Fault code P0128 – a step-by-step guide to diagnosing and fixing it

The powertrain DTC P0128 is triggered when the coolant temperature sensor readings indicate that the coolant hasn’t reached the correct regulating temperature within a certain timeframe. In other words, it hasn’t warmed up quickly enough. The powertrain control module (PCM) also takes into account the intake air temperature sensor readings. This code may indicate that there is a problem with the coolant thermostat.

• A thermostat that won’t close or is leaking. This is the most common cause and it can occur if the cooling system parts are not serviced on time or as a result of corrosion. This means the valve won’t be able to regulate the flow of coolant through the engine.
• A faulty coolant temperature sensor or IAT device. Due to faults, false information could be transmitted to the control module or the transmission could be interrupted.
• Wire damage
• A radiator fan malfunction. If the fan stays on all the time, the coolant will cool down quicker and take longer to warm up.
• A low coolant level
• PCM failure. The control module can malfunction, although this doesn’t occur frequently.

• The engine takes an unusually long time to warm up, especially in cold weather
• An increase in fuel consumption
• The check engine light is on
• The heater is noticeably less effective
• High idle speeds

Checking the trouble codes that have been registered by the control unit is often the first step for identifying the fault. You can perform a diagnostic scan by connecting an OBD2 scanner to the vehicle’s computer system. The DTCs listed may indicate a specific cause or other underlying problems. For example, if P0113 is on the list, you will need to check the IAT sensor before proceeding.

It might be a good idea to reset the DTCs after each step of the repair process and then do another scan after taking the vehicle for a test drive. This will help you to determine the effectiveness of your repair methods.

It is advisable to check the coolant level and the condition of the fluid in the tank. Contaminated fluid can cause the thermostat to stick and there should be enough coolant for the system to operate properly. You should top up, drain or change the fluid as required. You may also need to inspect the radiator fan.

You can check whether the sensors and wires are working properly by checking the voltage using a multimeter. Cleaning dirty electrical contacts with a special spray can help to restore the connection between components. If there is visible wire damage or a power supply issue, new wires should be installed. You will also need to replace the CTS and thermostat if they are broken.