P0117 on 2005-2016 Scion tC: Engine Coolant Sensor Circuit Low Causes and Fixes

What's Unique About the 2005-2016 Scion tC

On both the first-generation (2AZ-FE) and second-generation (2AR-FE) Scion tC, the P0117 code is a very common and straightforward issue. The Engine Coolant Temperature (ECT) sensor is a simple, accessible part that fails with age. There are no major platform-specific defects that cause this code; it's typically a standard wear-and-tear component failure. The OEM supplier is Denso, making it a reliable choice for replacement, and the part number 89422-33030 is shared across both tC generations and many other Toyota vehicles.

Generation note: This guide covers two generations of the Scion tC: - First Generation (2005-2010) with the 2.4L 2AZ-FE engine. The ECT sensor is located on the side of the cylinder head near the thermostat housing. A 19mm deep socket is typically required for removal. 🎬 Watch: First-gen Scion tC sensor location and replacement guide. - Second Generation (2011-2016) with the 2.5L 2AR-FE engine. The ECT sensor is on the side of the engine near the top, with a gray harness connector, and is very easy to access. 🎬 See this quick video for the 2011-2016 sensor location. The cause and symptoms are identical for both, but the sensor part number and exact location differ slightly. The same OEM part number 89422-33030 is used for both.

Symptoms You May Notice

• Check Engine Light is on
• Inaccurate or non-functional temperature gauge (may be stuck on cold or hot)
• Engine may be hard to start, especially when cold
• Rough idle
• Poor fuel economy.
• Black smoke from the exhaust due to a rich fuel mixture.
• Cooling fans running constantly, even when the engine is cold.

Most Likely Causes

1. Defective Engine Coolant Temperature (ECT) Sensor 🔴 High Probability → Shop Engine Coolant Temperature Sensor The sensor is a simple thermistor that wears out over time and with heat cycles, which is a common failure on most vehicles, including the tC.
   How to confirm: Use a scan tool to read live data. If the ECT reading is an extreme value (e.g., -40°F or 280°F+) when the engine is at a normal temperature, the sensor is likely bad. You can also test the sensor's resistance with a multimeter; at an ambient temperature of 68°F (20°C), the resistance should be between 2,000 and 3,000 ohms. If the reading is outside this range, the sensor has failed.
   Typical fix: Replace the Engine Coolant Temperature sensor.
   Est. part cost: $20-$60
2. Damaged Wiring or Connector 🟡 Medium Probability Wires can become brittle from engine heat over time, or damage can occur during other engine repairs. The ECT sensor circuit is a simple two-wire connection, but a short to ground on the signal wire will cause the P0117 code.
   How to confirm: Visually inspect the wiring harness and connector for the ECT sensor. Look for breaks, corrosion, or melted insulation. Check for a short to ground in the signal wire using a multimeter. A user on Reddit with a 2009 tC confirmed their P0117 was caused by damaged wiring found during a shop repair.
   Typical fix: Repair the damaged section of the wire or replace the connector pigtail.
   Est. part cost: $5-$25
3. Low Engine Coolant or Air in the System ⚪ Low Probability → Shop Engine Coolant / Antifreeze This is not a platform-specific issue but a general cooling system problem. If coolant is low, the sensor may be exposed to an air pocket, giving an inaccurate reading which can sometimes manifest as a circuit code.
   How to confirm: Check the coolant level in the radiator (when the engine is cold) and the overflow reservoir. If the system was recently serviced (e.g., thermostat or radiator replacement), it may need to be 'burped' to remove trapped air. A sloshing noise from the dashboard area upon startup is a common sign of air in the system.
   Typical fix: Top off the coolant and bleed the air from the cooling system. Find and repair any leaks.
   Est. part cost: $15-$30 for coolant

Rare But Worth Checking

• Faulty Powertrain Control Module (PCM): → Shop Engine Control Module (ECM) This is extremely rare. The PCM should only be considered after all other possibilities, including the sensor, wiring, and coolant system, have been thoroughly tested and ruled out.

Diagnosis Steps

1. Verify the Check Engine Light is on and code P0117 is present using an OBD-II scanner.
2. Using the scanner's live data function, observe the Engine Coolant Temperature reading. If it shows an illogical value (e.g., -40°F or 280°F+) with a cold engine, the sensor or circuit is faulty.
3. Turn the engine off. Visually inspect the ECT sensor and its electrical connector for any signs of damage, corrosion, or loose connections.
4. Disconnect the sensor. The scan tool reading should change to a different default value (often triggering P0118, Circuit High). If the reading does not change, a short to ground in the wiring is likely.
5. If the wiring appears intact, test the sensor. Remove it from the engine and use a multimeter set to Ohms. At 68°F, the resistance should be 2k-3k Ohms. You can also place it in hot water and watch for a smooth decrease in resistance.
🎬 Watch: How to test and replace the ECT sensor.
6. Check the coolant level in the radiator and overflow tank to ensure it's full. If it's low, top it off and check for leaks. Ensure no air is trapped in the system.
7. If the sensor and wiring test good, and the coolant is full, the problem may be a rare internal issue with the PCM.

Parts You'll Likely Need

• Engine Coolant Temperature Sensor (OEM #89422-33030) — This is the most common failure point for code P0117. The sensor's internal thermistor fails, sending an incorrect signal to the computer.
  Trusted brands: Denso (OEM), Aisin, Standard Motor Products
  OEM price range: $50-$80
  Aftermarket price range: $20-$50

Related Codes That Often Appear With This One

• P0118 — This is the opposite code, for 'Circuit High'. It can sometimes appear if you disconnect the sensor to test the circuit, or if the wiring has an intermittent short/open.

Platform-Specific Known Issues

• Persistent Code After Multiple Part Replacements: A user on ScionLife.com with a Scion xB (which shares the 2AZ-FE engine) reported a persistent P0117 and overheating condition even after replacing the ECT sensor, radiator, thermostat, and hoses. This indicates that in some rare cases, the code can be a symptom of a more complex issue like a failing water pump or even a head gasket problem, which requires more in-depth diagnostics.

Mechanic-Grade Diagnostic Values

• ECT Sensor Resistance vs. Temperature — expected: 2,200 - 3,000 Ω at 20°C (68°F); 250 - 350 Ω at 80°C (176°F); 180 - 250 Ω at 100°C (212°F).. Failure: Readings significantly outside these ranges, or an open/shorted reading (infinite/zero ohms).
• ECT Sensor Signal Voltage at ECM (2AZ-FE Engine) — expected: 0.2V to 1.0V with engine at normal operating temperature (80°C / 176°F).. Failure: Voltage below ~0.14V triggers the P0117 code. A reading near 5V indicates an open circuit (P0118).
• Sensor 5V Reference Voltage (VC) — expected: 4.5V to 5.5V at the sensor connector with key on, engine off, sensor unplugged.. Failure: No voltage indicates a wiring problem or an issue from the ECM.
• ECT Circuit Short to Ground Test — expected: Infinite resistance (O.L.) between the signal wire and chassis ground when the sensor and ECM are disconnected.. Failure: Low resistance (e.g., under 100 ohms) indicates a short to ground in the harness.

Scan Tool Commands That Help

• Techstream or professional OBD-II scanner: Live Data / Data Stream for ECT Sensor — This is the primary diagnostic step. With the engine cold, the ECT reading should be very close to the Intake Air Temperature (IAT) reading. A reading of -40°F or 280°F+ on a cold engine immediately points to a circuit or sensor fault.

Wiring & Ground Locations

• ECT Sensor Connector (2AZ-FE) — On the side of the cylinder head, near the thermostat housing.. This is where you test for voltage and resistance. For the 2007 tC, Pin E8-32 (THW, Green wire) is the signal wire, and Pin E8-28 (E2, Red wire) is the sensor ground return to the ECM.
• ECT Sensor Connector (2AR-FE) — On the side of the engine near the top, with a gray harness connector.. This is the physical connection point for testing. It connects to the ECM via connector C4.
• Ground Point EG / EH (2AZ-FE) — EG is on the left side of the cylinder head; EH is on the intake side of the cylinder block.. While the ECT sensor has a dedicated ground return to the ECM (E2), the ECM itself relies on these main engine grounds for a stable reference. A corroded or loose main ground can cause erratic behavior in multiple sensor circuits.

OEM Part Supersession History

• 89422-33030 → No supersession noted. This part number remains current. — N/A
  Heads up: This part number is used across a vast range of Toyota, Lexus, and Scion vehicles from 2003 to 2024, including both generations of the Scion tC, indicating wide compatibility.

Diagnostic Flowchart

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: A used wiring pigtail from a junkyard is a smart choice if your original connector is cracked, melted, or has corroded pins. The sensor itself is a wear item and inexpensive, so buying a new sensor is always the recommended path.

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 wiring pigtail, inspect for brittle or cracked plastic on the connector body.
• Ensure the wire insulation is flexible and not cracking.
• Check that the terminal pins inside the connector are clean, straight, and free of green or white corrosion.

Aftermarket brands forum-validated for this vehicle:

• Denso (OEM supplier)
• Aisin
• Standard Motor Products (SMP)

Brands owners have reported issues with on this vehicle:

• Unbranded, ultra-low-cost sensors from online marketplaces often have poor quality control and may fail prematurely or provide inaccurate readings out of the box.

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 companion code for Engine Coolant Temperature Sensor Circuit High. It is often triggered intentionally during diagnosis by disconnecting the sensor to see if the circuit changes from a shorted state (P0117) to an open state (P0118), which helps confirm the wiring is intact to the computer.
• P0172 — Often appears with P0117. The faulty sensor tells the ECU the engine is extremely cold (e.g., -40°F), causing it to inject far too much fuel. This creates a rich condition that can trigger a P0172 (System Too Rich) code.
• P0300 — A prolonged rich fuel mixture caused by P0117 can foul the spark plugs with carbon and unburnt fuel. This can lead to incomplete combustion and trigger P0300 (Random/Multiple Cylinder Misfire Detected) or specific cylinder misfire codes (P0301-P0304).
• P0480 — This code for 'Fan 1 Control Circuit' can be related because a common symptom of P0117 is the cooling fans running constantly. The ECU activates the fans as a fail-safe measure when it gets an invalid temperature reading. While P0117 is the cause, a related fault in the fan circuit itself could also be present.

Frequently Asked Questions