the O Ring had not been installed
 
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