VIEW LATEST
Written By: Gibson Hull
October 15, 2025
Corranta may earn a commission if you make a purchase through included links.
Engines like to run at a happy temperature, and engine manufacturers have become incredibly good at keeping engines reliably cool for typically the first 100,000 miles with no input from the owner (outside maybe a coolant flush). So much so that most of us ignore the oil temperature dial on the dash of most modern cars. The CTS gave codes P1258 (engine overheating) and P0117 (coolant temperature sensor) when scanned with my OBDII Scanner.
Overheating can be caused by a number of things, but fundamentally the liquid coolant was not carrying heat away from the engine and being recooled by the radiator. On a modern car like the CTS that could be as simple as the radiator fan not being plugged in or a bad sensor that needs replacing, to as complicated as a blocked coolant passage deep in the engine, or a faulty radiator fan relay. The complication on the CTS is that the thermostat (the likely culprit) and the coolant temperature sensor are located deep under the intake manifold at the middle top of the engine. Surgery time.
Fig. 01: Apprehensive James and the Cadillac CTS.
Step 02. Search 2017 Cadillac CTS P1258 and P0117. The internet and eventually some Cadillac bulletins showed that the common issue for the 2017 Cadillac CTS is a faulty engine thermostat, and the faulty thermostat leads to both codes. Engine thermostats use chemicals (like wax) inside a metal tube. When that chemical reaches a certain temp, it pushes open a valve letting engine coolant flow around the engine and keep the engine within it’s safe operating temperature range.
Step 04. Remove the plastic undertray/splash guard from under the engine. Same story here, use your plastic removal tools to remove clips from the undertray and work your way around until they are all off. Our friend Charlie got in the mix as well! We replaced the injectors on his BMW 135i a few weeks before this job on Jame’s Cadillac.
Step 05. Drain radiator. On the CTS, there is a nice yellow plastic knob you can turn (see in the picture) just above the drain port on the passenger side of the radiator which sits right in front of the engine. Open that yellow knob and radiator fluid will drain out. You only need to drain out enough to get the radiator fluid past the level of the top of the engine. If it has never been changed our flushed, now would be a good time to drain all of it, and replace it with fresh fluid while you are at it.
Step 06. Remove intake tubing. Simple to do, remove the band clamps from the throttle body on the front of the intake manifold and the air filter box and remove the intake tubing.
Step 07. Remove the intake manifold. There are bolts accessible from the top of the intake manifold that bolt it down to the engine. Unbolt these with a socket wrench and extension and remove from the engine. Be sure to stuff some shop rags down the intake ports to ensure that nothing falls into the engine. Trust me, you do not want to have to remove a head just to find the screw or nut you dropped into a cylinder or worse.
Step 03: Remove engine bay plastics.
Step 03 (cont.): Remove engine cover.
Step 04: Remove the plastic undertray/splashguard from under the engine.
Step 05: Drain radiator.
Step 06: Remove intake tubing.
Step 07: Remove the intake manifold bolts.
Step 09. Remove the engine thermostat. The Thermostat is held in with a few bolts easily removed with a socket wrench and extension.
Step 10. Remove the engine termperature sensor. Unplug the engine temperature sensor. The Engine Temperature sensor is difficult to get out without a super helpful wrench: the crowfoot. It is a box end wrench you can put on the end of an extension to get at a bolt you can't get to from the top with a socket.
Step 11. Inspect old vs new. James shows the difference between the shiny new thermostat and housing and the engine temperature sensor.
Step 12. Install the new parts. The new engine temperature sensor goes in, and then the new thermostat and housing. We were not sure the engine temperature sensor was even faulty, but given the difficulty in accessing and removing it, it best to go ahead and replace it as well. Use a torque wrench to properly torque the bolts to spec, and use the included gaskets as necessary.
Step 13. Reinstall the intake manifold, the intake piping, engine plastics and covers.
Step 08: Locate the engine thermostat and remove the radiator hose.
Step 09: Remove the engine thermostat.
Step 09 (cont.): Remove the engine thermostat.
Step 10: Remove the engine temperature sensor.
Step 11: Old vs. new thermostat and housing.
Step 11 (cont.): Old vs. new engine temperature sensor.
Step 15. Start your engine and check for leaks. Moment of truth, start your engine. Keep a close eye on leaks, anything you may have unplugged that needs to be plugged back in, anything out of place that may cause things not to work properly.
Step 16. Heat up engine. Wait for the engine to reach operating temperature to see if the new thermostat will open.
Step 17. Top off coolant. Add more coolant to top off the fluid that was pulled into the engine when the thermostat opened.
Step 18. Go for a test drive. Make sure to bring coolant with you! Go for a spin around the block. Get the engine hot and see if the thermostat will open properly to cool the engine off. Keep a close eye on the temperature gauge.
Step 19. Recycle Coolant and other fluids. Do not pour engine coolant down the drain! It is not good for the environment. Do not pour it in your yard or in the street. Same problem, not to mention it can kill pets or humans that may happen upon it and take a sip. It tastes sweet to animals. Your local municipal recycling center will take it free of charge, and you can also drop it off at most automotive chains like autozone or firestone for free.
Step 14: Refill the radiator fluid.
Step 15: Start your engine, and check for leaks.
This was James’s first time digging into an engine this deep, and with a bit of guidance he took to it like a fish to water. Suddenly engines are no big deal. This is what Corranta is all about. Grab some friends and some tools, go take something apart, learn how it works, put it back together, and burn some rubber.
Step 21. Subscribe Below! Be sure to subscribe below to get an email notice for each post or other updates and news so you don't miss out.
Step 20: Enjoy!