p0128
Easy Fix
Honda · Sedans
Coolant Thermostat Below Regulating Temperature
Quick Answer
Error code P0128 on your Honda Sedans means: Coolant Thermostat Below Regulating Temperature. This is usually fixable — see the step-by-step guide below.
Affected Models
Applies to all models
CV3CV4FC1FC4FE1FL5ZC5Accord EX 2024Accord EX-L 2024Accord Hybrid EX-L 2024Accord Hybrid Sport 2024Accord Hybrid Touring 2024Accord LX 2024Accord Sport 2024Accord Touring 2024Civic EX 2024Civic Hybrid Sport 2025Civic Hybrid Sport Touring 2025Civic LX 2024Civic Si 2024Civic Sport 2024Civic Touring 2024Civic Type R 2024ZE4
Likely Causes
How to Fix It
1
Allow the engine to fully warm up and monitor coolant temperature with a scan tool; it should reach 190-210 degrees F
2
Check coolant level in the reservoir and radiator when cold; top off if low
3
Test the ECT sensor resistance at known temperatures; compare to Honda specification chart
4
Remove and inspect the thermostat; test in hot water to verify it opens at the correct temperature (typically 170-180 degrees F)
5
Replace the thermostat and gasket if it fails to open at the correct temperature
Pro Tip
This is a common issue that most users can resolve without specialized tools. Try the first step before moving on to more complex solutions.
Parts You May Need
Thermostat
Thermostat gasket or O-ring
Engine coolant
ECT sensor (if faulty)
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