P0118: Engine Coolant Temperature Sensor Circuit High Input

What Is P0118?

P0118 is a generic diagnostic trouble code (DTC) defined as “Engine Coolant Temperature Sensor Circuit High Input.”

The engine coolant temperature sensor (ECT sensor or coolant temp sensor) is a thermistor in the coolant passage inside the cylinder head.

Your car’s Powertrain Control Module (PCM) uses the temperature sensor to monitor fluctuations in the temperature of your car’s engine coolant or cooling system. It does so by sending a 5-volt reference signal to the coolant temperature sensor.

The sensor resistance stays high when the coolant temperature is low, and the resistance drops when the coolant’s temperature increases. Your PCM reads this data and accordingly regulates the air-fuel injection, cooling fan, and ignition spark timing.

But if the ECT sensor signal circuit output is high (typically more than 4.7 volts) even when the engine has been running for several minutes, your PCM determines the sensor resistance is out of spec.

As a result, it registers the DTC P0118 code and activates the Check Engine Light.

Common Symptoms

One of the most common signs of code P0118 is an illuminated Check Engine Light. Some vehicles may also go into failsafe mode. 

Other symptoms you might experience with ECT sensor circuit high input code P0118 include: 

• Hard start when the engine is cold
• The engine runs rough or hesitates until it warms up
• Poor engine idling 
• Low fuel economy 
• Black smoke from the engine
• Misfires 
• Erratic temperature gauge (temp gauge) readings
• Malfunctioning air conditioner 
• A constantly running radiator fan

Note: Several vehicles may not experience any noticeable adverse conditions with an active DTC P0118.

Can I Still Drive?

While you may not notice any evident abnormalities initially, paying attention to the Check Engine Light triggered by the P0118 code is still important.  

If you continue driving with code P0118, you’ll be putting extra load on different engine components. Your engine’s cooling fan and radiator fan will continue to run non-stop and wear out prematurely. 

If your coolant sensor is defective, your PCM will also not know when the engine coolant temperature gets too high, which will prevent the engine from shutting down to protect itself from overheating. 

On the other hand, some vehicles may go into failsafe mode, which can cause your engine to run rough or hesitate until it warms up to the operating temperature (normal operating temp). It will also cause certain features of your vehicle to become limited. The car’s speed will be reduced, and you’ll find it hard to shift gears. 

The failsafe mode may even lead to excessive fuel consumption and carbon fouling of internal components. So, driving your vehicle for an extended time while in the failsafe mode will require additional repairs to remove the carbon build-up or fix possible ignition troubles.

P0118 Causes

Here are some of the possible reasons for your PCM activating the engine light and registering code P0118: 

• Rusted or dirty engine coolant
• Low engine coolant level 
• Air pocket in the cooling system
• Frayed temperature sensor wiring or wiring harness
• Bad coolant sensor connector
• Bad ECM connector or PCM connector
• Faulty engine coolant temp sensor 
• Faulty engine coolant thermostat
• Open circuit or short ECT signal circuit 
• A defective powertrain control module

Diagnosis

Diagnosing the P0118 (ECT sensor circuit high input) diagnostic trouble code requires several steps. 

Here’s a general guideline on how a mechanic will find the root cause for code P0118:

1. Verify the Code with a Scan Tool
The mechanic will first use an OBD-II scan tool to verify the P0118 code and ensure that no other error code is present. If other error codes are found, they may address those first.

2. Check Coolant
They’ll inspect the engine coolant system for air pockets. They’ll also ensure that the engine coolant is not rusty or dirty. In either case, they’ll flush or bleed the engine coolant.

3. Inspect the Wiring Harness
They’ll disconnect the engine coolant temp sensor and check its wiring harness for any fraying or disconnection.

4. Test Wire Resistance
They’ll also check the wire resistance of the engine coolant temperature sensor.  Under a normal operating temperature, you should have continuity and low resistance in the coolant temp sensor. But, if the resistance test shows an open circuit or high wiring resistance, the wiring harness needs to be repaired.

5. Check for a Bad Temperature Sensor
If the ECT signal circuit wiring is not the issue, the mechanic will remove the coolant temperature sensor and inspect it for corrosion or bent connector pins. 

If the engine coolant temperature sensor, wiring, and harness connector seem okay, a faulty PCM could be the reason.

Possible Repairs for P0118 & Costs

When it comes to fixing the P0118 code, some of the possible repairs include: 

• Flushing out dirty coolant or bleeding any air pocket
• Repairing or replacing the ECT sensor connector
• Repairing or replacing the open circuit wiring 
• Replacing the coolant temp sensor with a new sensor

Here are the average cost estimates for possible repairs (including labor charges): 

• Coolant flush: $100-$150
• Sensor wiring repair/replacement: $100-$1000
• Engine coolant temperature sensor (ECT sensor) replacement: $140-$200
• Powertrain control module replacement: $1000-$1200