P0128: Faulty Engine Coolant Temperature

What Is P0128?

P0128 is an OBD-II diagnostic trouble code (DTC) known as “Thermostat Rationality” or “Engine Coolant Temperature Below Thermostat Regulating Temperature”. 

It indicates an issue with the engine coolant temperature (ECT) system. 

Once you start the engine, the coolant should reach the thermostat-defined engine coolant temperature within a specific time. If it doesn’t, the Engine Control Module (ECM) or Powertrain Control Module (PCM) triggers a P0128 engine code.

Why does the thermostat regulating temperature matter?
The engine must reach the correct operating temperature so the fuel system can enter a closed loop and oxygen sensors can ensure proper air-to-fuel ratio for better performance. 

The operating coolant temp should typically be around 195-220°F (90.5-104°C). But it can vary depending on your car’s make and model and the driving conditions.

Common Symptoms of P0128

The most common symptom of this engine code is an illuminated check engine light or malfunction indicator lamp (MIL).

Other symptoms to look out for include:

• The engine takes too long to warm up (especially after a cold start)
• Poor heater performance
• Poor fuel economy
• Poor engine performance
• High idle

P0128 Causes

Anything from irregular coolant thermostat temp (from faulty sensor readings) to low engine coolant levels can trigger a DTC P0128 code. 

For example:

• A sticking or open thermostat: 

• • A thermostat regulates the engine temperature. You can find it inside the thermostat housing between the radiator and the engine.
  • When the engine is warm enough, or the temperature gauge reaches the required operating temp, the thermostat lets the coolant flow through the radiator to maintain the engine temperature.
  • So a sticking or open thermostat keeps circulating the coolant even when it’s not needed, causing the engine to operate below its normal operating temperature. 

• Low coolant level:

• • A coolant is an antifreeze solution that regulates the engine’s operating temperature to prevent overheating. It removes excess heat and protects important moving components like the water pump, cylinder, head gasket, etc. 
  • The coolant is a part of the car’s liquid cooling system, in addition to a water pump, radiator, cooling fan, thermostat, and hoses like the upper radiator hose. 

Other causes include:

• A defective engine coolant temp sensor (ECT sensor) 
• A faulty intake air temperature sensor (IAT sensor)
• Radiator fan malfunction
• Powertrain Control Module failure
• Cold weather conditions that prevent the engine from reaching its normal operating temperature

Is P0128 Serious? Can I Still Drive?

You can drive your car with the trouble code P0128 — it doesn’t immediately threaten your safety.

But we recommend fixing it soon to avoid complications like:

• Reduced fuel economy, increased emissions, and engine damage
• The check engine light may remain on
• The temperature gauge may not reach the expected levels
  

The extent of damage usually depends on what causes DTC P0128. For example, almost all coolant thermostat issues can contribute to engine damage if left unaddressed. 

Bottom line: Get a professional mechanic to check your car ASAP.

P0128 Diagnosis

Here’s how your mechanic will diagnose a P0128:

1. First, they will scan your vehicle with an OBD-II scanner for error codes.

2. If code P0128 is the only error present, your mechanic may check for an open thermostat since thermostat malfunctions usually cause DTC P0128. 

• To check your engine coolant thermostat, they will inspect the radiator hose once you start your car. Ideally, the hose should heat up when the hot coolant passes through. You may have a faulty thermostat if the radiator hose doesn’t warm up (or takes too long).

3. If your thermostat works fine, your mechanic may check the coolant level. They’ll remove the radiator cap to check for low coolant levels. They’ll also check for coolant leaks or pressure issues, which may damage oxygen sensors.

4. The mechanic may inspect the cooling system simultaneously, checking the wiring and circuitry. Rusty or dirty coolant can cause the engine coolant thermostat to stick.

5. Next, using a multimeter, they may check if the coolant temperature sensor works properly. Ideally, the ohm reading should change with temperature. If it doesn’t, it may indicate a coolant temp sensor problem.

6. Once the mechanic finishes the repairs, they will clear the P0128 code from your vehicle’s ECM using a scan tool and ensure it doesn’t reappear. This will reset the check engine light.

Possible Repairs for P0128 & Costs

Depending on the cause of the P0128 code, your mechanic may perform these repairs:

➤ Thermostat repairs/replacement: If you have a faulty thermostat, they may replace it. A new thermostat will regulate coolant flow and maintain the engine at the optimal temperature.

➤ Coolant flush & refill: The mechanic will check the coolant system for old, contaminated, or insufficient coolant and do a coolant flush and refill. This will ensure proper coolant performance, which will help the engine run at the correct temp.

➤ ECT sensor repairs/replacement: If the ECT sensor is faulty, they may replace it. The engine coolant temperature sensor may also malfunction due to sensor wiring and connector issues, which the mechanic will check and fix accordingly. 

If the error code persists even after the above repairs, the PCM could be the culprit and require replacement or repair.

Repair Costs for P0128

Fixing the P0128 engine code may cost roughly $150-$500 based on your car’s make and model, the repairs, labor rates, etc. 

For instance, replacing the ECT sensor may cost $200-$400. This includes labor, typically around $100-$150, and the cost of the sensor itself, which can range from $5-$250.