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 make and models, including brands like Honda and Chevrolet (Chevy). 

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) or Engine Control Unit (ECU) uses the temperature sensor to monitor temperature fluctuations in its engine coolant. The PCM sends a reference voltage (typically 5 volts) to the coolant temperature sensor, completing the circuit needed to read the sensor’s data.

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

If the engine has been running for several minutes and the ECT sensor reads a below-freezing temperature, the PCM or ECU will detect a circuit fault and register DTC P0118. The code can also be set if the ECT sensor circuit output is high (above 4.9 volts), triggering the Check Engine Light (CEL).

11 Crucial Symptoms of DTC P0118

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

However, various factors can trigger the CEL, so it’s important to watch for additional symptoms. For example, in some cases, vehicles may enter failsafe mode, a protective feature that restricts engine and transmission functions.

Other symptoms of ECT sensor circuit high input code P0118 include: 

1. Hard start when the engine is cold
2. Engine runs rough or hesitates until it warms up
3. Rough idling
4. Black smoke from the engine
5. Misfires
6. Engine overheating
7. Erratic temperature gauge (temp gauge) reading
8. Malfunctioning air conditioner 
9. Constantly running radiator fan
10. Failed emissions test due to increased carbon monoxide and nitrogen oxide emissions
11. Poor fuel economy

NOTE: Several vehicles may not have any noticeable symptoms with an active DTC P0118.

P0118 Causes

Here are some reasons for your PCM registering code P0118:

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

Is P0118 Code Serious & Can You Still Drive?

Yes, the error code P0118 code is serious and should be addressed ASAP.

Driving with the P0118 code can put your vehicle into failsafe mode. This will cause your engine to run rough or hesitate until it warms up to the normal operating temperature. The failsafe mode will also limit certain vehicle features like transmission functions, cruise control, and air conditioning. You may notice your car’s speed reducing and difficulty shifting gears.

But that’s not all!

Continuing to drive your vehicle for an extended time while in the failsafe mode will result in excessive fuel consumption and carbon fouling of internal components. You’ll need additional repairs to remove the carbon build-up or fix possible ignition troubles.

If left unresolved, this problem can further strain different engine components. For example, the engine’s cooling and radiator fans may run continuously, leading to premature wear and potential failure.

In severe cases, a defective coolant sensor can prevent the PCM from accurately detecting when the engine coolant temperature becomes too high. As a result, your engine won’t shut down to protect itself from overheating, potentially leading to costly repairs if not covered by warranty. 

Code P0118 Diagnosis: Step-by-Step Process

Diagnosing and troubleshooting the DTC P0118 (ECT sensor circuit high input) fault code involves several steps and is best left to a professional mechanic.

Why?

Misdiagnosis can result in unnecessary ECT sensor replacement, even if the issue is due to a dirty coolant, bad connection, or faulty thermostat. Instead of attempting DIY checks, it’s best to visit a mechanic to identify the root cause and get a second opinion before installing a new sensor.

Here’s how an automotive mechanic will diagnose engine code P0118:

Step 1: Verify the Fault Code with a Scan Tool

The first thing an automotive mechanic will do is use an OBD-II scan tool to read stored information and verify the P0118 code, ensuring no other fault code is present. If they find other engine codes, they will address those first.

Step 2: Check Coolant

The mechanic will inspect the engine coolant system for air pockets and ensure it’s not dirty. If it is or there are air pockets, they will flush and bleed the coolant system. If the engine is not reaching the normal operating temperature (195°F to 220°F), they’ll inspect the thermostat for faults.

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 bad connection. They’ll also examine the ECM connector for damage, corrosion, or loose pins, as a good connection is essential for accurate readings.

Step 4: Test Wire Resistance

They’ll also check the wire resistance of the engine coolant temperature sensor.

Under a normal range (operating temperature), there should be continuity and low resistance in the coolant temp sensor, indicating a good connection.

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 isn’t 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 the temperature reading on the tool fluctuates within the normal range of 195°F and 220°F, the problem might be intermittent. The mechanic will perform a ‘wiggle test’ by gently twisting the ECT sensor connector and harness while monitoring the scan tool. If there are significant drops or changes in the reading, you may have a poor connection. 

• If there are no changes and the sensor is within the specified voltage range, the mechanic will check the temperature reading. They’ll unplug the sensor and attach the two terminals of the sensor connector using a fused jumper wire. The temperature reading on the scan tool should now be above 250°F. If not, your ground circuit or voltage supply might have a problem.

• 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 engine coolant temperature sensor, wiring, and harness connector seem okay, a faulty PCM could be the culprit and will need to be replaced.

Possible Repairs for P0118 & Costs

Here are the potential repairs for error code P0118 and their estimated costs (including labor charges):

• ECT sensor connector and wiring replacement: $100 to $350
• Coolant flush: $140 to $250
• Thermostat replacement: $200 to $750
• ECT sensor replacement: $400 to $500
• Powertrain control module (PCM) replacement: $750 to $1500

NOTE: P0118 repair costs can vary depending on your repair shop’s location and the time a mechanic takes to complete the repairs — ranging from 30 minutes to several hours.