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”. It applies to all vehicles from 1996 to current models. 

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

Your car’s Powertrain Control Module (PCM) uses the temperature sensor to monitor temperature fluctuations in your car’s engine coolant or cooling system. 

It does so by sending a 5-volt reference signal to the coolant temperature sensor and providing a ground for a low reference circuit. 

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 regulates the air-fuel injection, cooling fan, and ignition spark timing accordingly.

But if the ECT sensor signal circuit output is high (typically more than 4.9 volts), your PCM will determine the sensor resistance is out of spec.

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

Common Symptoms

A common sign of the P0118 trouble code is an illuminated Check Engine Light. 

Some vehicles may also go into a failsafe mode — a protective feature that limits engine and transmission function.

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
• Rough 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 have 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 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 an automotive mechanic will find the root cause for engine code P0118:

Step 1: Verify the Code with a Scan Tool

The automotive mechanic will first use an OBD-II scan tool to verify the P0118 code and ensure no other error code is present. If other error codes are found, they may address those first.

Step 2: Check Coolant

The mechanic will inspect the engine coolant system for air pockets. They’ll also ensure that the engine coolant is not dirty. They may flush or bleed the engine coolant.

Step 3: Inspect the Wiring Harness

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

Step 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.

However, if the resistance test shows an open circuit or high wiring resistance, the wire harness needs to be repaired.

Step 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. 

They’ll also check the coolant sensor with an OBD-II scan tool. 

If it fluctuates within the normal range of 195°F and 220°F, the problem might be intermittent. Your mechanic will wiggle test the ECT sensor connector and harness while monitoring the scan tool. If there’s a significant drop, you may have a bad connection. 

If the scan tool reads a temperature below 195°F or above 220°F, they’ll check the resistance of the temperature sensor. If it’s not within the specified resistance range, the mechanic will replace it with a new sensor. 

If the sensor is within the specified range, they’ll unplug it and attach the two terminals of the connector together using a fused jumper wire. 

The temperature reading should now be above 250°F. If not, your ground circuit or voltage supply might have a problem. 

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

Possible Repairs for P0118 & Costs

Potential repairs for an engine code P0118 can include: 

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

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

• Coolant flush: $100 to $150
• Sensor wiring: $90 to $1100
• ECT sensor replacement: $399 to $441
• Powertrain control module replacement: $500 to $1200