On the 9th-gen Accord the thermostat lives in a small housing where the lower radiator hose meets the block, not up top by the upper hose like many older cars. That means the job is done from underneath: drop the splash shield, drain the radiator, pull two 10 mm bolts, swap the thermostat and its O-ring, and put it back. Mechanically it is easy; the fiddly part is access around the lower hose, which is tighter on the V6.
The mistake that bites people is the refill. These cars trap air readily and the bleeder screw on the upper hose/housing has to be opened while filling, followed by a long idle with the heater on full hot until the fans cycle twice. Skip that and you get a cold heater, a wandering temp gauge, or an overheat on the first highway run. The other common error is overtightening the two cover bolts into aluminum threads, they only need about 9 ft-lb.
| Item | Type | |
|---|---|---|
| Floor jack 2 ton minimum Lift the front of the car to reach the drain and splash shield. | Required | shop |
| Jack stands 2 ton pair, under pinch weld jack points Support the car while you work underneath. | Safety | shop |
| Socket set 10 mm 6-point socket, 3/8 in drive Removes the under-engine splash shield bolts and the thermostat housing bolts. | Required | shop |
| Ratchet and extensions 3/8 in drive, 3 in and 6 in extensions plus swivel Reaches the housing bolts tucked behind the lower radiator hose. | Required | shop |
| Trim clip pliers Plastic push-pin removal tool Pops the splash shield push clips without breaking them. | Required | shop |
| Drain pan 2 gallon low-profile Catches roughly 1.5-2 gallons of old coolant. | Required | shop |
| Hose clamp pliers Spring (constant-tension) clamp pliers, cable type helpful Honda uses spring clamps on the lower radiator hose and bypass hoses. | Required | shop |
| Pliers, needle nose Handy for smaller bypass hose clamps and the bleeder area. | Recommended | shop |
| Flat screwdriver Small, and a #2 Phillips Opens the bleeder screw and pries the old thermostat/O-ring free. | Recommended | shop |
| Gasket scraper Plastic or brass scraper Cleans the housing sealing face without gouging aluminum. | Recommended | shop |
| Torque wrench 3/8 in drive, 5-80 ft-lb Thermostat cover bolts thread into aluminum and strip easily. | Required | shop |
| Spill-free funnel kit Universal radiator funnel with Honda adapter Makes refilling and burping air out far easier and cleaner. | Recommended | shop |
| Thermostat with O-ring Honda/OEM-equivalent, K24W or J35Y specific The replacement unit; O-ring is usually included and must be used. | Parts | shop |
| Coolant Honda Type 2 blue, premixed, 1-2 gallons Honda's silicate-free formula; do not substitute green universal coolant. | Consumables | shop |
| Distilled water 1 gallon For flushing/topping if you use concentrate. | Consumables | shop |
| Shop rags and parts cleaner Clean the sealing surface and wipe up spills. | Consumables | shop |
| Nitrile gloves and eye protection Coolant is toxic and hot coolant burns. | Safety | shop |
| Infrared thermometer or scan tool OBD2 with live ECT data Confirms the thermostat opens and the system is fully bled. | Recommended | shop |
Both engines use 10 mm housing bolts and 10 mm splash shield bolts, so the socket set is the same. On the 2.4 K24W the thermostat housing is on the transmission side of the block at the lower radiator hose and is reachable with a 6 inch extension. On the 3.5 J35Y it sits lower and further forward under the intake area, so plan on a swivel joint and possibly unclipping a coolant pipe bracket; the V6 also takes more coolant, buy 2 gallons of premix.
| Thermostat housing/cover bolts (6 mm) | about 9 ft-lb (12 Nm) - verify in the service manual |
| Radiator drain plug (plastic) | Hand tight only, do not use a tool |
| Under-engine splash shield bolts | about 7-9 ft-lb (10-12 Nm) |
| Cooling system bleeder screw | Snug by hand, roughly 2-4 ft-lb - verify in the service manual |
Bolt sizes (10 mm for the housing, splash shield and clips) are consistent across the 9th-gen Accord, but the thermostat housing location differs between the 2.4 and 3.5 and the exact cover bolt torque should be confirmed against the Honda service manual for your specific year.
It is in a housing at the lower radiator hose connection on the engine block, not at the upper hose. On the 2.4 it is on the transmission side of the block; on the 3.5 V6 it is low on the front of the engine, both reached from underneath after removing the splash shield.
Honda Type 2 blue coolant, ideally premixed. It is silicate and borate free; mixing in green or orange universal coolant risks water pump seal and gasket damage. The 2.4 takes roughly 1.5 gallons total, the V6 a bit more.
Open the bleeder screw on the upper hose/housing and fill the radiator until coolant runs out clean, then close it. Use a spill-free funnel, idle the engine with the heater on full hot through at least two fan cycles, then cool, top off the radiator and fill the reservoir to MAX.
No. Honda sells the thermostat with its O-ring separately and the aluminum housing is reusable unless it is corroded or the sealing face is pitted. Always use a new O-ring and never reuse the old one.