P0118 on 2012-2017 Toyota Prius v: Engine Coolant Temperature Sensor Causes and Fixes

What's Unique About the 2012-2017 Toyota Prius v

The Toyota Prius hybrid system is highly dependent on accurate engine temperature data to manage the switch between the gasoline engine and the electric motor. A P0118 code will typically prevent the vehicle from entering its fuel-saving EV (electric vehicle) mode because the system believes the engine is too cold to shut off. It's also critical to know that the 2ZR-FXE engine in this Prius has two coolant temperature sensors; the one that triggers P0118 is the main sensor located in the cylinder head, not the secondary sensor used for the exhaust heat recovery system. This is a very common point of confusion, and replacing the wrong sensor will not fix the code.

Symptoms You May Notice

• Check Engine Light is on
• Vehicle will not enter EV mode
• Noticeably reduced fuel economy
• Radiator fans may run constantly or not at all
• High temperature warning light may flash on the dash
• Engine may hesitate or run rough until it warms up
• Black smoke from the exhaust due to a rich fuel mixture

Most Likely Causes

1. Failed Engine Coolant Temperature (ECT) Sensor 🔴 High Probability → Shop Engine Coolant Temperature Sensor The sensor is a thermistor that can fail internally, creating an open circuit. This is the most common failure point for this code on most vehicles, including the Prius v.
   How to confirm: With a scan tool, check the live data for the coolant temperature. If it reads -40°F or a similarly low, fixed number even when the engine is warm, the sensor is almost certainly bad. You can also test the sensor's resistance with a multimeter; it should be between 2,000 and 3,000 ohms at room temperature (around 68°F/20°C). An open circuit (infinite resistance) confirms a failed sensor.
   Typical fix: Replace the Engine Coolant Temperature sensor. It is located on the driver's side of the engine, under the EGR cooler assembly. This often requires partially draining the coolant and removing or shifting the EGR cooler for access, which may require new gaskets.
   Est. part cost: $25-$70
2. Wiring or Connector Issue 🟡 Medium Probability The sensor's location can make its wiring susceptible to damage or being left unplugged during other engine repairs, especially head gasket replacement or EGR system service. The connector itself can also become loose, corroded, or filled with debris.
   How to confirm: Visually inspect the wiring harness and the connector going to the ECT sensor for any signs of damage, corrosion, or loose pins. With the sensor unplugged, use a jumper wire to short the two terminals in the harness connector; the temperature reading on a scan tool should jump to a very high value (e.g., 250°F+). If it does, the wiring and ECM are good, pointing back to the sensor. If the reading does not change, the wiring is faulty.
   Typical fix: Repair the damaged section of wiring or clean/replace the connector pigtail.
   Est. part cost: $5-$30

Rare But Worth Checking

• Faulty Engine Control Module (ECM): This is extremely rare. The ECM should only be considered after all other possibilities, including the sensor and its entire wiring circuit, have been thoroughly tested and ruled out.

Diagnosis Steps

1. Connect an OBD-II scanner and confirm the P0118 code is present. Check for any other codes.
2. Using the scanner's live data function, view the Engine Coolant Temperature reading. If it shows -40°F or a similar, static low number, this strongly suggests an open circuit.
3. Turn the vehicle off and locate the ECT sensor on the driver's side of the engine, underneath the EGR cooler.
4. Inspect the sensor's electrical connector and wiring for any visible damage, corrosion, or looseness. This is a common failure point after other engine service.
5. If the visual inspection is clear, disconnect the connector. Check for 5V reference voltage on one pin and a good ground on the other with the key in the 'ON' position.
6. If voltage and ground are present, the issue is likely the sensor itself. Consider testing the old sensor's resistance with a multimeter before replacing.
7. If voltage or ground is missing, there is a fault in the wiring harness between the sensor and the ECM that needs to be traced and repaired.
8. After repair (e.g., replacing the sensor), clear the code with the scanner and drive the vehicle to ensure the fix was successful and that the car can enter EV mode.

Parts You'll Likely Need

• Engine Coolant Temperature Sensor (OEM #89422-33030) — This sensor is the primary cause of P0118. It fails internally, creating the open circuit that the ECM detects as a high voltage input.
  Trusted brands: Denso (OEM Manufacturer), Toyota (OEM)
  OEM price range: $50-$70
  Aftermarket price range: $20-$40
• EGR Cooler Gasket / EGR Pipe Gasket (OEM #25634-37010 (Cooler to Head), 25628-37010 (Pipe to Cooler)) — Accessing the ECT sensor requires moving the EGR cooler. It is highly recommended to replace these single-use gaskets to prevent coolant or exhaust leaks upon reassembly.
  Trusted brands: Toyota (OEM)
  OEM price range: $5-$15 each
  Aftermarket price range: $3-$10 each

Related Codes That Often Appear With This One

• P0117 — If there is an intermittent short or a failing sensor, you might see P0117 (Circuit Low) along with P0118 (Circuit High) as the sensor fails.
• P0A0F / P3191 — 'Engine Failed to Start'. These can be triggered alongside P0118 because the incorrect temperature reading causes the ECM to use the wrong fuel mixture for starting.

Platform-Specific Known Issues

• The primary challenge of this repair on the Prius v is the location of the ECT sensor. It is buried underneath the EGR cooler assembly on the driver's side of the engine. While not technically difficult, removing or shifting the cooler to gain access adds time and requires care, along with the replacement of associated gaskets to prevent leaks.

Mechanic-Grade Diagnostic Values

• ECT Sensor Resistance vs. Temperature — expected: Approx. 2,000-3,000 Ω at 20°C (68°F); Approx. 250-350 Ω at 80°C (176°F).. Failure: Infinite resistance (open circuit) or near-zero resistance (short circuit) at any temperature.
• ECT Sensor Circuit Voltage at ECM — expected: A range between 0.14V (hot) and 4.91V (cold). Voltage decreases as temperature increases.. Failure: A sustained voltage reading greater than 4.91V for 0.5 seconds will trigger the P0118 code.
• Scan Tool Live Data (ECT) — expected: Should reflect the actual engine coolant temperature.. Failure: A static reading of -40°C (-40°F) indicates an open circuit, consistent with P0118.

Scan Tool Commands That Help

• Toyota Techstream: Data List -> Coolant Temp — This is the primary screen to watch for the live temperature reading from the ECT sensor. A reading of -40°F/-40°C confirms the ECM is seeing an open circuit, which is the fault condition for P0118.
• Toyota Techstream: Utility -> All Readiness — After replacing the sensor and clearing the codes, this function can be used to check the status of the DTC monitor without needing to perform a full drive cycle. It helps confirm if the repair was successful more quickly.

Wiring & Ground Locations

• ECT Sensor Connector — On the driver's side of the engine, screwed into the cylinder head area, underneath the EGR cooler assembly.. This is the primary connection point. It can be left unplugged after major service (like a head gasket job) or become corroded, causing an open circuit and triggering P0118.
• Engine Ground Points (EA, EB, EC, ED, EE, EF) — Various points on the engine block and cylinder head in the engine compartment. For the 2ZR-FXE, key grounds are on the left side of the engine compartment.. The ECT sensor circuit relies on a clean ground path back to the ECM. A corroded or loose engine ground strap can introduce high resistance into the circuit, potentially contributing to or causing a P0118, although a sensor failure is more common.
• ECM (Engine Control Module) — Located in the left side of the engine compartment.. This is the termination point for the ECT sensor wiring. The THW (coolant temp) and E2 (sensor ground) pins at the ECM are the final test points to confirm continuity of the entire circuit from the sensor.

Real Owner Repair Stories

• YouTube user comment on a P0117 video (2012 Toyota Prius) — Car started to overheat, high-temperature coolant light came on, and Check Engine Light with code P0117 (low circuit, but diagnosis is similar for high circuit).
  ❌ Tried (didn't work) The issue appeared shortly after a head gasket job where the thermostat was replaced, but the original ECT sensor was reused.
  ✅ What actually fixed it Replacing the Engine Coolant Temperature sensor resolved the overheating and the trouble code. The user notes the difficult access under the inverter and brackets was the main challenge of the 3-hour DIY job.

OEM Part Supersession History

• 89422-35010 → 89422-33030 — Standard part consolidation and updates by Toyota over time. Both part numbers refer to the same type of ECT sensor used across many Toyota and Scion models.
  Heads up: While 89422-33030 is the most commonly cited part for this application, 89422-35010 was also used and is functionally identical. Ensure the part cross-references to the 2012-2017 Prius v specifically when ordering.

Diagnostic Flowchart

Other Known Issues on This Vehicle

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

• Head Gasket Failure 🔴 High — Commonly occurs between 120,000 and 200,000 miles. Often preceded by a clogged EGR system. Symptoms include a cold-start rattle, coolant loss, and misfire codes (P030x).
• EGR System Clogging (Cooler, Valve, and Intake Manifold Passages) 🟠 Medium — Very common, typically requires cleaning every 100,000-150,000 miles to prevent rough idle, misfires, and contributing to head gasket failure. (Ref: T-SB-0116-15 addresses a related rough running condition by replacing the intake manifold.)
• Brake Booster / Actuator Assembly Failure 🔴 High — A well-known issue causing loss of brake assist, multiple warning lights (ABS, Brake, Slip), and a buzzing/whirring noise from the pump. Can occur at various mileages. (Ref: Customer Support Program (CSP) ZJB extended warranty coverage for this part.)
• Excessive Oil Consumption 🟠 Medium — Some earlier models in this generation may experience high oil consumption, sometimes linked to piston ring design.
• Hybrid Inverter (IPM) Failure 🔴 High — Less common than on earlier generations but can still occur, leading to sudden power loss or stalling. (Ref: Recalls were issued for some model years to update software to prevent overheating of the Intelligent Power Module (IPM).)

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: A used part is generally not recommended for this repair. The cost of a new, high-quality aftermarket or OEM sensor is low ($25-$70), and the labor to access it (moving the EGR cooler) is significant enough that you do not want to risk having to do the job twice. A used wiring pigtail from a junkyard is a viable option if only the connector is damaged.

Donor-vehicle mileage cap: roughly under 100000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• For a wiring pigtail, ensure the plastic is not brittle and the locking tab is intact.
• Check for any signs of green corrosion on the metal pins inside the connector.
• Ensure at least 4-6 inches of wire is attached to the pigtail for easier splicing.

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

• EGR Cooler Gaskets: These are single-use metal gaskets. Always use new OEM gaskets to prevent coolant or exhaust leaks, as the cost is minimal.

Aftermarket brands forum-validated for this vehicle:

• Denso (This is the Original Equipment Manufacturer for Toyota)
• NTK

Brands owners have reported issues with on this vehicle:

• Ultra-cheap, unbranded sensors from online marketplaces. Failure rates are high, and the savings do not justify the risk of a repeat repair.

Real Owner Stories

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

Related OBD-II Codes

• P030X — Misfire codes that often accompany the common head gasket failures and clogged EGR systems seen on this vehicle platform.
• P0420 — Often discussed alongside engine issues like oil consumption and coolant leaks on high-mileage Prius v engines.
• C1391 — Though not an engine code, this relates to the brake actuator failure which is a known high-severity issue for this model year range.
• P0A94 — Related to Hybrid Inverter (IPM) failure, which can occur on this platform and lead to sudden power loss.

Frequently Asked Questions