P0118 – Engine coolant temperature sensor 1 circuit high

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The P0118 OBD2 code triggers when the engine coolant temperature sensor reads at too high a resistance for the engine conditions. While this may result from a bad sensor, it’s often a more simple issue with the wiring or coolant.

While P0118 is a generic powertrain code, the exact symptoms and repair steps can vary widely from one manufacturer to another. Because of this, it’s recommended to seek out repair bulletins specific to your vehicle before you begin your diagnosis.

P0118 code definition

P0118 (generic): Engine Coolant Temperature (ECT) sensor circuit has higher than normal voltage

Here's where you can get a thorough understanding of the P0118 OBD2 code
If you want to learn about the P0118 OBD2 code, this is the right place

What does P0118 mean?

The engine coolant temperature (ECT) sensor is built into the cylinder head. It’s a thermistor, meaning it changes resistance depending on the temperature it reads. Resistance is highest when the coolant temperature is low and drops as the temperature goes up.

Information from the ECT sensor is sent to the powertrain control module (PCM) or engine control module (ECM). The computer sends a ground and a 5-volt reference to the ECT sensor. It then monitors the drop in voltage to determine the temperature of the coolant. 

Data collected through the ECT sensor is important to many aspects of your engine’s operation. It helps the computer regulate the air-to-fuel ratio and the activation and speed of cooling fans. Left unchecked, problems with the ECT sensor can disrupt the spark ignition’s timing and lead to misfires or false starts.

If the ECT sensor resistance indicates a coolant temperature below freezing during engine operation, the PCM or ECM determines there’s a fault in the circuit and triggers the P0118 trouble code. The code will also trigger when the resistance is determined to be outside standard operating specifications. 

Activation of the P0118 OBD2 code may put your engine into failsafe mode. Failsafe mode protects your engine from overheating, but it’s not good from a driver’s standpoint. It greatly reduces your engine’s performance, increasing emissions and gas consumption. Finding the root cause of the P0118 trouble code is key to restoring your usual engine performance. 

What are the symptoms of the P0118 code?

There are often no drivability symptoms when P0118 activates. In other cases, the symptoms are severe and can include:

  • Activation of the check engine light
  • Engine idling rough
  • Reduced fuel economy
  • Reduced performance
  • Failure to start or hard starts, especially in cold weather
  • Engine misfires
  • Black smoke from the tailpipe

What are the causes of P0118?

The engine coolant temperature sensor is triggered by the P0118 OBD2 code when it reads at too high a resistance for the engine conditions.
The P0118 OBD2 code triggers when the engine coolant temperature sensor reads at too high a resistance for the engine conditions.
  • Engine coolant is old or dirty
  • Air pockets in coolant lines
  • Damaged or loose wires/connections around ECT sensor
  • Internal short in ECT sensor
  • Open circuit in ECT sensor
  • Faulty ECM or PCM (rare)

How serious is the P0118 code?

The P0118 OBD2 code is of high severity. Driving with a malfunctioning ECT sensor can lead to long-term engine damage from overheating and misfires. Aside from this is the issue of driving in failsafe mode, which is frustrating and potentially dangerous. Because of this, you should avoid driving your vehicle until the code has been cleared. 

How to diagnose and fix the P0118 code

Tools you’ll need:

  1. Use an OBD2 scan tool to read the trouble codes for your vehicle. Make a note of any other codes that appear alongside P0118, as these can help guide your diagnosis.
  2. Inspect the wires around the ECT sensor. Replace any that are damaged or fraying, and make sure all connections are secure.
  3. Check the coolant. A healthy coolant should be clear with a bright color. If it’s dark, cloudy, smells bad, or has visible debris in it, you should replace it. Flush and bleed the coolant system, which will also release any air pockets in the lines, then replace it with fresh coolant.
  4. Wiggle the connector and wiring harness while watching the ECT sensor readings on a scan tool. If the reading drops out, check and tighten all the connections. 
  5. Clear the codes and test drive your vehicle, then scan again with the OBD2 scanner. If the P0118 code comes back, remove the ECT sensor and check its resistance. The easiest way to do this is to test the resistance at room temperature, then slowly raise the sensor’s temperature using a hairdryer. The resistance should go down as the temperature rises, then rise as the sensor cools. If the readings don’t support this, replace the ECT sensor. 
  6. Inspect the sensor connector for bent pins. If you see signs of corrosion or another build-up, clean with a solvent designed for electronic components.
  7. If the ECT sensor seems to be functioning, the problem may be an open circuit in the wiring connecting the sensor to the engine computer. Test each wire individually to verify it reads a continuous low resistance, replacing any that show an open circuit or high resistance. 
  8. If the code still doesn’t clear, you may have a more serious issue with your PCM or ECM. Take your vehicle to a mechanic for further diagnosis. 

Common mistakes to avoid while diagnosing the P0118 code

Many inexperienced mechanics immediately replace the ECT sensor in response to a P0118 trouble code. While a faulty sensor can be the issue, it can also be caused by issues with the coolant lines and wiring. Conduct a thorough diagnosis to pinpoint the source of the problem before beginning any repairs. 

Tips to avoid P0118 in the future

One common cause of P0118 is a wiring issue, often caused by loose connections or sensors. Make sure you firmly connect all wires and bolts any time you make modifications or repairs to your engine. The vibrations caused by driving can easily dislodge components that aren’t held insecurely. 

You can also help prevent the P0118 OBD2 code by keeping up with your regular maintenance schedule. Changing your coolant when it turns is an important part of maintaining a healthy engine temperature and will prevent a range of pesky trouble codes. 

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