This procedure covers the removal and replacement of the transmission oil cooler on a 2016-2021 Honda Civic with the 1.5L turbo engine and CVT transmission.

Warnings

Tools required

Parts

• Transmission oil cooler assembly × 1 — Use OEM specification
• Transmission cooler line O-rings × 2 — Use OEM specification
• Transmission drain plug crush washer × 1 — 94109-20000 or equivalent

Fluids

• Honda DW-1 ATF — 8 qt

Preparation

1. Park vehicle on level ground and apply parking brake
2. Allow engine and transmission to cool completely (minimum 2 hours)
3. Raise front of vehicle and secure on jack stands at manufacturer-specified lift points
4. Remove front splash shield and undertray fasteners to access transmission
5. Have drain pan positioned under transmission drain plug

Procedure

1. 1

   Drain transmission fluid
   Locate the transmission drain plug on the bottom of the transmission case. Position drain pan underneath. Remove the drain plug using a 14mm socket and allow fluid to drain completely. This will take 5-10 minutes. Once drained, inspect the drain plug and crush washer.
2. 2

   Access transmission oil cooler
   The transmission oil cooler is mounted on the driver's side of the transmission case, near the differential housing. Remove any heat shields or protective covers that may obstruct access. You may need to remove the left front wheel well liner for better access depending on your approach.
3. 3

   Disconnect cooler lines from radiator
   Trace the cooler lines from the transmission oil cooler to where they connect to the main radiator's integrated ATF cooler section (typically on the driver's side of radiator). Place a catch container underneath. Use pliers to compress the quick-disconnect fittings and pull the lines free. Expect residual fluid to drain. Cap or plug the radiator ports to prevent contamination.
4. 4

   Disconnect cooler lines from transmission oil cooler
   Locate where the metal cooler lines attach to the transmission oil cooler assembly. Note the orientation and routing for reinstallation. Release the quick-disconnect fittings or unbolt the line connections depending on design. Have a rag ready to catch any remaining fluid.
5. 5

   Remove transmission oil cooler mounting bolts
   The transmission oil cooler is secured to the transmission case with mounting bolts. Identify all mounting points (typically 2-3 bolts). Remove these bolts and keep them organized. Support the cooler while removing the final bolt.
6. 6

   Remove transmission oil cooler
   Carefully pull the transmission oil cooler away from the transmission case. There may be O-rings or gaskets that seal the cooler to the transmission housing. Note their positions and condition. Remove and discard old O-rings.
7. 7

   Prepare new transmission oil cooler
   Clean the mounting surface on the transmission case thoroughly with a clean rag. Inspect for any damage or debris. Install new O-rings onto the new transmission oil cooler, lubricating them lightly with clean ATF to ensure proper seating and prevent pinching during installation.
8. 8

   Install new transmission oil cooler
   Position the new transmission oil cooler onto the transmission case, ensuring O-rings are properly seated. Align the mounting holes and hand-thread all mounting bolts before tightening any. Tighten mounting bolts evenly in a cross pattern to the appropriate torque specification depending on bolt size (either 26 Nm or 58 Nm based on bolt diameter).
   Torque spec

   Mounting Bolts58 Nm (43 lb-ft)
9. 9

   Reconnect cooler lines to transmission oil cooler
   Connect the cooler lines to the new transmission oil cooler assembly. Ensure quick-disconnect fittings click into place securely, or torque threaded fittings appropriately. Verify lines are not kinked or stressed.
10. 10

    Reconnect cooler lines to radiator
    Remove caps or plugs from the radiator ATF cooler ports. Connect the cooler lines back to the radiator, ensuring proper routing matches original configuration. Verify quick-disconnect fittings are fully seated and locked.
11. 11

    Install transmission drain plug
    Clean the drain plug threads and sealing surface. Install a new crush washer onto the drain plug. Thread the drain plug into the transmission case by hand, then torque to specification.
    Torque spec

    Transmission Drain Plug30 Nm (22 lb-ft)
12. 12

    Refill transmission with ATF
    Locate the transmission fill plug (located on the side of transmission case, requires removal to fill). Remove the fill plug. Using a fluid pump or funnel with long hose, add approximately 3.5-4.0 quarts of Honda DW-1 ATF through the fill hole. This is initial fill only; final level will be verified while running.
13. 13

    Initial fluid level check
    With vehicle still raised, start the engine and allow it to idle. Shift through all gear positions (P-R-N-D-L), pausing 2-3 seconds in each. Return to Park. Check fluid level at the fill plug opening with engine running. Fluid should be just at the bottom of the fill hole. Add additional fluid in small increments as needed.

Reassembly

1. Reinstall transmission undertray and splash shields using existing fasteners
2. Reinstall wheel well liner if removed
3. Lower vehicle from jack stands
4. Perform final transmission fluid level verification per Honda procedure (fluid temperature must be 95-113°F for accurate check)

Verification

• Start engine and verify no ATF leaks from cooler mounting points or line connections
• Allow engine to reach normal operating temperature and verify no leaks at cooler lines
• Test drive vehicle for 10-15 minutes, including highway speeds if possible
• Recheck transmission fluid level following Honda's warm fluid checking procedure
• Verify smooth transmission operation with no slipping, harsh shifts, or unusual noises
• Inspect underneath vehicle one final time for any signs of ATF leakage

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