Your car’s Powertrain Control Module (PCM) illuminates a check engine light and records a diagnostic trouble code after detecting an engine problem. If you see this light flashing on the dash, it’s necessary to scan your car for the responsible trouble code.
One common DTC you may encounter is the P0128 code. You may also notice several P0128 code symptoms indicating your engine needs attention. So, what does this code mean? And how long can you drive with a P0128 code?
Herein are the answers to these questions. We’ll also look into the symptoms, causes, and fixes of this trouble code.
Question: What Does the Code P0128 Mean?
The P0128 is a DTC that stands for “Engine Coolant Temperature Below Thermostat Regulating Temperature.” It occurs due to several issues with the engine cooling system or PCM failure.
When you start your engine, the coolant must reach a predetermined thermostat regulating temperature to heat the engine within a specified time. In a normal situation, the operating coolant temperature should range between 195 and 220°F. However, this may vary depending on a vehicle’s make and model.
Once your coolant reaches this temperature, it warms up the engine, allowing it to reach its correct operating temperature. When this happens, your car enters a “closed-loop” mode, allowing the PCM/ECM to use oxygen sensors to regulate the air-to-fuel ratio. This process helps to ensure fuel economy and minimize emissions.
If the PCM/ECM detects that your coolant temperature is below the thermostat regulating temperature, it’ll trigger the P0128 code. In other words, this code appears when the engine runs cooler than it should.
This code helps you know there’s an issue causing the coolant not to get significantly warm within the expected time.
Question: What Are the Symptoms of Code P0128?
When the onboard computer records the P0128 code, you may notice the following symptoms:
- Check engine light on the dash
- The engine takes longer to warm up
- Reduced fuel economy
- Fluctuating engine temperature while driving
- High idle
- Poor heater performance
- Poor engine performance
Question: How Long Can You Drive With a P0128 Code?
If your car has the P0128 DTC, you can continue driving for as long as you want. You can drive at least 500 miles without worrying about the car stopping suddenly. The truth is the P0128 code is not a serious issue and should not deter you from driving.
Though you can continue driving with this code, you should not ignore it completely. The DTC indicates issues with the cooling system that can affect your engine in the long run.
Ignoring this code can cause a decrease in fuel economy and increased emissions. Moreover, prolonged coolant temperature issues can cause performance issues or severe engine damage. If you scan this code with an OBD-II scanner or notice the above symptoms, it’s wise to diagnose and fix the root cause.
Answer: Common Causes of Code P0128 and Fixes
Below are the common causes of the P0128 code and what to do about them. But before determining the P0128 code causes, use your OBD-II scanner to check if there are other stored error codes. Clear these codes with the scanner or fix what caused them to occur.
Stuck Open Thermostat
The thermostat prevents coolant flow into the radiator until your engine attains its ideal operating temperature. It opens when the engine warms up or reaches its normal operating temperature. This allows coolant to enter the radiator and maintain engine temperature.
Issues with the thermostat can cause it to stick in the open position, triggering the P0128 code. When this happens, coolant will flow continuously and over-cool the engine. Consequently, the engine will operate below its ideal temperature, leading to a reduced fuel economy and excessive wear of engine components.
To diagnose a stuck open thermostat, locate and touch the radiator hose. Be careful when touching the hose as it could be excessively hot and burn you.
The goal is to check how hot the coolant is as it starts to flow through the hose. If the hose doesn’t get warm as the coolant passes through, you might have a stuck open thermostat. Replacing a stuck open thermostat will help resolve this problem.
Poor Coolant Condition
Contaminated/dirty coolant will alter the engine’s operating temperature and trigger the P0128 code. Dirt or contaminants in the coolant can clog up the thermostat and cause it to stick in the open position. As a result, coolant continues to flow into the radiator and overcool the engine.
Thankfully, dirty or contaminated coolant is an issue you can fix at home. Flush the coolant system by draining the old coolant. You’ll need to locate the radiator drain valve to drain the dirty coolant into a container.
Then, close the drain valve and pour a radiator cleaner/flush and distilled water into the radiator. Start the engine and allow it to run for 10-15 minutes. Drain the radiator again and flush it with distilled water to remove all the dirt.
Then, refill the radiator with fresh coolant and add the leftover coolant into the reservoir.
Faulty Coolant Temperature Sensor
The coolant temperature sensor measures the coolant’s temperature and sends this data to the PCM. Then, the PCM uses this information to adjust ignition timing and fuel injection. Some vehicles may use this data to switch on the cooling fans.
Over time, this sensor can go bad due to faulty connections, corrosion on its terminals, or a low coolant level. If your coolant temperature sensor is defective, it may send incorrect data to the PCM, leading to the P0128 code.
To fix a faulty coolant temperature sensor, check the coolant level and refill if it’s low. If this issue persists, locate this sensor and examine it for corrosion. Clean or replace this sensor if it looks corroded. Check the connectors and wiring for signs of damage and repair or replace them.
If the sensor doesn’t show signs of damage, use a digital multimeter to check its readings. Compare the readings displayed on the multimeter with the service manual specs. The sensor is faulty and needs replacing if the readings don’t match.
Malfunctioning Radiator Fan
Your vehicle’s radiator fan blows cool air through the radiator to remove heat from the hot coolant. It also keeps the engine cool while driving.
Unfortunately, several issues can cause the radiator fan to stay on and run continuously. When this happens, the engine coolant will heat slowly and trigger the P0128 code.
A radiator fan that sticks in the on position may result from a faulty temperature sensor, a short in the wires, or a damaged fan clutch.
Replace the temperature sensor if it’s faulty and fix issues with the wiring. A professional can help inspect the fan clutch or other defects with the radiator fan and fix them.
PCM Failure
If the P0128 code remains after fixing the above, your PCM could be to blame. A faulty PCM may throw several error codes, including the P0128.
Besides these fault codes, you may notice several symptoms of a bad PCM to help you during diagnosis. Some of them include:
- Engine’s poor performance
- Misfires
- Shifting problems
- Starting difficulties
- Increased emissions
- Dashboard warning light
If the PCM is to blame, allow a professional to examine and repair or replace it.
When you resolve the root problem, the P0128 code should disappear after driving for a few miles. If the code remains, erase it using your scanner.
Wrapping Up
The P0128 may occur if there are issues with the engine cooling system. This code indicates that the coolant temperature is below the thermostat regulating temperature.
But how long can you drive with a P0128 code? Luckily, this diagnostic trouble code is not among the most serious DTCs your car may have. With this code, you can drive as long as you want. However, it’s wise to troubleshoot issues leading to this code. Failure to fix this code can cause performance issues or severe engine damage.