Fix P0115: Mechanic’s solutions that WORKED

P0115: Engine Coolant Temperature (ECT) Sensor Circuit Malfunction points out an issue with your ECT (engine cooling temperature sensor) and most likely is caused by a faulty sensor or damaged wiring. Other cooling system issues like faulty cooling fans, low coolant levels, or malfunctioning thermostats could also be causing this fault.

P0115 quick overview

Meaning	Engine Coolant Temperature (ECT) Sensor Circuit Malfunction
Is it serious?	Yes, it can be serious as it relates to the engine’s cooling system, and prolonged issues may lead to overheating.
Possible causes	– Faulty ECT sensor. – Wiring or connector issues in the ECT circuit. – Engine cooling system problems, such as a malfunctioning thermostat or cooling fans. – ECM/PCM (faulty, or needs update)
How to diagnose?	Check ECT Sensor: Inspect the ECT sensor for any signs of damage or wear. Test the sensor’s resistance using a multimeter to ensure it’s within the specified range. Inspect Wiring and Connectors: Examine the wiring and connectors related to the ECT circuit for any damage or loose connections. Repair or replace damaged wiring or connectors. Cooling System Check: Inspect the cooling system components, including the thermostat and cooling fans, for proper operation. Ensure that the engine is reaching normal operating temperature. Diagnostic Tools: Use an OBD-II scanner to monitor live data, especially the Engine Coolant Temperature (ECT) reading. Replace ECT Sensor: If the ECT sensor is found to be faulty, replace it with a new, compatible sensor. Clear the fault codes and monitor the system for any recurrence.
What is ECT?	Engine cooling temperature / Engine cooling temperature sensor

How to fix P0115 – learn from mechanics

I looked into P0115 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Toyota

Vehicle Information:
2005 Toyota RAV4 2.4L

Issue + Repair History:

• The engine overheated due to a broken radiator cap.
• The vehicle experienced engine power loss and compression issues, leading to a complete engine overhaul. All components were replaced except the Crankshaft Position Sensor (CKP).
• After the overhaul, the vehicle had a no-start issue and no spark on coils 2 and 4.

Suggestions from the Community:

• Check if the P0115 code reappears immediately after clearing.
• Perform electrical testing, check all grounds and power connections.
• Investigate possible ECM damage.
• Check the temperature reading on the scan tool.
• Address the P0115 code first by replacing the coolant sensor and retesting.
• Verify the operation of the Immobilizer system.
• Check for injector pulse on all cylinders and inspect the CKP ring for broken teeth.

Final Fix:
The issue was traced to a burnt-out terminal socket on the Engine Coolant Temperature (ECT) sensor. The socket was replaced, and the spark angle was adjusted from 5BTDC to 7BTDC. This resolved the no-spark issue on coils 2 and 4, and the P0115 code was cleared.

Case #2: Nissan

Vehicle Information:
1997 Nissan Altima GXE 2.4L
Engine: 2.4L / 4 cyl / Gas / DOHC

Issue + Repair History:

• The vehicle came to the shop for a smog test failure and MIL on with code P0115 (ECT sensor circuit problem).
• Wiring checks showed ground was okay, power was at 4.98V, and ECT sensor resistance and temperature were normal.
• Replaced the ECT sensor, thermostat, and used ECM, but the issue persisted.
• Disconnecting the ECT sensor caused the data to display 176°F instead of -40°F.
• Another shop had previously worked on a similar problem.

Suggestions from the Community:

• Check coolant temp resistance at both computer terminals.
• Test voltage at the ECT sensor at the ECM to identify a possible short to ground.
• Replace the ECT sensor and thermostat with OEM parts.
• Ensure the coolant level is okay.
• Consider that the vehicle might be in limp-in mode if a sensor is far off, leading to default settings.
• In generic OBDII, view the actual temperature value, not the calculated or failsafe value.
• Perform a paper clip test to see if the computer reads max positive temperature.
• Run redundant grounds to the ECM and Transmission Control Unit.
• Voltage drop the grounds for the ECM at the intake manifold.
• Ensure the temp gauge is functioning properly.
• Consider that Nissans may display a guessed number when detecting a problem with the ECT.

Final Fix:
A ground issue was identified on the ground side of the ECT sensor. A wiggle test revealed 4.8 volts on the ground side, affecting multiple components. Further inspection showed 0 volts after the wiggle test on the harness from the ECM. Due to time constraints, the complete wiring harness was replaced with one from a junkyard. The vehicle ran fine and passed smog after the harness replacement.

Case #3: Subaru

Vehicle Information:
1995 Subaru Legacy LS 2.2L

Issue + Repair History:

• The vehicle came in with a hard start and MIL lamp issue.
• Code P0115 (ECT circuit malfunction) was present.
• Another technician found a bad thermostat and replaced it.
• The coolant temp sensor was replaced based on scope readings.
• Despite these repairs, the vehicle continued to overheat.

Suggestions from the Community:

• Perform leakdown tests, both stone cold and very hot, to check for head gasket issues.
• Consider block issues if the leakdown test fails when hot, indicating a cracked block.
• Ensure the radiator flow is adequate.
• Perform a cooling system pressure test, reverse leakdown, and dye test.
• Check for bubbles or rising coolant levels during leakdown tests.
• Conduct a chemical test (Block Check) and sniff test with a gas analyzer.
• Verify the thermostat’s functionality and replace it if defective.

Final Fix:
The thermostat was identified as defective. Upon checking by immersing it in water, it was found not to open. The thermostat and gasket were replaced with dealer parts, resolving the overheating issue, and the MIL lamp turned off.

Case #4: Mercedes

Vehicle Information:
1997 Mercedes-Benz E420 4.2L

Issue + Repair History:

• Initially had a dead cylinder #4 and associated P0304 code.
• Found water in the cylinder #4 tube, with a carbon-tracked plug; plugs and coil boots were replaced.
• Cleaned a carboned-up throttle body with a suspected stuck plate.
• The misfire issue was resolved, but subsequent codes P0133 (right O2 sensor before cat slow response) and P0140 (right O2 sensor after cat no activity) appeared.
• Both right O2 sensors and the ECT sensor were replaced.

Suggestions from the Community:

• Check freeze frame data, short-term and long-term fuel trims, speed, and load.
• Noticed right O2 readings at cruise and idle were low (around 50 mV) due to an exhaust leak found in the front of the cat.
• Fixed the exhaust leak, which improved the right O2 readings, but lambda control after the cat remained abnormal.
• Continued test driving; the light came back on, indicating an unresolved issue.

Final Fix:
After fixing the exhaust leak, fuel trim values slowly returned to normal. After two days of driving, the MIL light went off, and the vehicle passed emissions with no further codes.

Case #5: Jaguar

Vehicle Information:
2002 Jaguar X-Type 3.0L

Issue + Repair History:

• The car ran fine initially, but after driving about 120 miles, the cruise control disconnected, and the engine overheating message appeared.
• The temperature gauge rapidly moved from 1/3 to 1/2, then slowly rose to hot.
• The check engine light came on with code P0115 (temp sensor).
• Turning off and restarting the engine temporarily resolved the issue, but it reoccurred after a few more miles, triggering fail-safe mode.
• The ECU was reprogrammed, and the temperature sensor was replaced, but the issue persisted.

Suggestions from the Community:

• Replaced the temperature sensor and reprogrammed the ECU.
• Discovered a melted fan module on the shroud, which was replaced.
• Suspected an intermittent break in the wiring to the temp sensor.
• Considered potential ground path issues and suggested running new wires to the temp sensor.
• Inquired about the loss of heat when the temp gauge rises.
• Suggested monitoring the ECT PID with a scanner and checking for fan control module failures.
• Raised the possibility of a thermostat issue contributing to overheating and fail-safe mode.
• Highlighted the importance of checking for chafed wires in the engine harness and asked about the reprogramming process.
• Mentioned the common problem of cooling fans on X-Type models.

Final Fix:
The PCM was replaced with a used one, resolving the issue.