Error Code P0117: Engine Coolant Temperature (ECT) Sensor Circuit Low Input

Error Code P0117 is described as Engine Coolant Temperature (ECT) Sensor Circuit Low Input. Meaning, the PCM (powertrain control module, also referred as ECM or engine control module) determined the ECT sensor output going less than 0.14V or above 284˚ F (140˚ C).

This error code is a generic trouble code, meaning it’s applicable to all vehicles made since 1996 up to present. Different makes and models have different specifications in their definition and troubleshooting of this trouble code.

This code is related to other ECT sensor and circuit related trouble codes, such as P0115, P0116, P0118, P0119, P0125 and P0128.

Definition

The job of the ECT sensor is to measure the rise and fall of the ECT, sending that critical data to the PCM, which allows the latter to provide better control for many components of Emission Control Systems, such as air fuel ratio, cooling fans, ignition spark timing, etc. It can be found in the coolant passage or in the block. It’s a two-wire sensor; with one wire providing 5V reference voltage for the PCM, and the other for ground. The ECT sensor resistance will appear high when the coolant temperature is low and vice versa.  The PCM monitors these readings and determines the coolant temperature to be able to provide good fuel management.

Basically, the ECT is a thermistor which changes its resistance based on the coolant’s temperature it’s connected to. The PCM expects slow change of resistance from the ECT signal circuit. As the temperature of the coolant changes, the sensor’s resistance also changes. If the PCM detects that the signal voltage is lower than the normal operating range of the sensor, then it triggers the error code P0117.

Common Symptoms

Aside from the activation of the Check Engine light which puts the vehicle to failsafe mode, substituting its input to 176˚ F. some vehicles may not show noticeable abnormalities in drivability. In many cases however, common symptoms include:

  • Hard starting when cold, starts normal when warm
  • Poor fuel mileage
  • Poor drivability
  • Rough idle
  • Engine hesitates until warm
  • Black smoke coming from tailpipe

Note that these symptoms may be lessened once the engine warms up.

Possible Causes

Shorted wiring is the most common cause of this error code. Other possible causes include:

  • Damaged ECT sensor
  • High resistance in sensor wiring
  • Damaged or faulty connectors
  • Damaged wiring harness or loose terminals at ECT or PCM
  • Overheated engine
  • Defective PCM (rare)

How to Check

Diagnosis for this error code is similar to most error codes concerning the ECT – through visual inspection of electrical connectors, wirings and sensors that concerns the Emissions Control Systems.

Next, the technician will use their scan tool, take note of the codes received, and check the freeze frame data and see when the code was set.

They will then reset the codes and clear the scan tool, and then retest the vehicle through a test drive, to see if the code comes back.

Then they will unplug the ECT sensor and observe the live stream data to see if the reading goes beyond 284°F or -40°F.

If you see the reading going down to 40°F, then it’s time to replace the ECT sensor. If it doesn’t, then look to see if the harness connector to the PCM has shorted.

How to Fix

Repair for this error code is pretty easy. As said above, you may have to replace the ECT connector if necessary. Also, you may need to repair, if not replace wirings or connectors as necessary. You may have to replace the whole ECT sensor.

Make sure you conduct the vehicle manufacturer’s pinpoint test procedures first and follow the troubleshooting and repair step-by-step. Never replace the ECT sensor unless you’re 100% sure it’s the source of the problem.

Also, before you install a new ECT sensor, try to connect it and view the PCM data and see if the expected temperature output comes back.

As with many error codes, the P0117 will set the PCM to failsafe mode, which may cause the engine to run erratically, with hesitation or run rough until the engine is properly warmed up.

 

 

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