How To Test: P0118 OBD II Trouble Code (Honda 2.2L, 2.3L)

OBD II trouble code P0118 ECT Sensor Circuit High Voltage is telling you that your Honda's fuel injection computer is seeing a continuous engine coolant temperature of -4°F (20°C) or higher.

The most common causes of a P0118 trouble code are: a bad ECT sensor or a bad ECT sensor connector (creating a false contact), or an ‘open’ in the sensor's wires.

In this tutorial, I'm gonna' help you diagnose this diagnostic trouble code (DTC) in a step-by-step way.

P0118 Basics You Need To Know

Engine temperature, via the engine coolant temperature (ECT) sensor, has a direct impact on the amount of fuel your Honda's fuel injection computer injects into the engine's cylinders.

Why is the engine coolant temperature info so important? Because the colder the engine is, the more fuel the fuel injection computer (known as the PCM = Powertrain Control Module) needs to inject. The warmer the engine is, the less fuel the PCM needs to inject.

So when the PCM gets the wrong engine coolant temperature (either because the ECT sensor has failed or there's a problem in the sensor's wires), it can no longer control fuel injection to maximize performance and decrease emissions.

Now, when a code P0118 ECT Sensor Circuit High Voltage lights up the check engine light (CEL) on your Honda's instrument cluster, the PCM is letting you know that it sees an engine coolant temperature of -4°F (20°C) or colder.

How The ECT Sensor Works

Here's a very brief description of how the engine coolant temperature (ECT) sensor works:

1. The engine coolant temperature (ECT) sensor is a 2 wire sensor.
   1. One wire (circuit) supplies power.
      • Power is in the form of 5 Volts DC and is supplied only with Key On Engine Off (KOEO) or Key On Engine Running (KOER).
      • Power comes directly from the PCM.
   2. The other wire (circuit) supplies Ground.
      • This Ground is fed to the ECT sensor by the PCM (internally).
2. The ECT sensor is a resistor (thermistor). Its resistance changes in response to changes in the engine's coolant temperature.
   • The cooler the engine coolant, the higher the resistance of the ECT sensor.
   • The warmer the engine coolant, the lower the resistance of the ECT sensor.
3. When the 5 Volts pass through the engine coolant temperature (ECT) sensor, the resistance of the sensor produces a voltage drop. This in plain English means that only a certain amount of the 5 Volts are let through to the Ground wire.
   1. The cooler the engine coolant is, the less voltage is let thru' onto the Ground wire (due to the ECT sensor's higher resistance).
      • The PCM translates this higher voltage drop into a colder temperature value.
   2. The warmer the engine coolant, the more voltage is let thru' onto the Ground wire (due to the ECT sensor's lower resistance).
      • The PCM translates this lower voltage drop into a warmer/hotter temperature value.

With this bit of information, let's move on to the next subheading.

Symptoms Of A P0118 Diagnostic Trouble Code

When the PCM thinks that the engine coolant temperature is around -4°F (-20°C), your Honda isn't gonna' run at its best.

You may see one or more of the following symptoms when the engine coolant temperature (ECT) sensor fails:

1. Check engine light (CEL) is on.
2. DTC P0118 is present.
3. Won't pass the state mandated emissions test.
4. Gas mileage will suffer.
5. Hard start and/or extended cranking time (after shut off).
6. Black smoke coming out of the tailpipe.

Let's find out what are the common causes of a P0118 DTC, in the next subheading.

Common Causes Of A P0118 Trouble Code

The 3 most common cause of trouble code P0118 are:

1. A bad engine coolant temperature (ECT) sensor.
2. A broken ECT sensor connector.
3. A problem in the sensor 2 wires. Specifically, an ‘open’ in one of them.

Although extremely rare for this to happen, a bad PCM can also cause a false P0118 trouble code.

In this tutorial, I'll help you troubleshoot all three of the above. With this basic info under our belts, let's turn the page and get testing!