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2003 Ford Focus 2.3L Engine Code P0128 - Thermostat Replacement Failed, What's Next?

Model: 2003 Ford Focus Fault Code: P0128 Posted: 2017-07-10 13:23

I own a 2003 Ford Focus 2.3L and have been experiencing a persistent check engine light with code P0128. I purchased the vehicle one year ago and have already replaced the thermostat twice — once at an auto parts store and once at a Ford dealership. Despite these replacements, the P0128 code continues to appear. The issue occurs both in winter and summer conditions. I'm looking for help understanding what else could be causing this fault or how to properly diagnose it.

Related fault codes

P0128

Comments (4)

Anonymous 2017-07-10 15:42

Are there any other diagnostic trouble codes present? You should check the Intake Air Temperature (IAT), Engine Coolant Temperature (ECT), and Cylinder Head Temperature (CHT) sensors. A dedicated OBD2 scanner is required to monitor real-time sensor readings. The Thermostat Monitor relies on ECT or CHT sensor data to confirm that the engine warms up according to a predictable cycle when generating sufficient heat.

Anonymous 2017-07-10 15:47

Only P012-than code is present, and it appears consistently regardless of season — whether in winter or summer. This suggests the issue may not be related to ambient temperature alone.

Anonymous 2017-07-10 16:06

The Thermostat Monitor evaluates multiple sensors. If the engine isn't following its expected warm-up cycle, other sensors must be investigated. Start by comparing readings after a long soak — for example, overnight — when all sensors should show values within a few degrees of each other. A scanner is essential to capture accurate sensor data during this test.

Anonymous 2017-07-10 16:16

Thanks! Are there any specific components or procedures I can try before replacing more parts? Any recommendations for next steps?