Hi, welcome to Justanswer.com my name is XXXXX XXXXX X will be answering your question.
The most likely cause of this is a stuck open thermostat replace the thermostat. If you had replaced the temp sending unit which is different from the temp sensor and the thermostat is okay then it is a problem with the wiring and guage. Most of the time it is the thermostat.
I hope my assistance helped you and resolved your concern, if you need further assistance please ask.
Please note, your Deposit doesn't constitute a payment. Please click the 'ACCEPT' button to allow me to receive credit for my work. Positive feedback and Bonuses are appreciated. Thank you, Shawn.