P0118 on 2014-2021 Mazda 6: Engine Coolant Temperature Sensor Circuit High Causes and Fixes

What's Unique About the 2014-2021 Mazda 6

The 2014-2021 Mazda 6, equipped with Skyactiv engines, has two coolant temperature sensors. P0118 specifically refers to ECT Sensor 1, the primary sensor located on the cylinder head that measures engine temperature for fuel mixture, ignition timing, and cooling fan control. A second sensor (ECT Sensor 2) is used for other functions, like monitoring heater core temperature for the climate control system. While a related code, P011A, indicates a performance problem or discrepancy between the two sensors (often due to a clogged heater core as per TSB 07-007/17), P0118 is a direct electrical fault indicating an open circuit for the primary sensor.

Generation note: This range covers the entire third generation (GJ/GL) of the Mazda 6, including the 2018 facelift. The location and function of the ECT sensor and the causes of code P0118 are consistent across this generation and its platform mates (Mazda 3, CX-5) using the Skyactiv-G engine.

Symptoms You May Notice

• Check Engine Light is on
• Cooling fans run constantly at high speed, even when the engine is cold
• Difficulty starting the engine, especially when cold
• Rough idle or stalling
• Reduced fuel economy
• Black smoke from the exhaust on startup due to a rich fuel mixture
• Inaccurate or non-functional temperature gauge on the dashboard (may stay on 'C')
• Strong smell of gasoline when starting

Most Likely Causes

1. Failed Engine Coolant Temperature (ECT) Sensor 🔴 High Probability The sensor is a thermistor that can fail internally over time, creating an open circuit which directly results in the high voltage signal. This is the most common failure point for this code.
   How to confirm: Use a scan tool to view live data. With a cold engine, compare the ECT reading to the Intake Air Temperature (IAT). They should be nearly identical. If the ECT reading shows an extreme negative temperature (e.g., -40°F) while the IAT is ambient, the sensor has an internal open circuit. You can also test the sensor's resistance with a multimeter; an infinite resistance reading (OL) confirms an open circuit and failure.
   Typical fix: Replace the ECT sensor. On the Skyactiv-G engine, it is located on the driver's side of the cylinder head, near the transmission and thermostat housing.
   Est. part cost: $25-$70
2. Damaged Wiring or Connector 🟡 Medium Probability Engine bay heat and vibration can cause wires to break or insulation to chafe, especially right at the connector. The connector pins can also become corroded, bent, or loose, creating an open circuit.
   How to confirm: Visually inspect the wiring harness and two-pin connector going to the ECT sensor for any signs of damage, corrosion, or loose pins. Wiggle the connector while watching live data on a scan tool; if the temperature reading flickers between -40°F and a real value, it indicates a bad connection. Check for 5V reference voltage at the connector with the key on.
   Typical fix: Repair the damaged section of wire or replace the connector pigtail if the break is too close to the connector body.
   Est. part cost: $10-$30

Rare But Worth Checking

• Faulty Powertrain Control Module (PCM): This is extremely rare and should only be considered a last resort. The PCM should only be suspected after the sensor and its entire circuit (power, ground, and signal wires) have been thoroughly tested and confirmed to be in good working order.

Diagnosis Steps

1. Connect an OBD-II scanner and confirm P0118 is the active code. Note any other codes present.
2. View the live data stream for Engine Coolant Temperature (ECT). If it shows a static value of -40°F or another extreme low, it confirms the fault.
3. For comparison, check the Intake Air Temperature (IAT) on the scanner. On a cold engine, ECT and IAT should be within a few degrees of each other. A large discrepancy points to the ECT circuit.
4. Turn the engine off and let it cool. Locate ECT Sensor 1 on the driver's side of the cylinder head.
5. Inspect the sensor's electrical connector and wiring for any visible damage, corrosion, or loose connections. Pay close attention to the area where wires enter the connector.
6. Disconnect the sensor. With the ignition key on (engine off), use a jumper wire to short the two terminals in the electrical connector pigtail. The temperature reading on the scan tool should now jump to an extreme high value (e.g., 250°F+).
7. If the reading jumps to high, the wiring and PCM are good. The ECT sensor itself has failed and must be replaced.
8. If the reading remains at -40°F with the connector jumped, there is an open circuit in the wiring between the connector and the PCM, or a faulty PCM. Test for 5V reference and ground at the connector to further isolate the fault.
9. If the sensor is confirmed to be the issue, replace it. A small amount of coolant will leak when the old sensor is removed, so be prepared to top it off with Mazda FL-22 coolant.
10. Clear the trouble code with the scanner and test drive the vehicle to ensure the Check Engine Light and symptoms (like constant fan operation) do not return.

Parts You'll Likely Need

• Engine Coolant Temperature Sensor (ECT) (OEM #SH01-18-840) — This sensor is the most common failure point for a P0118 code, as an internal open circuit directly causes the 'Circuit High' condition.
  Trusted brands: Mazda (OEM), NTK, Denso, Standard Motor Products, Duralast (SU4007)
  OEM price range: $50-$70
  Aftermarket price range: $25-$50

Related Codes That Often Appear With This One

• P0117 (Engine Coolant Temperature Sensor Circuit Low Input): The opposite code, which can sometimes appear intermittently if there is a short in the wiring.
• P011A (Engine Coolant Temperature Sensor 1/2 Correlation): May appear alongside P0118 if the PCM detects a major discrepancy between the two ECT sensors, though P011A is more commonly associated with a clogged heater core.

Technical Service Bulletins (TSBs) & Recalls

• 07-007/17 & 07-007/19: Discusses DTC P011A:00 (ECT sensor No.1/No.2 performance problem) related to poor heater performance on 2014-2019 Mazda 6 models, caused by a clogged heater core. The fix involves replacing the heater core and flushing the coolant twice with FL-22.

Platform-Specific Known Issues

• A Technical Service Bulletin (TSB 07-007/17, later superseded by 07-007/19) exists for 2014-2019 Mazda 6 models for a related code, P011A. This TSB addresses poor heater performance caused by a clogged heater core, which creates a temperature discrepancy between ECT Sensor 1 and ECT Sensor 2. While P0118 is a direct electrical fault, this TSB is relevant because it confirms the two-sensor setup and highlights the complexity of the cooling system. An independent mechanic might misdiagnose the issue as a thermostat if unaware of the TSB and the dual-sensor system.

Mechanic-Grade Diagnostic Values

• ECT Sensor Resistance vs. Temperature — expected: At 20°C (68°F): ~2,200 - 3,000 Ω. At 80°C (176°F): ~250 - 350 Ω. At 90°C (194°F): ~200 - 300 Ω.. Failure: Infinite resistance (OL) indicates an open internal circuit (the direct cause of P0118). Zero resistance indicates an internal short (which would cause P0117). A reading that doesn't change with temperature also indicates failure.
• ECT Sensor Connector Voltage (Key On, Engine Off) — expected: One pin should have a 5V reference from the PCM. The other is the sensor ground/signal return.. Failure: No voltage or voltage significantly different from 5V points to a wiring or PCM issue, not a sensor failure.
• PCM Fail-Safe Strategy (Live Data) — expected: When P0118 is active, the PCM ignores the sensor and substitutes a fixed default temperature value, often around 80°C (176°F) for engine control calculations.. Failure: This is not a failure indicator, but an important diagnostic note. If you see the ECT value locked at this temperature after startup, it confirms the PCM is in fail-safe mode due to the circuit fault.
• P0118 Trigger Condition — expected: The PCM will set code P0118 if it detects a sensor voltage above 4.9V for more than 5 seconds.. Failure: This high voltage is what the PCM interprets as an extreme cold temperature, like -40°F/°C.

Scan Tool Commands That Help

• Mazda M-MDS (or equivalent professional scanner): KOEO/KOER Self Test (Key On Engine Off / Key On Engine Running) — Used after a repair to verify the fix. Clearing the code and running the KOER self-test will confirm if the malfunction is gone without needing a full drive cycle.
• Mazda M-MDS: Access ECT PID — This is the live data parameter for the Engine Coolant Temperature. It is the primary tool for diagnosis, used to observe the faulty -40° reading and to confirm the fix by watching the temperature rise normally.

Wiring & Ground Locations

• ECT Sensor Connector (C119) — On the driver's side of the cylinder head, typically a 2-pin black or gray connector.. This is the most common point for wiring failure. Wires can break internally right at the connector body from strain and heat cycles, causing the open circuit that triggers P0118.
• PCM Connector Pin 1AK (SKYACTIV-G 2.0/2.5) — This is the specific pin on the Powertrain Control Module that receives the signal from ECT Sensor No. 1.. When diagnosing a persistent P0118 where the sensor is good, a continuity test must be performed from the sensor connector (harness side) to this pin to find an open in the wiring harness.
• Engine Ground Points — For the SKYACTIV-G engines, a primary ground point is located on the left side of the engine compartment. The sensor itself is grounded through the PCM, so the PCM's own ground connection is critical.. While a bad ground for the sensor circuit itself would typically cause a P0117 (low input), a poor main engine or PCM ground can cause floating voltages and unpredictable electrical issues, including a P0118 in rare cases.

Real Owner Repair Stories

• Mazda Forum User (2000 Mazda Protege 1.8L (Similar 2-wire ECT sensor circuit)) — Hard to start when cold, runs fine when warm. P0118 code present. Incorrect voltage reading at the sensor connector.
  ❌ Tried (didn't work) Replacing the ECT sensor, Replacing the ECU (PCM)
  ✅ What actually fixed it The user suspected a short in the wiring harness between the sensor and the PCM, as they were reading 12V at the connector instead of the expected 5V. The resolution, though not explicitly stated as 'fixed', points directly to running new wires as the final step after all other components were swapped without success.

OEM Part Supersession History

• SH01-18-840 → SH01-18-840 (current) — No supersession found. This part number appears to be consistent for the entire generation and its platform mates.
  Heads up: The same part number is used across the Mazda 3, 6, CX-3, CX-5, CX-9, and MX-5 from roughly 2016 onwards, indicating wide compatibility.

Model Year Variations Within This Range

• 2018-2021: The introduction of the 2.5T Skyactiv-G Turbo engine in 2018 did not change the primary ECT sensor part number or its function regarding code P0118. The diagnostic procedure remains the same for both the naturally aspirated and turbocharged 2.5L engines.

Diagnostic Flowchart

Other Known Issues on This Vehicle

Issues unrelated to this code that are worth knowing about as an owner of this generation:

• Infotainment System Glitches (Ghost Touch, Freezing, Rebooting) 🟠 Medium — Common across the generation. Can occur at any mileage. Often requires a software update or a hard reset (holding Nav+Back+Mute). A class action lawsuit was filed regarding these issues. (Ref: Multiple software updates have been released by dealers.)
• Carbon Buildup on Intake Valves (Skyactiv-G) 🟠 Medium — An inherent trait of all Gasoline Direct Injection (GDI) engines. Not a widespread problem causing failure on Mazda's Skyactiv engines, but can cause rough idle or reduced performance at higher mileage (80k+ miles).
• Prematurely Worn Wheel Bearings 🟠 Medium — A notable number of owners report noisy wheel bearings requiring replacement earlier than expected, sometimes under 60,000 miles.
• Warped Brake Rotors 🟡 Low — Some owners report that the OEM brake rotors are prone to warping, causing vibration during braking.
• Leaking Belt Tensioner 🟠 Medium — The hydraulic belt tensioner is known to leak oil over time, requiring replacement.
• Cracked/Leaking Valve Cover Gasket 🟡 Low — Can occur at higher mileage, leading to oil smells or visible leaks around the engine.

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: For this repair, a used part is only recommended for the **connector pigtail**. If the wiring is broken right at the connector, sourcing a used pigtail from a junkyard harness is an excellent and cost-effective repair method, often better than trying to re-pin the old connector.

Donor-vehicle mileage cap: roughly under 120000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• For a connector pigtail, ensure the plastic isn't brittle or cracked.
• Check that the locking tab clicks firmly.
• Inspect the wire insulation for at least 4-6 inches back from the connector; it should be flexible and free of cracks or oil contamination.

OEM-only on this vehicle (don't cheap out):

• While not strictly OEM-only, using a known-good brand for the sensor itself is critical. Cheap, unbranded sensors are a common source of repeat failures.

Aftermarket brands forum-validated for this vehicle:

• NTK
• Denso
• Standard Motor Products (SMP)

Brands owners have reported issues with on this vehicle:

• Avoid generic, no-name brands from online marketplaces, as their thermistor accuracy and longevity are often poor, leading to incorrect fuel trims even if they don't fail completely.

Real Owner Stories

Aggregated from forums and TSBs cited above. Mileages and costs reflect what owners reported in those sources.

Related OBD-II Codes

• P011A — A performance problem between ECT Sensor 1 and Sensor 2, often caused by a clogged heater core in 2014-2019 Mazda 6 models.
• P0117 — The opposite of P0118, representing a circuit low input, often involving the same ECT sensor (SH01-18-840) or wiring harness.
• P0128 — Coolant thermostat failure; often misdiagnosed in place of ECT sensor issues or the heater core TSB issues.
• P0113 — Intake Air Temperature (IAT) circuit high; the IAT is used as a comparison value to diagnose a faulty ECT sensor during cold starts.

Frequently Asked Questions