What Does P0172 Mean on Your 2021 Toyota Highlander?
The 2021 Toyota Highlander is available with a 3.5L V6 producing 295 hp or a 2.5L Atkinson-cycle inline-4 with electric motors in the hybrid (243 combined hp). P0172 means the ECM detected a rich fuel condition on Bank 1. On the V6, Bank 1 is typically the front bank of cylinders. On the hybrid's inline-4, Bank 1 is the only bank.
If you have the V6 and also see P0175, both banks are rich, which points to a shared cause like the MAF sensor. P0172 alone suggests a Bank 1-specific issue.
Common Symptoms You Might Notice
- Check engine light on
- Fuel economy below the 21/29 MPG (V6) or 36/35 MPG (hybrid) ratings
- Fuel or sulfur smell from exhaust
- Black soot on tailpipe tips
- Rough idle or vibration at stops
- Sluggish throttle response
Top Causes of P0172 on the 2021 Highlander
- Dirty MAF Sensor (35% likelihood): The MAF sensor collects contamination from PCV oil vapor and dust. A dirty element underreports airflow, causing over-fueling.
- Bank 1 O2 Sensor Issue (25% likelihood): The upstream O2 sensor on Bank 1 may be reading inaccurately, causing the ECM to add excess fuel to that bank.
- EVAP Purge Valve Problem (20% likelihood): A stuck-open purge valve sends fuel vapors continuously into the intake, enriching the overall mixture.
- Intake or Vacuum Leak (20% likelihood): A cracked hose, loose clamp, or intake gasket issue can introduce unmetered air that confuses the MAF.
How to Diagnose P0172 on Your 2021 Highlander
- Check for companion codes. P0175 alongside P0172 on the V6 means both banks are rich, pointing to a common cause like the MAF sensor.
- Clean the MAF sensor. Remove from the intake tube, spray with MAF cleaner, and let dry. This is the most effective first step.
- Compare bank fuel trims. With a scan tool, check Bank 1 vs. Bank 2 LTFT. If only Bank 1 is negative, focus on bank-specific components.
- Inspect the air filter. The Highlander's air box is accessible from the top of the engine bay.
- Test the purge valve. Disconnect the purge hose at idle. A notable idle change suggests the valve is stuck open.
Repair Options and Costs
- MAF sensor cleaning: $10 DIY or $60-$100 at a shop
- MAF sensor replacement: $150-$290 parts and labor
- O2 sensor replacement: $180-$380 parts and labor
- EVAP purge valve replacement: $110-$260 parts and labor
- Vacuum leak repair: $75-$250 depending on location
Can You DIY This Repair?
The 2021 Highlander has decent engine bay access for a three-row SUV. MAF cleaning and air filter replacement are easy DIY tasks. The EVAP purge valve is moderately accessible. O2 sensor replacement on the V6 front bank is manageable but may require a flex-head sensor socket. Rear-bank work is more challenging due to space constraints.
Prevention Tips
- Replace the air filter every 15,000-20,000 miles
- Clean the MAF sensor every 30,000 miles
- Use top-tier fuel
- Follow Toyota's maintenance schedule
- Address check engine lights promptly to protect expensive V6 catalytic converters