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2003 Honda Civic EX P0128 Code: Diagnosis After Thermostat and Head Gasket Replacement

Model: 2003 Honda Civic EX Fault Code: P0128 Posted: 2016-10-21 13:13

I recently purchased a 2003 Honda Civic EX with 104,000 miles and have been experiencing persistent issues related to the P0128 diagnostic trouble code. The car has been running perfectly otherwise, but the engine is now showing signs of running cooler than normal, which negatively impacts fuel efficiency—currently around 26 mpg on highway drives. Shortly after purchase, I replaced the timing belt and noticed the check engine light came on with codes P0128 and P0420. My initial steps were to replace both the thermostat and the primary oxygen sensor. The previous owner had bypassed the thermostat entirely, so I installed a new Honda OEM thermostat—verified as correct for this model year. Subsequently, I began experiencing overheating symptoms, including coolant pressure buildup in the reservoir. After ruling out common causes, I concluded that the head gasket was leaking and replaced it along with installing a brand-new radiator. Despite these repairs, the P0128 code persists. The typical symptoms associated with this code include: - Low engine coolant level - Leaking or stuck-open thermostat - Faulty cooling fan operation (running excessively) - Malfunctioning engine coolant temperature (ECT) sensor - Intake air temperature (IAT) sensor failure After the head gasket and radiator replacements, I no longer observe low or leaking coolant. The ECT sensor was recently replaced this week, and now the temperature gauge reads that the engine is running cooler than normal—this is inconsistent with expected operating temperatures. My cooling fan operates correctly, and I do not believe the intake air temperature (IAT) sensor requires replacement. A coworker suggested that the new thermostat might be damaged or stuck open, so this weekend I plan to open the housing and visually inspect it. Given its age—less than four months—I find this unlikely. I’ve been advised to try replacing the secondary oxygen sensor, but I’m unclear how this would impact the cooling system, as P0128 is related to engine temperature regulation rather than emissions. What should I do next? Any insights or diagnostic steps would be greatly appreciated.

Related fault codes
P0128
Comments (4)
Anonymous 2016-10-21 13:31

You used the correct thermostat—was it not a lower-rated version? The PCM compares ECT and IAT during cold start; using a scan tool to monitor live data will show exactly what the engine control module is detecting. I’ve found aftermarket thermostats often fall short of OEM quality—even if labeled 'OEM.' Always use a scanner before replacing any component.

Anonymous 2016-10-21 13:35

I ordered the Honda OEM thermostat from Amazon to ensure compatibility and quality, as I've seen many aftermarket parts fail in older models like this one.

Anonymous 2016-10-21 13:53

Check live data from the OBD2 scanner—this is key for diagnosing P0128. We need to understand what the PCM is actually sensing, especially during cold starts. Is this a 1.7L engine? I’ll verify specifications once confirmed.

Anonymous 2016-10-21 14:04

The term 'OEM' can be misleading—what's labeled as 'OEM' may actually be a brand-name part, not the original manufacturer’s component. In older vehicles like this, many users have found that so-called OEM parts from third-party suppliers are often of lower quality, such as Chinese-made replacements, which don’t meet original specifications.