Thermostat

Replace the thermostat on a 2016-2021 Honda Civic 1.5L turbo engine to restore proper coolant temperature regulation.

Warnings

Tools required

Parts

• Thermostat assembly × 1 — Use OEM Honda thermostat
• Thermostat housing O-ring × 1 — Use OEM specification
• Thread sealant (for temperature sensor) × 1 — Honda thread sealant or equivalent

Fluids

• Honda Type 2 Coolant (Blue) — 2 qt

Preparation

1. Ensure engine is completely cold (2+ hours since last operation)
2. Park vehicle on level surface and engage parking brake
3. Remove engine cover by pulling upward on front clips
4. Place drain pan under radiator drain plug location
5. Locate thermostat housing on driver's side of engine block, below intake manifold and near turbocharger

Procedure

1. 1

   Drain coolant from system
   Remove the radiator cap slowly to release any residual pressure. Locate the radiator drain plug at the bottom of the radiator on the driver's side. Open the drain plug and drain approximately 2 quarts of coolant into the drain pan. This is enough to lower the coolant level below the thermostat housing.
2. 2

   Disconnect battery negative terminal
   Disconnect the negative battery terminal to prevent electrical issues when working near sensors. Use a 10mm socket to loosen the terminal clamp and move the cable away from the battery post.
3. 3

   Remove air intake components
   Loosen the clamp on the turbo inlet pipe and disconnect it from the turbocharger. Remove the air intake tube assembly to provide clear access to the thermostat housing area. Set aside components in a clean area.
4. 4

   Disconnect coolant temperature sensor
   Locate the coolant temperature sensor on the thermostat housing. Press the locking tab and disconnect the electrical connector. Move the wiring harness aside to prevent damage during thermostat removal.
   Torque spec

   Coolant Temperature Sensor20 Nm (15 lb-ft)
5. 5

   Remove coolant hoses from housing
   Use pliers to compress and slide the spring-type hose clamps away from the thermostat housing. Carefully twist and pull the upper radiator hose and bypass hose from the thermostat housing. Be prepared for residual coolant spillage.
6. 6

   Remove thermostat housing bolts
   Remove the two 10mm bolts securing the thermostat housing to the engine block. Work carefully as the housing is plastic. Keep track of bolt locations as they thread directly into the aluminum engine block.
   Torque spec

   Thermostat Housing Bolts15 Nm (11 lb-ft)
7. 7

   Remove old thermostat and housing
   Carefully pull the thermostat housing away from the engine block. Remove the old thermostat from the housing or engine block (depending on design). Note the orientation of the thermostat with the spring element facing toward the engine block. Remove and discard the old O-ring seal from the housing.
8. 8

   Clean mating surfaces
   Use a plastic scraper or wire brush to thoroughly clean the mating surface on the engine block where the thermostat housing seals. Remove all old gasket material, corrosion, and debris. Wipe clean with shop towels. Ensure the surface is completely clean and dry for proper seal.
9. 9

   Install new thermostat and O-ring
   Install the new O-ring onto the thermostat housing, ensuring it seats properly in the groove. Install the new thermostat into the housing or engine block (depending on design) with the spring element facing toward the engine block and the air bleed valve at the highest point. Verify correct orientation before proceeding.
10. 10

    Install thermostat housing
    Position the thermostat housing onto the engine block, ensuring the O-ring remains properly seated. Thread in both housing bolts by hand to prevent cross-threading. Torque the bolts in a crisscross pattern to the specified value of 14.0 Nm (10 lb-ft). Do not overtighten the plastic housing.
    ⚠Critical torque specification. Overtightening will crack the plastic housing. Undertightening will cause coolant leaks.
    Torque spec

    Thermostat Housing Bolts15 Nm (11 lb-ft)
11. 11

    Reinstall coolant temperature sensor
    Apply a small amount of thread sealant to the coolant temperature sensor threads. Thread the sensor into the thermostat housing by hand, then torque to 20.0 Nm (15 lb-ft) using a torque wrench. Reconnect the electrical connector until it clicks into place.
    Torque spec

    Coolant Temperature Sensor20 Nm (15 lb-ft)
12. 12

    Reconnect coolant hoses
    Reconnect the upper radiator hose and bypass hose to the thermostat housing. Ensure hoses are fully seated onto the housing nipples. Slide the spring clamps back into position over the hose connection points. Verify clamps are properly positioned.
13. 13

    Refill and bleed cooling system
    Close the radiator drain plug. Locate the coolant bleed screw on top of the thermostat housing or nearby coolant passage. Open the bleed screw to 7.0 Nm (5 lb-ft) loose position. Slowly fill the radiator with Honda Type 2 coolant until coolant flows from the bleed screw without air bubbles. Close the bleed screw to 7.0 Nm (5 lb-ft). Continue filling to the radiator neck. Install the radiator cap.
14. 14

    Complete reassembly and initial run
    Reinstall the air intake tube and connect to the turbocharger inlet. Tighten the clamp securely. Reconnect the negative battery terminal. Start the engine and allow it to reach operating temperature while monitoring for leaks around the thermostat housing. The cooling fans should cycle on when the thermostat opens (approximately 180-190°F).

Reassembly

1. After initial warm-up, turn off engine and allow to cool for 30 minutes
2. Check coolant level in radiator and reservoir when cool, top off as needed
3. Inspect all connections at thermostat housing for any signs of coolant leakage
4. Reinstall engine cover by pressing down firmly on mounting clips until seated

Verification

• Start engine and verify it reaches normal operating temperature (middle of gauge) within 5-10 minutes
• Confirm cooling fans activate when engine reaches operating temperature
• Check for any coolant leaks around thermostat housing, hoses, and bleed screw
• Verify heater produces hot air when engine is at operating temperature
• After test drive, recheck coolant level when engine is cool and top off if necessary
• Monitor temperature gauge during first few drives to ensure stable operation

Related trouble codes on this vehicle

Codes that commonly send this job to the bay — tap one for symptoms, causes, and diagnostic steps.

Chasing one of these codes and not sure which part is guilty? Ask a master mechanic about YOUR car →

More procedures for this vehicle