The Quick Answer above is the generic definition. What follows is the model-specific detail for the Honda models that log P0128.
DTC/P0128·Honda
P0128 on Honda: Causes, Symptoms and Fix Cost
Quick Answer
Low SeveritySafe to drive (short-term)Honda
P0128 on a Honda means the engine coolant stayed below the thermostat's regulating temperature for longer than the ECU allows, so the engine takes too long to warm up. The most common cause is a thermostat stuck open, which lets coolant circulate before the engine is warm. Expect lukewarm cabin heat, a low temperature gauge, and worse fuel economy. A replacement thermostat typically costs 80 to 300 euros parts and labour combined.
Typical repair cost, all makes€10 - €260
Connect any standard Bluetooth OBD2 adapter to your Honda and Skanyx tells you what P0128 means on that engine, whether it is safe to keep driving, and what the fix should cost. Free on iPhone and Android.
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Honda Civic
2006-2024Honda thermostats open at 76 to 80C depending on the model year, and the R18A unit is easy to reach. On 2016 and newer 1.5T models the thermostat housing is plastic and can crack during removal, so work carefully and always renew the O-ring seal. A common false lead is the ECT sensor, so verify with live data before parts swapping.
What causes P0128 on a Honda?
Beyond the generic causes listed on the main P0128 page, these are the patterns noted for Honda models:
Honda CivicThermostat degradation at 100,000 km and above
Honda CivicPlastic thermostat housing cracking on the 1.5T during service
How to diagnose P0128 on a Honda with OBD2
Follow these steps to pinpoint the root cause of P0128:
- Start the engine from cold and monitor ECT live data on a scan tool. Temperature should climb steadily to 80 to 95C within 5 to 10 minutes of driving
- If the temperature plateaus at 60 to 70C or drops while driving, the thermostat is stuck open
- Feel the upper radiator hose on a cold start. It should stay cool for the first few minutes. If it gets warm immediately, the thermostat is not closing
- If the thermostat behaves normally, compare the scanner ECT value to an infrared thermometer reading at the thermostat housing. A gap over 5 to 10C points to a faulty sensor
- Check coolant level in the reservoir when cold. If it is below minimum, top up and find the source of the loss before clearing the code
- Inspect the ECT connector and wiring for corrosion or a loose pin, and confirm the correct-spec thermostat was fitted if one was recently replaced
How much does P0128 cost to fix on a Honda? (EUR)
Estimated repair costs for P0128 in EUR, compiled across makes. Honda parts and labour usually land towards the lower end of each range.
Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.
Codes related to P0128
These codes are often seen alongside P0128 or point to related issues, on any make:
Looking for the full P0128 reference (all makes, full diagnosis flow, complete repair cost matrix)?
See the main P0128 guideSkanyx · Honda
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