Tools you need for thermostat on a 2016-2021 Honda Civic

On the 10th-gen Civic the thermostat is not up on top where you can see it - it sits in a small housing on the lower front of the engine block, right where the lower radiator hose attaches. That means jacking the car, dropping the plastic splash shield and working by feel with a short extension. The bolts themselves are only two M6 flange bolts, so the mechanical part of the job is easy once you can reach it.

The part people get wrong is the refill. Air pockets in this cooling system are stubborn and will give you a lukewarm heater and a temp gauge that creeps, which people then blame on the new thermostat. Use a spill-free funnel, run the engine with the heater on full hot until the fans cycle, and recheck the level cold the next morning. Also use Honda Type 2 blue coolant, not a universal mix.

Complete tool and parts list

Differences within the 2016-2021 generation

Both the 2.0L K20C2 and the 1.5L turbo L15B7 use the same basic setup: a thermostat in a bolt-on housing at the lower radiator hose with two M6 bolts, usually 8 mm heads (some housings use 10 mm). The 1.5T is tighter to reach because of the turbo plumbing and charge pipe, so removing the driver-side front wheel and inner fender flap helps a lot. Si and Type R (K20C1) cars have a similar layout but more hoses in the way; budget extra time.

Torque specs

How the job goes

1. Park on level ground, let the engine cool completely, chock the rear wheels. Never open a hot cooling system.
2. Remove the radiator cap (or expansion tank cap) to let the system vent and drain faster.
3. Raise the front of the car and set it on jack stands. Removing the driver-side front wheel gives much better access on the 1.5T.
4. Remove the underbody splash shield: 10 mm bolts plus plastic push-pin clips. Set the hardware aside in order.
5. Place the drain pan under the radiator, open the plastic drain petcock at the bottom corner by hand and drain the coolant. Expect roughly 4-5 quarts from the radiator alone.
6. Locate the thermostat housing on the lower front/side of the engine block where the lower radiator hose connects.
7. Use hose clamp pliers to slide the spring clamp back, then twist and pull the lower radiator hose off the housing neck. Have the pan positioned; more coolant will come out.
8. Remove the two housing cover bolts (8 mm head, some units 10 mm). Support the cover as the last bolt comes out.
9. Pull the thermostat and its rubber O-ring/seal out of the block. Note the orientation - the jiggle valve/air bleed pin goes up.
10. Clean the mating surface on the block with a rag and cleaner. Do not gouge the aluminum and do not use a metal scraper.
11. Fit the new seal to the new thermostat, install it in the block with the jiggle valve at the 12 o'clock position, and refit the cover.
12. Start both bolts by hand to avoid cross-threading the aluminum, then torque to 9 ft-lb (12 Nm).
13. Reinstall the lower radiator hose and reposition the spring clamp fully onto the neck.
14. Close the radiator drain plug hand tight. Reinstall the splash shield and the wheel if removed, then lower the car.
15. Fit the spill-free funnel to the radiator neck and fill with Honda Type 2 premixed coolant.
16. Start the engine with the heater set to max heat and highest fan speed. Let it idle and watch the funnel; squeeze the upper hose occasionally to push air out. Keep adding coolant as the level drops.
17. When the thermostat opens you will see flow in the funnel and the upper hose gets hot. Let the cooling fans cycle on at least once, then shut off and let it cool enough to remove the funnel.
18. Install the radiator cap, fill the overflow/expansion tank to the MAX line, and clean up spills.
19. Drive the car, then recheck the coolant level cold the next day and top the reserve tank as needed. Confirm the temp gauge sits normal and the heater blows hot.

Watch out

The 8 mm/10 mm bolt heads, 10 mm splash shield bolts and the 12 Nm M6 torque are standard Honda practice for this generation, but confirm your specific engine's housing bolt head size and the exact thermostat part number against your VIN before starting.

Other jobs on the 2016-2021 Honda Civic

Thermostat Replacement on other vehicles