P2186 – Engine coolant temperature (ECT) sensor 2 – circuit intermittent/erratic

By Reinier (Contact Me)
Last Updated 2017-05-22
Automobile Repair Shop Owner

Table of Contents

1. What Does Code P2186 Mean?
2. What are the common causes of code P2186?
3. What are the symptoms of code P2186?
4. How do you troubleshoot code P2186?
5. Codes Related to P2186
6. Get Help with P2186

What Does Code P2186 Mean?

OBD II fault code P2186 is a generic code that is defined as “Engine coolant temperature (ECT) sensor 2 “circuit intermittent/erratic”, and is set when the PCM detects an erratic or intermittent voltage in the control circuit of the #2 engine coolant sensor. Note that some applications use two engine coolant sensors; one sensor to operate the temperature gauge on the dashboard, and another that supplies the PCM with input data that is used to calculate fuel delivery and ignition timing strategies. Code P2186 refers specifically to the #2 engine coolant sensor, which is the sensor that supplies the PCM with input data.

NOTE: On applications with only one engine coolant sensor, this sensor is almost invariably of the three-wire type, one wire being a ground, and the other two delivering input data to the temperature gauge and the PCM respectively.

Since engine temperature plays a vital role in how sufficiently the engine operates at various temperatures, many manufacturers use a dedicated engine coolant temperature sensor to supply the PCM with input data directly. For instance, at low engine temperatures, combustion of the air/fuel mixture is relatively difficult and incomplete, and under these conditions, the PCM may enrich the air/fuel mixture and advance the ignition timing to ensure smooth engine operation.

As the engine warms up (and the oxygen sensors enter closed loop operation) the engine coolant temperature sensor’s resistance decreases, which allows a higher signal voltage to reach the PCM. Based on this (changing) voltage, the PCM will lean out, and retard the ignition timing progressively, thereby keeping the engine running smoothly until it reaches full operating temperature.

In terms of operation, an engine coolant temperature sensor is always placed so that the thermistor, which is the sensing part of the sensor, is in full contact with the engine coolant. As the engine warms up, the coolant starts to circulate, which transfers heat to the sensor, whose resistance changes in direct proportion to the temperature of the engine coolant. The PCM recognizes, or interprets these changes in voltage as degrees of temperature, and adapts fuel delivery and ignition timing settings in response to the changing signal voltage to keep the engine operating smoothly at all times.

The image below shows a typical engine coolant sensor, but take note that the actual appearance and location of engine coolant sensors vary greatly between applications. Refer to the manual for the affected application to locate and identify the #2 engine coolant sensor to ensure that the correct sensor and control circuit is investigated. Failure to do this will result in a misdiagnosis, wasted time, and the unnecessary replacement of parts.

What are the common causes of code P2186?

Common causes of code P2186 could include the following-

• Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors
• Defective #2 engine coolant temperature sensor
• Low coolant level
• Poor coolant circulation due to a defective thermostat, or clogged radiator
• Failed or failing PCM. Note that this is a rare event, and the fault must be sough elsewhere before any control module is replaced.

What are the symptoms of code P2186?

Common symptoms of code P2186 could include the following-

• Stored trouble code, and possibly an illuminated warning light
• Engine idle may be rough, or erratic after cold starts
• Idling speed may fluctuate after cold starts
• Black smoke may be visible from the tail pipe after a cold start
• The strong odor of fuel may be present after a cold start
• Engine may stumble or hesitate upon acceleration after a cold start
• Fuel consumption may increase considerably over extended periods
• In some cases, especially where the #2 engine coolant temperature sensor also controls the radiator fan(s), engine overheating may occur if the fan(s) operation is interrupted.
• In some cases, the radiator fan(s) may run continuously.

How do you troubleshoot code P2186?

NOTE #1: The diagnostic and repair procedures for code P0119 are identical to those for code P2168, but with the significant difference that code P2168 refers to the #2 engine coolant temperature sensor, whereas P0119 refers to the control circuit of the #1 engine coolant temperature sensor. To avoid confusion, wasted time and a possible misdiagnosis, make absolutely sure to identify the #2 engine coolant temperature sensor correctly before starting a diagnostic procedure for P2168.

NOTE #2: An infrared or laser based thermometer may prove useful during a diagnostic procedure for this code.

Step 1

Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be confirmed later on.

WARNING:  Note that this code can be caused by a low coolant level. Before starting an electrical diagnosis for this code, verify that the engine coolant is up to the proper mark, and that there are no coolant leaks. Top off the coolant level as required, or trace and correct all coolant leaks before continuing the diagnostic procedure to prevent a recurrence of the problem, or worse, engine failure due to overheating.

Step 2

Assuming that the engine coolant level is up to the mark and that there are no coolant leaks present, refer to the manual to identify and locate the #2 coolant temperature sensor. Also use this opportunity to locate and identify the relevant associated wiring, as well as the function, color-coding and routing of all relevant wiring.

Perform a thorough visual inspection of all associated wiring. Look for damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors. Make repairs, or replace wiring and/or connectors as required. Clear all codes after repairs are complete, and operate the vehicle for at least one complete drive cycle to see if the code returns.

Step 3

If the code persists but there is no visible damage to wiring, prepare to perform ground integrity, reference voltage, resistance, and continuity tests on all associated wiring, including fuses and relays but be sure to disconnect the coolant temperature sensor from the PCM to prevent damage to the controller.

Pay particular attention to the reference voltage circuit. If the voltage is abnormally low or absent, refer to the manual on the correct procedure to follow to test the reference voltage circuit at the PCM connector. If the voltage, (usually 5 volts, or close to it) checks out at the PCM connector, suspect an abnormal resistance in this circuit.

Compare all obtained readings with the values stated in the manual, and make repairs to wiring, or better yet, replace all wiring as required to ensure that all electrical values fall within the ranges specified by the manufacturer.

NOTE: Be sure to test the internal resistance of the engine coolant temperature sensor as well, since its resistance is a reasonably good indicator of its overall condition. Replace the sensor with an OEM part if its resistance deviates from the stated value by more than a few percent. Clear all codes after repairs are complete, and operate the vehicle for at least one complete drive cycle to see if the code returns.

Step 4

If the code persists, use the scanner to obtain a temperature reading of the engine coolant while the engine is running. Confirm this reading with the thermometer at a suitable point on the cooling system, but bear in mind that it is highly unlikely that you will obtain identical readings.

Nonetheless, the two readings should not differ by more than two or three degrees. If a bigger difference exists, suspect a poor electrical connection in the wiring that had escaped your first inspection, or a defective coolant temperature sensor. While it is not altogether impossible for the PCM to be defective, this is an exceedingly small possibility, so either replace the coolant temperature sensor, or inspect the wiring again until the fault is both found and repaired or until it is certain that the wiring is in perfect condition.

WARNING: Do NOT open the radiator or expansion tank cap to obtain a direct coolant temperature reading from the engine coolant itself while the engine is hot. Use a suitable spot on the radiator tanks or thermostat housing (if metal), or at a point on the cylinder head close to the engine coolant temperature sensor to avoid sustaining severe personal injuries.  

Step 5

If the fault persists despite several repair attempts, bear in mind that poor coolant circulation can also cause this code. However, this will generally become evident if large temperature differences exist between different parts of the cooling system, such as between the top and bottom radiator hoses if the engine is hot.

This is usually caused by a defective thermostat, but it can also be caused by a clogged radiator. Replace the thermostat, and/or have the radiator cleaned out chemically at a reputable radiator repair shop.

If the fault persists despite the replacing the thermostat and cleaning out the radiator, suspect a defective PCM or other control module. If this is suspected, refer the vehicle to the dealer or other competent repair shop for professional diagnosis and repair since cheap generic code readers can generally not diagnose faults in control modules.

Codes Related to P2186

• P2182 – Relates to “Engine Coolant Temperature Sensor 2 Circuit”
• P2183 – Relates to “Engine Coolant Temperature Sensor 2 Circuit Range/Performance”
• P2184 – Relates to “Engine Coolant Temperature Sensor 2 Circuit Low”
• P2185 – Relates to “Engine Coolant Temperature Sensor 2 Circuit High”

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