Quick answer: Thermostat Replacement on a 2017–2020 Hyundai Elantra is rated Moderate and takes about 1-1.5 hours. You'll need 13 tools for this job, and parts cost about $53. This guide walks you through the full job step by step, including the mistakes to avoid.
| Part Name |
Part Number |
Price |
Buy |
| Thermostat with Housing Assembly |
25500-2E085 |
$52.95 |
|
| Green phosphate-based EG coolant (Asian formula) |
— |
— |
|
About prices: shown prices are typical retail averages and change frequently; cheaper alternatives may be available. Confirm the current price before buying.
💡 Pro tip: Order all parts at once to save on shipping
Step-by-Step Instructions
Follow these steps carefully
1
Prepare (electronic stat, 212°F!)
OE: Hyundai 25500-2E085. You'll need: new electronic thermostat assembly (25500-2E085), green phosphate-based EG coolant, 10mm socket (7 undercover bolts), trim clip tool (12 clips), drain pan, torque wrench. The 2-pin electrical connector on the housing is the giveaway — it's electronic.
🔧 Mechanic's note — Tom Becker
I never buy the $20 conventional stat for this car — the 2.0L Nu needs the electronic 25500-2E085 assembly, and the cheap one will not even plug in. The 2-pin connector is the giveaway.
2
Remove Undercover, Drain Coolant
Let the engine cool. Remove the undercover: 7× 10mm bolts + 12 trim clips. Drain ~1/2 gallon of coolant via the radiator petcock. The 2.0L Nu's electronic thermostat housing has a 2-pin electrical connector — disconnect it.
3
Remove the Electronic Assembly
Disconnect the 2-pin connector fully and clean the mating surface. OE is 25500-2E085 (TSB 16-01-034-2). If you are here for P0128, this electronic unit is the fix; the older Elantra's conventional 192°F stat will not work.
4
Install New Electronic Stat
Install the new electronic thermostat assembly (25500-2E085). Reconnect the 2-pin electrical connector. Torque the housing bolts to spec (service data — the exact torque wasn't verified; snug + verify). The electronic stat must be connected — the ECM needs the heater circuit. A disconnected or faulty heater sets P0597/P0598. The 212°F opening is the ECM's target — don't install a conventional stat.
5
Bleed the cooling system
Bleed: run to operating temp with the cabin heat on max, let the 212°F stat open (upper hose gets hot), cool fully, and recheck the level. The higher 212°F target means the gauge may read slightly higher than the old Elantra — that is normal.
6
Verify, Check Codes, Recheck
Start the engine — check for leaks at the housing. Verify the 2-pin connector is seated (no P0597). If P0128 returns, the new unit may be faulty or there's an air pocket. Confirm the upper hose gets hot (212°F opening).
All Done!
Test your repair before closing everything up. If something doesn't work right, double-check your steps.
⚠️ Common Mistakes to Avoid
- • Not properly draining the coolant before starting the replacement process.
- • Failing to replace the thermostat gasket, leading to potential leaks.
- • Over-tightening the thermostat housing bolts, which can crack the housing.
