What Does P0174 Mean on Your 2021 Toyota Highlander?
When your 2021 Toyota Highlander triggers a P0174 diagnostic trouble code, the engine control module (ECM) has detected that the air-fuel mixture on Bank 2 is running too lean. This means there is an excess of air or a shortage of fuel in the combustion process. The 2021 Highlander is available with a 3.5-liter V6 engine producing 295 horsepower or as a Highlander Hybrid with a 2.5-liter four-cylinder engine paired with electric motors for 243 combined hp. On the V6 model, Bank 2 is a distinct cylinder bank, making P0174 a clear indicator of a bank-specific issue. On the four-cylinder hybrid, the code may indicate a sensor-related concern.
A lean condition means the engine is not getting the proper fuel mixture, which leads to increased exhaust temperatures, potential misfires, and premature catalytic converter wear. Since the Highlander is a family vehicle, addressing this promptly ensures safe and reliable transportation.
Common Symptoms You May Notice
- Check engine light illuminated
- Rough or unsteady idle
- Reduced power during acceleration
- Decreased fuel economy
- Engine hesitation or stumbling
- Occasional misfires or vibration
Most Likely Causes of P0174 on the 2021 Highlander
- Vacuum Leak on Bank 2 - A cracked vacuum hose, leaking intake manifold gasket on the Bank 2 side, or a loose intake boot can introduce unmetered air. On the 3.5-liter V6, Bank 2 vacuum leaks are a common source of this code.
- Dirty or Failing MAF Sensor - The mass airflow sensor can become contaminated with oil residue or debris, causing it to underreport air volume and resulting in insufficient fuel delivery.
- Faulty Bank 2 Oxygen Sensor - The upstream O2 sensor on Bank 2 provides critical feedback for fuel trim calculations. A sluggish or failing sensor can cause the ECM to lean out the mixture.
- Low Fuel Pressure - A weakening fuel pump or partially clogged fuel filter can restrict fuel flow, creating a lean condition that is more noticeable under load or at higher RPMs.
How to Diagnose the Problem
- Scan for all codes - Confirm P0174 and check for companion codes. If P0171 is also present, the issue is likely system-wide rather than bank-specific.
- Check fuel trims - Monitor STFT and LTFT for both banks. LTFT above 10 percent on Bank 2 specifically confirms a bank-specific lean condition.
- Inspect for vacuum leaks - Focus on the Bank 2 side of the V6 engine. Check vacuum hoses, the intake manifold gasket area, and any connections specific to Bank 2.
- Test the MAF sensor - Clean the sensor first and recheck fuel trims. Compare MAF readings to Toyota specifications.
- Verify fuel pressure - Use a fuel pressure gauge to check pressure at idle and under load. The 3.5-liter V6 should maintain 40 to 50 psi at idle.
Repair Options and Cost Breakdown
The 2021 Highlander is a mid-range SUV with parts priced accordingly. Some repairs may still fall under the factory warranty depending on mileage.
- Vacuum hose replacement - $10 to $50 for parts. Straightforward DIY repair.
- Intake manifold gasket replacement - $100 to $200 for gaskets, plus $200 to $400 in labor on the V6.
- MAF sensor cleaning - $8 to $15 for MAF cleaner.
- MAF sensor replacement - $120 to $220 for the part, plus $60 to $120 in labor.
- O2 sensor replacement - $120 to $280 for the part, plus $100 to $180 in labor.
- Fuel pump replacement - $280 to $500 for the pump, plus $180 to $350 in labor.
Can You Fix This Yourself?
Some repairs are DIY-friendly on the Highlander, though the V6 engine bay is tighter than a four-cylinder. Cleaning the MAF sensor and replacing vacuum hoses are easy tasks. O2 sensor replacement requires a sensor socket and some patience with the exhaust components. Intake manifold gasket replacement on the V6 is more involved and may be best left to a shop. If your 2021 Highlander is still under warranty, take advantage of dealer service.
Prevention Tips
- Replace the engine air filter every 15,000 to 20,000 miles
- Use quality fuel from reputable gas stations
- Inspect vacuum hoses and connections during routine maintenance
- Follow the Toyota recommended service schedule
- Address check engine lights promptly to avoid secondary damage to the catalytic converter