Rick's Free Auto Repair Advice

Jeep P0128

Diagnose and fix Jeep P0128

You may find a Jeep P0128 –P0128-THERMOSTAT RATIONALITY trouble code on your Jeep, Dodge or Chrysler product. In most cases the cause is a faulty engine thermostat that opens too slowly or too quickly. To determine when to set the Jeep P0128 trouble code, the PCM tries to predict what engine coolant temperature should be based on the engine temperature and ambient temperature at cold start-up. Then the PCM compares the predicted temperature to the actual temperature from the engine coolant temperature sensor (ECT).

For example, with an ambient temperature between 17.6°F and 122°F and an engine coolant temp less than 122°F along with an average vehicle speed of more than 10-MPH, the coolant should reach 185° the PCM tries to determine if the coolant temp is above or below the predicted temperature before the coolant reaches the target 185°F.

Diagnose Jeep P0128

Start by checking the coolant level. If it is low, that will set the P0128 code. Next, check thermostat operation using a noncontact infrared thermometer. Compare the ECT reading to actual engine temperature.

If you replace the thermostat and the Jeep P0128 code returns, check both the ECT and ambient air temperature (AAT) sensor readings to make sure they’re within an acceptable range based on actual engine and ambient air temperatures. This is critical since the predicted coolant temp is based in part on the AAT.

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Posted on by Rick Muscoplat

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