P0135 on the 2023 Toyota Highlander
The 2023 Toyota Highlander introduced a significant powertrain change, replacing the 3.5L V6 with the T24A-FTS 2.4-liter turbocharged inline-four engine producing 265 horsepower. A P0135 code on this new powertrain means the heater circuit in the Bank 1, Sensor 1 oxygen sensor has malfunctioned. Since this is an inline-four, there is one bank, and the upstream O2 sensor sits in the exhaust manifold before the turbocharger's catalytic converter.
Turbo Engine and O2 Sensor Placement
The turbocharged Highlander has a different exhaust layout than the previous V6. The upstream O2 sensor is positioned between the exhaust manifold and the turbocharger. Exhaust gas temperatures in this area run extremely high, especially during boost conditions, placing more thermal stress on the sensor and its heater element than in a naturally aspirated engine. The heater must reach 600°F operating temperature quickly for accurate readings in this demanding environment.
What Causes P0135 on the 2023 Highlander
- Premature heater element failure (High Likelihood): The extreme exhaust temperatures near the turbocharger can accelerate heater element wear even on a new vehicle. The Denso OEM sensor may have experienced thermal shock during aggressive driving or heavy towing. On a 2023 model, this likely points to a manufacturing defect or design margin issue.
- Heat-damaged wiring (Medium Likelihood): The turbo location means exhaust temperatures exceeding 1,600°F are nearby. The sensor wiring and connector must have proper heat shielding to survive. A gap in heat protection or a misrouted wire can cause insulation failure.
- Blown fuse or relay failure (Medium Likelihood): The engine compartment fuse box houses the O2 heater circuit fuse. A blown fuse is always worth checking first since it is the simplest and cheapest possible fix.
- ECU calibration or driver issue (Low Likelihood): The 2.4L turbo engine is relatively new for Toyota. Early production vehicles can sometimes have ECU calibration issues that set codes prematurely. Check for technical service bulletins at the dealer.
Diagnostic Approach
Dealer Warranty Repair
Your 2023 Highlander is under full warranty, including the 8-year/80,000-mile federal emissions warranty for O2 sensors. Take it directly to a Toyota dealer for diagnosis and repair at no cost. The turbo engine's sensor location and new platform make dealer repair the smart choice even for experienced DIY mechanics.
Understanding the Turbo Complication
On the turbocharged engine, the upstream O2 sensor may be harder to access than on the previous naturally aspirated V6. The turbocharger, intercooler piping, and associated hardware occupy space in the engine bay. The dealer technicians have the specific tools and access procedures for this newer platform.
If Verifying the Code
Use an OBD-II scanner to confirm P0135. Check the O2 heater fuse in the fuse box. Visually inspect the sensor connector if accessible. Do not attempt to remove the sensor yourself on a warranty vehicle, as improper procedures on the turbocharged exhaust system could damage expensive components.
Repair Cost Reference
Under warranty: free. For future reference when warranty expires, an OEM Denso sensor for the 2.4L turbo runs $90 to $170. Shop labor is $100 to $180 due to the turbo engine's tighter access. Total out-of-pocket would be $190 to $350. The turbocharged engine requires more care during sensor replacement to avoid disturbing turbo connections.
Do Not Wait
The 2.4L turbo engine relies on precise fuel management for both performance and turbocharger longevity. Running rich during cold starts can affect turbo spool-up behavior and deposit carbon in the intake. Since the repair is free under warranty, schedule your dealer visit promptly. Most dealers can complete this repair in one to two hours.