The most difficult part of replacing a PT Cruiser thermostat is gaining access to its location. Workspace is at a premium and the upper intake manifold plenum must be removed. With this much work involved, it's a good idea to not only replace the thermostat, but service the thermostat housing, inspect the coolant sensor, and replace all the gaskets as well. You don't need any special tools to carry out this task successfully.
Jack the car up, place jack stands under the frame and lower the Cruiser onto the stands. From under the vehicle, open the drain valve and drain the coolant into a drain pan.
Remove the five bolts attaching the upper plenum to the lower manifold. Remove the one rear support bolt and the throttle body intake hose and gently tilt the plenum up and out of the way.
Remove the upper radiator hose from the radiator filler neck. Remove the two bolts that secure the filler neck to the thermostat housing. Remove the thermostat. To replace the thermostat, proceed to step 7. To perform additional cooling system service, proceed to step 4.
Unplug the coolant sensor that is installed in the thermostat housing. Remove the thermostat housing from the engine block by removing its three securing bolts. Clean the housing and remove the old gasket. Remove any gasket material from the engine block.
Remove the coolant sensor, clean it and check the electrical connections for corrosion. Replace it if there is any doubt of its serviceability. Inspect the filler neck for heat cracks. Replace it if needed.
Fit new gaskets to the thermostat housing and install it on the engine block. Install the thermostat, thermostat housing and upper radiator hose. Close the coolant drain valve and, while the radiator fill is easily accessible, add coolant before installing the plenum. Proceed to step 8.
Install the new thermostat into the housing. Install the radiator filler neck, and attach the upper radiator hose. Fill the radiator with new coolant while the filler is easily accessible.
Install the upper plenum. Start the Cruiser and let it run for five minutes. Check the coolant after the engine cools for 30 minutes and refill the radiator and overflow container as needed. This process insures there are no air pockets in the engine that could lead to overheating. Repeat this process before driving.