P0117 on 1997-2001 Lexus ES300: Engine Coolant Temperature Sensor Circuit Low Causes and Fixes

What's Unique About the 1997-2001 Lexus ES300

On this generation of Lexus ES300 with the 1MZ-FE engine, it's crucial to distinguish between two different temperature sensors located on the coolant filler neck. The code P0117 specifically refers to the two-wire Engine Coolant Temperature (ECT) sensor that sends information to the engine computer. This sensor typically has a green plastic top. There is a separate, single-wire sensor nearby that operates the temperature gauge on the dashboard. Owners frequently confuse these two parts; replacing the single-wire gauge sender will not fix a P0117 code.

Symptoms You May Notice

• Check Engine Light is on
• VSC (Vehicle Stability Control) light may also illuminate.
• Poor fuel economy.
• Engine runs rough, especially when cold
• Hard starting or may start and then die.
• Black smoke from the exhaust
• Cooling fans run continuously, even when the engine is cold.
• Engine may overheat if the fans fail to activate correctly.
• A/C may blow warm air as the system tries to reduce engine load.
• In some cases, the dashboard temperature gauge may behave erratically or drop to 'C' (Cold) as a fail-safe indication, even though the code indicates a high temperature condition.

Most Likely Causes

1. Failed Engine Coolant Temperature (ECT) Sensor 🔴 High Probability → Shop Engine Coolant Temperature Sensor The ECT sensor is a thermistor that degrades over 20+ years of heat cycles. It's a very common failure point on the 1MZ-FE engine and related Toyota/Lexus models of this era.
   How to confirm: With the engine cold, use a scan tool to view live data. If the coolant temperature reads an abnormally high value (e.g., 250°F or higher), the sensor has internally shorted. You can also test the sensor's resistance with a multimeter. 🎬 Watch: How to test and replace the ECT sensor At 68°F (20°C), it should be approximately 2,000-3,000 Ohms. At operating temperature of 176°F (80°C), it should be around 200-300 Ohms. A reading near zero ohms indicates a dead short.
   Typical fix: Replace the two-wire ECT sensor (with the green connector). A 19mm deep socket is typically required. It's wise to apply thread sealant to the new sensor's threads and install a new gasket to prevent leaks.
   Est. part cost: $20-$60
2. Damaged Wiring or Connector 🟡 Medium Probability Engine bay heat and vibrations can cause the 20+ year old wiring to become brittle and crack, leading to a short circuit. The connector itself can also become corroded or the locking tab may break, causing an intermittent connection.
   How to confirm: Visually inspect the wiring harness leading to the ECT sensor for any signs of cracking, fraying, or melting. Check the connector pins for corrosion. Wiggle the connector and wiring with the engine running and live data displayed to see if the temperature reading fluctuates wildly. A persistent P0117 even with the sensor unplugged points to a short to ground in the signal wire.
   Typical fix: Repair the damaged section of wire or replace the pigtail connector. The connector can be difficult to release due to age and location; needle-nose pliers may be helpful.
   Est. part cost: $10-$30

Rare But Worth Checking

• Faulty Engine Control Module (ECM): → Shop Engine Control Module (ECM) This is extremely rare. The ECM should only be considered after the sensor and wiring have been thoroughly tested and proven to be good.
• Air Pocket in the Cooling System: If the coolant level is very low or if air is trapped near the sensor after a coolant service, it can't get an accurate reading. This usually causes other symptoms like overheating and gurgling sounds from the reservoir. Properly 'burping' the cooling system after service is critical.
• Actual Engine Overheating: While the code indicates a circuit fault, a severe overheating event can sometimes damage the sensor or wiring, causing the P0117 code to appear simultaneously with the mechanical issue.

Diagnosis Steps

1. Connect an OBD-II scanner and confirm P0117 is the active code. 🎬 See this walkthrough for diagnosing and fixing P0117 codes Note any other codes.
2. View the live data stream for the Engine Coolant Temperature (ECT). With a cold engine, the reading should be close to the ambient air temperature. If it reads a very high, fixed temperature (like 275°F or 135°C), the sensor or its circuit is shorted.
3. Turn the engine off and let it cool completely. WARNING: Never open a hot cooling system.
4. Locate the two-wire ECT sensor. On the 1MZ-FE engine, it is located on the coolant filler neck housing, between the cylinder heads, near the passenger side of the engine. It has a green electrical connector.
5. Inspect the sensor's connector and wiring for any visible damage, corrosion, or loose connections.
6. Disconnect the sensor. With the key on (engine off), check the live data again. The temperature reading should now show a very low value (like -40°F), and a P0118 code may appear. This test confirms the wiring to the ECM is likely okay and the sensor itself is the problem.
7. If the high-temperature reading persists even with the sensor disconnected, inspect the wiring harness for a short to ground between the sensor connector and the ECM.
8. If desired, test the old sensor's resistance with a multimeter. It should be around 2000-3000 ohms at room temperature (approx. 70°F). A reading near zero confirms it has failed.
9. If the wiring and sensor test good, the final, though unlikely, possibility is a fault within the ECM.

Parts You'll Likely Need

• Engine Coolant Temperature Sensor (for ECM) (OEM #89422-35010) — This sensor is the most common cause of a P0117 code due to internal failure from age and heat cycles. It provides the critical temperature reading to the engine computer.
  Trusted brands: Denso (OEM supplier), NGK/NTK, Delphi
  OEM price range: $45-$70
  Aftermarket price range: $20-$40

Related Codes That Often Appear With This One

• P0118 — P0118 is 'ECT Circuit High Input', the opposite of P0117. Seeing them together or intermittently could point to a loose connection, damaged wiring, or internal sensor failure where the circuit alternates between open and shorted.
• P0115 — P0115 is a general 'ECT Circuit Malfunction' code. It can appear alongside P0117 if the sensor's readings are erratic or nonsensical to the ECM.

Platform-Specific Known Issues

• Sensor Location and Access: The ECT sensor is located on the coolant water neck/filler housing between the V of the engine. While accessible from the top, it is somewhat buried under the air intake snorkel and other components. Removing the air intake assembly provides much better access for the replacement.
• Coolant Loss During Replacement: → Shop Engine Coolant / Antifreeze When the sensor is unthreaded, a small amount of coolant will leak out. This is normal. Have the new sensor ready to install quickly to minimize loss. Afterwards, top off the coolant and properly 'burp' the system to remove any air that was introduced.

Mechanic-Grade Diagnostic Values

• ECT Sensor Resistance vs. Temperature — expected: At 20°C (68°F): ~2,000-3,000 Ω. At 80°C (176°F): ~200-300 Ω. At 100°C (212°F): ~180-250 Ω.. Failure: A resistance reading near zero ohms indicates a short circuit. An infinite or extremely high reading indicates an open circuit.
• ECT Sensor Signal Voltage at ECM (THW pin) — expected: Cold engine (~20°C): ~3.0-3.5V. Warm engine (~80°C): ~1.0-1.3V.. Failure: Voltage below 0.14V for more than 0.5 seconds triggers P0117.
• Scan Tool Live Data (Coolant Temp) with Short Circuit — expected: A fixed, maximum value, such as 135°C (275°F) or higher.. Failure: The temperature reading does not change and is pegged at an impossibly high value, even with a cold engine.
• Scan Tool Live Data (Coolant Temp) with Open Circuit — expected: A fixed, minimum value, such as -40°C (-40°F).. Failure: This reading will appear if the sensor is disconnected and confirms the wiring to the ECM is likely intact.

Scan Tool Commands That Help

• Toyota Techstream: Data List -> Coolant Temp — This is the primary function used to diagnose the ECT circuit. By observing the live temperature reading, a technician can instantly determine if the circuit is open (reads -40°C), shorted (reads >135°C), or providing a plausible but incorrect reading.

Wiring & Ground Locations

• ECT Sensor Pins at ECM — At the main Engine Control Module (ECM), typically located behind the glove compartment.. The two wires from the ECT sensor connect to specific pins on the ECM. The signal wire connects to pin THW, and the ground reference wire connects to pin E2. Testing for voltage at the THW pin or checking for continuity from the sensor connector to these pins can isolate a wiring fault from a sensor or ECM fault.
• Ground Point EC / ED — On the 1MZ-FE, ground point 'EC' is on the right-hand side of the surge tank (intake manifold), and 'ED' is on the rear side of the surge tank.. The engine wiring harness, which includes the ECT sensor circuit, relies on clean engine-to-chassis ground connections. While the ECT sensor grounds through the ECM (E2 pin), a poor main engine ground can introduce electrical noise and potential voltage offsets, leading to erratic sensor behavior. These are the closest major ground points to the sensor's harness.

Real Owner Repair Stories

• ClubLexus Forum User (Lexus RX (with same 1MZ-FE engine family)) — Check Engine Light and VSC light on, A/C blowing warm air, engine temperature gauge rose to the red line, small amount of steam/smoke from the front of the engine bay.
  ❌ Tried (didn't work) Letting the car cool down temporarily helped but the issue returned.
  ✅ What actually fixed it Replacing the two-wire Engine Coolant Temperature (ECT) sensor resolved the P0117 code and all associated symptoms.

OEM Part Supersession History

• 89422-35010 → Still current, but often cross-references with older numbers. — N/A
  Heads up: This part number is widely used, but may be cross-referenced with 89422-20010 or 89422-30030. Ensure the replacement has the correct green two-pin connector for the ECM, not the single-pin connector for the gauge sender.

Model Year Variations Within This Range

• 1999-2001: The 1MZ-FE engine was updated with VVT-i (Variable Valve Timing with intelligence) starting in the 1999 model year. While the ECT sensor and its function remain the same, it reports to a more complex ECM designed to manage the VVT-i system. The ECU part numbers and some wiring harness routing differ between the 1997-1998 non-VVT-i and 1999-2001 VVT-i models.

Diagnostic Flowchart

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: For a low-cost electronic sensor like the ECT, buying a used part is generally not recommended. The primary failure mode is degradation from age and heat cycles, so a used part has unknown and likely limited remaining life. It should only be considered as a temporary, emergency fix to get the vehicle running.

Donor-vehicle mileage cap: roughly under 80000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• Connector plastic is not brittle or cracked.
• Electrical pins are clean, straight, and free of corrosion.
• Part comes from a vehicle that was not in a front-end collision or engine fire.

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

• Engine Control Module (ECM)

Aftermarket brands forum-validated for this vehicle:

• Denso (OEM supplier)
• NGK / NTK

Brands owners have reported issues with on this vehicle:

• Unbranded, 'white-box' parts from online marketplaces with no warranty or brand reputation.

Real Owner Stories

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

Related OBD-II Codes

• P0118 — This is the direct opposite of P0117. P0118 (ECT Circuit High Input) indicates an open circuit, often caused by an unplugged sensor or broken wire. It may appear during diagnosis if you unplug the sensor to test the circuit, which is a valid troubleshooting step.
• VSC/TRAC SYSTEM CODES (E.G., C1201) — The Vehicle Stability Control (VSC) light often illuminates along with the Check Engine Light for a P0117 code. This is because the Engine Control Module (ECM) detects a critical sensor failure and disables the VSC system as a fail-safe precaution. The P0117 is the root cause, and the VSC light is a secondary effect.

Frequently Asked Questions