P0118 on 2004-2015 Scion xB: Engine Coolant Temp Sensor Causes and Fixes

What's Unique About the 2004-2015 Scion xB

The Scion xB was produced in two distinct generations with different engines, which is critical for this repair. The first generation (2004-2006) uses a 1.5L 1NZ-FE engine, while the asecond generation (2008-2015) has a 2.4L 2AZ-FE engine. These engines use different ECT sensors with unique part numbers and have them in different locations, so verifying your engine is the first step. Failures are almost always due to the sensor itself aging rather than a systemic design flaw.

Generation note: The 2004-2015 range covers two generations: - 1st Gen (2004-2006): Uses a 1.5L 1NZ-FE engine. The ECT sensor is located toward the center of the engine, behind the throttle body, and can be harder to access. The OEM part number is 89422-16010. 🎬 Watch: 1st Gen Scion xB thermostat and sensor replacement walkthrough. - 2nd Gen (2008-2015): Uses a 2.4L 2AZ-FE engine. The ECT sensor is on the side of the cylinder head near the upper radiator hose connection and is much easier to access. The OEM part number is 89422-33030.

Symptoms You May Notice

• Check Engine Light is on.
• Poor fuel economy because the engine runs in a rich fuel condition.
• Engine overheating, especially in traffic, because the cooling fans may not activate.
• Hard starting, particularly when the engine is warm.
• Rough idle or stalling.
• Black smoke from the exhaust due to the rich fuel mixture.
• Inaccurate or non-functional temperature gauge on the dashboard.

Most Likely Causes

1. Failed Engine Coolant Temperature (ECT) Sensor 🔴 High Probability This is a standard wear-and-tear item. After many years and heat cycles, the internal thermistor fails, causing an open circuit which results in a high voltage signal.
   How to confirm: With a scan tool, check the live data for the ECT sensor with the engine cold. If it reads an absurdly low temperature (e.g., -40°F), the sensor is bad. You can also test the sensor's resistance with a multimeter; an open circuit (infinite resistance) confirms failure. A good sensor should have resistance that changes with temperature. 🎬 Watch: How to test for ECT fault codes with a multimeter.
   Typical fix: Replace the ECT sensor. It's a small, threaded sensor that screws into the engine's coolant passages. It's wise to also replace the O-ring or gasket at the same time.
   Est. part cost: $20-$60
2. Damaged Wiring or Connector 🟡 Medium Probability The wiring harness and connector can become brittle from age and engine heat, leading to cracks, corrosion, or a loose connection. An open circuit in the wiring will produce the same P0118 code as a failed sensor. Wires can also chafe against other components, like radiator hoses, causing a break in the circuit.
   How to confirm: Visually inspect the wiring leading to the ECT sensor for any signs of damage, melting, or corrosion in the connector pins. With the connector unplugged and key on, you can check for 5V reference and ground at the connector with a multimeter. Wiggle the harness while watching live data on a scan tool to see if the reading fluctuates, indicating a loose connection.
   Typical fix: Repair the damaged section of wire or replace the connector pigtail.
   Est. part cost: $10-$25

Rare But Worth Checking

• Low Engine Coolant Level / Air Pocket: If the coolant level is so low that the sensor is exposed to air instead of coolant, it can't read the temperature correctly. This usually happens after a coolant service if the system wasn't bled properly. You would likely have other overheating symptoms as well. One owner on a forum reported getting a P0118 after a thermostat change, suggesting an air pocket could be the cause.
• Faulty Engine Control Module (ECM): This is extremely rare. The ECM itself could be failing to read the sensor input correctly. This should only be considered after all other possibilities (sensor, wiring, coolant level) have been exhaustively ruled out.

Diagnosis Steps

1. Check Live Data: Use an OBD-II scanner to view live data. With a cold engine (off for several hours), the ECT reading should be close to the ambient air temperature. If it shows an extremely low value (like -40°F), the sensor has failed or the circuit is open.
2. Inspect the Connector and Wiring: Disconnect the sensor and inspect the electrical connector and nearby wiring for any signs of corrosion, damage, or loose pins. Pay close attention to where the harness might rub against other engine components.
3. Test the Sensor: You can test the sensor itself with a multimeter set to Ohms. A good sensor will have a specific resistance at a given temperature (refer to a service manual for specs). An infinite resistance reading means the sensor is open and bad.
4. Verify Reference Voltage: With the key on and engine off, check the electrical connector (vehicle side). One pin should have a 5-volt reference signal from the ECM, and the other should be a ground. If either is missing, you have a wiring or ECM issue.
5. Check Coolant Level: Ensure the coolant level is full and there are no air pockets in the system. An air pocket around the sensor can cause incorrect readings, especially after recent cooling system work.
6. Replace the Sensor: If the sensor is confirmed bad, replace it. Be prepared for a small amount of coolant to spill. It's best to do this on a cold engine.

Parts You'll Likely Need

• Engine Coolant Temperature Sensor (OEM #89422-16010 (1.5L) / 89422-33030 (2.4L)) — This sensor is the most common failure point for a P0118 code. It directly measures the coolant temperature and is prone to failure from age and heat cycles.
  Trusted brands: Denso, NTK, Standard Motor Products
  OEM price range: $50-$80
  Aftermarket price range: $20-$45

Related Codes That Often Appear With This One

• P0117 — P0117 is 'ECT Circuit Low Input'. Seeing both codes intermittently could point to a severe wiring issue or a failing sensor that is fluctuating between an open and a short.
• P0128 — P0128 means 'Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature)'. If the ECT sensor is providing faulty data, the ECM may incorrectly believe the engine isn't warming up properly, which can trigger this code as well.

Technical Service Bulletins (TSBs) & Recalls

• No Technical Service Bulletins (TSBs) were found for this specific trouble code (P0118) on the Scion xB. However, several TSBs exist for other platform issues.

Platform-Specific Known Issues

• Owner Experience with Persistent P0118: A 2004 Scion xB owner on RepairPal reported a recurring P0118 code even after replacing the sensor, water pump, thermostat, and radiator. The issue, which caused the car to shut down while driving, pointed towards an intermittent wiring harness problem that was difficult to diagnose, as a simple meter check on the wiring seemed fine initially. This highlights the importance of checking for intermittent open circuits in the harness, not just a complete failure.

Mechanic-Grade Diagnostic Values

• ECT Sensor Resistance (Cold) — expected: 2,200 - 3,000 Ω (2.2k - 3.0k Ω) at 20°C (68°F). Failure: Infinite resistance (Open Loop) or significantly outside this range. This is the most common test.
• ECT Sensor Resistance (Hot) — expected: 250 - 350 Ω at 80°C (176°F). Failure: Resistance does not drop as the engine warms, or is wildly different from this value.
• ECT Sensor Connector Voltage (Key On, Engine Off, Sensor Unplugged) — expected: One pin should show ~5V (Reference from ECM), the other should show near 0V (Sensor Ground).. Failure: No voltage on the reference pin indicates a wiring or ECM problem. P0118 is specifically triggered when the signal wire *reads* high voltage, which happens when this 5V reference has no path to ground (i.e., an open circuit).
• ECT Live Data on Scan Tool (Circuit Open) — expected: -40°C (-40°F). Failure: This reading confirms the ECM is seeing an open circuit, which is the condition for P0118.
• ECM Fault Trigger Voltage — expected: Voltage at the ECM from the ECT sensor is > 4.91V for 0.5 seconds or more.. Failure: This is the specific threshold the Toyota ECM uses to set the P0118 code.

Scan Tool Commands That Help

• Toyota Techstream: Data List > All Data > Coolant Temp — Use this to view the live temperature the ECM is reading. With the engine cold, it should match ambient air temp. If it reads -40°F/°C, it confirms an open circuit fault. When the engine is warm, it should read between 80-100°C (176-212°F).
• Toyota Techstream: Utility > All Readiness — After a repair, you can use this function to check the status of the diagnostic monitor for the ECT circuit to confirm the fix without waiting for a full drive cycle.

Wiring & Ground Locations

• Engine Ground (2nd Gen) — Under the factory air box, on top of the transmission case, just behind the starter.. A poor main engine ground can cause fluctuating voltages and incorrect readings on various sensors that use the engine block as a ground path, potentially leading to intermittent sensor codes.
• ECT Sensor Connector (2AZ-FE) — On the cylinder head, near the upper radiator hose.. This is the primary point of failure for wiring issues. The two wires are THW (signal) and ETHW (sensor ground), which both run directly to the ECM.
• ECM / ECU Location — Beneath the passenger side glovebox. A plastic cover must be unsnapped to gain access.. In very rare cases of persistent issues after replacing the sensor and checking the harness, the pins at the ECM connector itself could be corroded or loose.

Real Owner Repair Stories

• ScionLife.com user (2012 Scion xB (2nd Gen)) — Vehicle would not start.
  ❌ Tried (didn't work) Replaced battery, Replaced alternator, Replaced battery ground cable
  ✅ What actually fixed it The main engine-to-chassis ground cable was the actual problem. It is located under the airbox on top of the transmission. Cleaning/replacing this ground resolved the no-start condition. While not a P0118 story, it highlights the location and importance of a critical ground that can cause widespread electrical issues.

OEM Part Supersession History

• 89422-16010 → No official supersession found. This part number remains active for the 1.5L 1NZ-FE engine. — N/A
• 89422-33030 → No official supersession found. This part number remains active for the 2.4L 2AZ-FE engine and many other Toyota models. — N/A

Diagnostic Flowchart

Other Known Issues on This Vehicle

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

• Excessive Oil Consumption (2.4L 2AZ-FE) 🔴 High — Very common on 2008-2015 models. Can start as early as 60,000 miles. Caused by faulty piston ring design. (Ref: Toyota issued a Warranty Enhancement Program (ZE7) and TSB S-SB-0024-11R2 (later superseded) to address this. The repair involves replacing pistons and rings.)
• Broken/Cracked Rear Hatch Handle 🟡 Low — Extremely common across both generations (2004-2015). The plastic handle becomes brittle and breaks off with normal use. (Ref: Scion issued a warranty enhancement (ZTW) and service bulletin S-SB-0012-13 to cover replacements for a limited time.)
• Oil Consumption (1.5L 1NZ-FE) 🟠 Medium — Common on the first generation (2004-2006), especially at higher mileage. Often attributed to stuck piston rings, but less severe and not covered by the same TSB as the 2AZ-FE.
• Check Engine Light for VVT-i Controller (1st Gen) 🟠 Medium — A widely reported issue on 2004-2006 models where the VVT-i controller (solenoid) fails or gets clogged, causing rough running and a CEL.
• Squealing Brakes in Reverse (1st Gen) 🟡 Low — A common complaint on 2004-2006 models with rear drum brakes. Often caused by an accumulation of rust or brake dust inside the drum.

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: For the connector pigtail ONLY. If the original connector is cracked or the locking tab is broken, sourcing a used pigtail from a junkyard harness is a cost-effective and reliable repair. Do NOT buy a used ECT sensor itself.

Donor-vehicle mileage cap: roughly under 150000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• For a connector pigtail, ensure the plastic is not brittle or cracked.
• Check that the locking tab still clicks firmly.
• Inspect the wire insulation for at least 4-6 inches back from the connector to ensure it's not cracked or peeling.
• Look for any signs of green corrosion on the metal pins inside the connector.

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

• While not strictly OEM-only, using a known quality brand for the sensor is critical. Avoid generic, unbranded sensors.

Aftermarket brands forum-validated for this vehicle:

• Denso (often the Original Equipment Manufacturer)
• NTK
• Standard Motor Products (SMP)

Brands owners have reported issues with on this vehicle:

• Generic, no-name sensors from online marketplaces (e.g., eBay, Amazon) that lack brand identification. These often have poor accuracy and a high failure rate.

Real Owner Stories

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

Related OBD-II Codes

• P0117 — The direct opposite of P0118. P0117 indicates a circuit low input (short circuit), while P0118 indicates a high input (open circuit). Both point to a failure of the same sensor or wiring.
• P0172 — System Too Rich (Bank 1). The ECM interprets the P0118's high voltage signal as an extremely cold engine (e.g., -40°F) and commands a very rich fuel mixture, which can trigger this code.
• P0116 — Engine Coolant Temperature Sensor Circuit Range/Performance. This code can be set if the sensor's reading is irrational or doesn't change as expected, which can happen with an intermittent wiring issue that also causes P0118.
• P0128 — Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temperature). While P0118 is an electrical fault, it can be confused with or occur after work on the cooling system, like a thermostat replacement, which can also trigger P0128 if an air pocket is introduced.
• P0300 — Random/Multiple Cylinder Misfire Detected. The severe rich fuel condition caused by the ECM's reaction to a P0118 fault can lead to fouled spark plugs and misfires.
• P0115 — Engine Coolant Temperature Sensor Circuit Malfunction. This is a more generic fault code for the ECT circuit and may be stored if the signal is erratic before it fails completely into a high-input (P0118) or low-input (P0117) state.

Frequently Asked Questions