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Did you replace the water pump and t-stat for an overheating concern?
Did you do the work yourself? If so did you fill the engine with new coolant first through the t-stat housing cap? Do you know how hot the engine got?
You already have replaced the usual parts that cause those engines to overheat. It may just be an air pocket in the system from them not filling the engine block first. May be you can talk to the tech and ask if he filled the system through the cap on top of the t-stat housing then filled the expansion tank with the bleeder screw loosened. If the engine did get too hot it is always possible that it's a bad head gasket.
Let me know.
The t-stat cap is on top of the housing and is removed with a 10mm hex key. The bleeder is a small plastic screw on the bleed hose near the expansion tank. The t-stat housing is number 7 in the pic. The bleeder is on the hose in the second pic.
It's possible that the coolant temp sensor is bad but the gauge does that on those cars. If the temp was around 200 degrees and the fans came on then I would say that the sensor is working as it should. The gauge will go from the middle to the red in seconds. Did you ask how they filled the system?
Talk to you later.
Ok. Let me know if you need further assistance. Good luck.