P0174 Code: Toyota Tacoma (2016-2023) - Causes, Symptoms & Fixes

COMPREHENSIVE DIAGNOSTIC GUIDE: P0174 ON 2016-2023 TOYOTA TACOMA

Note on data limitations

• REAL NHTSA DATA FOR Toyota Tacoma (2016-2023): OWNER COMPLAINTS show no NHTSA complaints for this make/model/year/issue combination. OFFICIAL RECALLS: No recalls found in NHTSA database.
• This guide combines the limited NHTSA data with general automotive knowledge. P0174 means a lean condition on Bank 2 and is a common DTC across many vehicles, including Tacomas with 2.7L I4 or 3.5L V6 engines. Use live data and flow-tested diagnostics to confirm a root cause for your specific vehicle.

CODE MEANING AND SEVERITY

• Code: P0174 — System Too Lean Bank 2
• What it means: The engine computer (ECU/PCM) detected that the air-fuel mixture on Bank 2 is lean (too much air, not enough fuel) based on sensor readings (typically the upstream O2 sensor and fuel trims).
• Bank 2: In a V6 configuration like the Tacoma, Bank 2 is the side of the engine opposite Bank 1 (cylinders on that bank share sensor data and fuel trim).
• Severity: Moderate. A lean condition can cause rough idle, poor acceleration, reduced power, and degraded fuel economy. Prolonged lean operation can heat exhaust components and may trigger misfires or catalyst damage if the condition persists. It’s not usually an immediate “catastrophic” failure code, but left unaddressed it can lead to drivability problems and potential catalyst issues.

COMMON CAUSES ON TOYOTA TACOMA

Note: Causes listed below are typical for P0174 on Tacomas (2016–2023) and may apply to both 2.7L I4 and 3.5L V6 models. Start with the most common and easiest-to-diagnose items.

• Vacuum leaks and cracked/loose hoses
  • Cracked intake boot, cracked vacuum lines, or loose fittings.
  • PCV system leaks (PCV valve, hoses, valve cover grommets).
• Mass Air Flow (MAF) sensor issues
  • Dirty or contaminated MAF, faulty wiring, or a damaged MAF sensor.
• Air intake and filtration issues
  • Clogged or dirty air filter, intake hose restrictions, or improper aftermarket intake components.
• Oxygen sensor (O2 sensor) problems
  • Faulty upstream (pre-cat) O2 sensor on Bank 2 or slow response, leading to incorrect fuel trim corrections.
• Fuel delivery problems
  • Low fuel pressure or insufficient fuel volume due to a weak fuel pump, clogged fuel filter, or failing fuel pressure regulator.
  • Dirty or leaking fuel injectors (or injector filtration issues).
• Exhaust leaks and oxygen sensor location
  • Exhaust leaks before the Bank 2 O2 sensor (manifold/gasket leaks, cracked exhaust components) can cause a lean reading.
• Intake manifold or gasket leaks (especially on V6)
  • Leaks at intake manifold gaskets or around ports can introduce unmetered air.
• MAP sensor and/or related sensors
  • Map sensor or related manifold pressure reading anomalies that skew air-fuel calculations (less common than MAF issues but possible).
• EVAP and related systems (secondary contributors)
  • Small vacuum leaks or small EVAP system leaks can contribute to perceived lean condition, particularly when the engine is under certain load or temperature conditions.
• Engine mechanical issues
  • Cylinder compression issues or timing concerns can contribute to lean conditions, though less common than sensor/air/fuel delivery problems.
• PCM/software
  • Infrequent software calibration or update requirements can cause inaccurate fuel trim behavior in some cases.

SYMPTOMS

• Check Engine Light (CEL) illuminated.
• Rough idle or irregular idle speed.
• Engine hesitates or sputters during acceleration; reduced power.
• Dropped or inconsistent fuel economy.
• Possible misfire symptoms (P0300 family) if the lean condition causes combustion irregularities.
• O2 sensor readings (data stream) show lean trim values for Bank 2 (positive LTFT/STFT).
• Occasionally, no obvious symptom other than a stored P0174 in the ECU.

DIAGNOSTIC STEPS

Goal: Confirm P0174 while identifying the root cause. Use a systematic, evidence-based approach.

Step 0: Gather data

• Verify the exact DTC: P0174 (Bank 2 Lean).
• Retrieve Freeze Frame data: engine load, RPM, coolant temp, LTFT/STFT at the time of fault (helps indicate how long the lean condition has existed and under what conditions it occurred).
• Check for related codes (P0171 for Bank 1 lean, P0172 for Bank 1 rich, P030x misfire codes, misfire-related codes, etc.).
• Note driving conditions when the code occurs (cold start vs after warm-up, idle vs acceleration).

Step 1: Visual inspection

• Inspect all vacuum hoses and intake plumbing for cracks, loose connections, or disconnections (including PCV hoses and valve).
• Inspect the air intake tube, air filter housing, and intercooler piping (if applicable) for leaks or obstructions.
• Check for exhaust leaks near Bank 2 before the O2 sensor.

Step 2: Check service data and live data

• Scan/monitor LTFT and STFT for Bank 2. If LTFT is consistently positive (e.g., +12% to +30% over a number of drive cycles), fuel delivery or air metering is under-correcting lean.
• Review upstream O2 sensor (Bank 2) data: does it switch lean/rich as you drive? Slow response or perpetually lean readings point toward sensor issues or pre-sensor air leaks.
• Check MAF sensor reading vs. engine load and RPM from live data. A noticeably high MAF reading with little RPM change can indicate a dirty/failed MAF.
• Check MAP sensor readings (if applicable) for expected pressure at idle and at load. Abnormal MAP readings can indicate a faulty MAP sensor or plumbing issue.
• If available, perform a smoke test to quickly identify vacuum leaks that are not obvious visually.

Step 3: Inspect and test the MAF sensor

• Clean the MAF with a spray designed for MAF sensors if you suspect contamination (do not touch the sensing element with tools).
• Inspect the MAF wiring harness for damage or poor connections.
• If cleaning and inspection do not improve readings, consider replacing the MAF sensor.

Step 4: Inspect the fuel system

• Test fuel pressure with a proper gauge to verify it meets Toyota specifications for your engine (idle and under load; refer to service manual for exact numbers; typical ranges are in the 50–65 psi range for many modern Toyota systems, but verify).
• If fuel pressure is low or unstable, inspect:
  • Fuel pump operation and electrical circuit.
  • Fuel filter (if applicable) and fuel pressure regulator (or any integrated regulator in the fuel rail).
  • Fuel injectors for proper spray pattern and leaks.
• Consider a fuel system cleaner only as a temporary measure; if issues persist, service or replace failing components.

Step 5: Check the O2 sensor and downstream sensors

• Inspect Bank 2 upstream (pre-cat) O2 sensor and its wiring for damage or poor connections.
• Check the downstream O2 sensor (Bank 2) if present; a faulty downstream sensor can affect fuel trim feedback.
• Replace if foul or slow to respond (after confirming there are no leaks or air intrusion issues).

Step 6: Inspect for intake leaks and gasket integrity

• Look at the intake manifold gaskets for signs of leaks; a leak here can introduce extra air and lean the mixture.
• Inspect throttle body and idle air control paths for cleanliness and proper operation.
• If you suspect a leak around the throttle body, gasket, or intake plenum, perform a careful inspection or perform a smoke test.

Step 7: Evaluate exhaust and EVAP interactions

• Ensure there are no exhaust leaks before the O2 sensor on Bank 2, as these leaks can create lean readings.
• If the EVAP system shows symptoms or related codes, address those as they can indirectly influence engine breathing and fuel trim.

Step 8: Consider software and mechanical condition

• If all hardware checks out, review recent software updates or TSBs for your year/model (Toyota dealers may have reflash/updates). Note: Only use official updates when advised by Toyota.
• In rare cases, internal engine mechanical issues (low compression on Bank 2) can mimic lean conditions; perform a compression test if other causes are not found and symptoms persist.

Step 9: Plan of action

• After identifying a likely cause, address it and re-test under multiple driving conditions (idle, light throttle, and highway). Confirm that LTFT/STFT on Bank 2 return to near-zero values and that the P0174 clears.

RELATED CODES

• P0174: System Too Lean Bank 2 (the primary code)
• P0171: System Too Lean Bank 1 (common companion on vehicles with dual banks)
• P0172: System Too Rich Bank 1
• P0300 through P0306: Random/Multiple/Specific cylinder misfire (can occur if lean conditions cause misfires)
• P1133 or similar heater/air-fuel sensor related codes (less common; depends on sensor hardware)
• P0420/P0430: Catalyst efficiency (may appear if prolonged lean condition damages catalysts)

REPAIR OPTIONS AND COSTS (2025 PRICES)

Note: Prices vary by region, shop, and exact parts. All ranges are approximate household figures to help planning. Labor rates commonly range from about $80–$150 per hour in many areas; Toyota OEM parts may cost more than aftermarket equivalents.

• Vacuum leaks and PCV system repair

  • Parts: vacuum hoses, PCV valve/grommets: $5–$60
  • Labor: 0.5–2.0 hours
  • Typical total: $60–$250

• MAF sensor replacement (Bank 2 upstream MAF)

  • Parts: $60–$230 (aftermarket) or $120–$350 (OEM)
  • Labor: 0.5–1.0 hour
  • Typical total: $180–$410

• Upstream O2 sensor (Bank 2) replacement

  • Parts: $50–$120 (aftermarket) or $100–$300 (OEM)
  • Labor: 0.5–1.0 hour
  • Typical total: $120–$420

• Fuel delivery issues (low pressure or failing pump/regulator)

  • Parts: fuel pump or regulator: $100–$400 (pump often on the higher end)
  • Labor: 1.0–3.0 hours
  • Typical total: $350–$1,600
  • Note: If only regulator or filter is replaced, costs may be lower.

• Fuel injector service or replacement

  • Parts: injectors or cleaning service: $0–$350 per bank
  • Labor: 0.5–2.0 hours
  • Typical total: $100–$1,200

• Intake manifold gasket or gasket repair

  • Parts: $20–$150
  • Labor: 2.0–8.0 hours
  • Typical total: $260–$1,000

• Throttle body cleaning or replacement

  • Parts: $0–$260 (if replacement)
  • Labor: 0.5–1.5 hours
  • Typical total: $80–$450

• PCV valve and related hardware

  • Parts: $5–$20
  • Labor: 0.1–0.5 hours
  • Typical total: $20–$120

• ECU/PCM software update or reflash

  • Parts: Typically none or minimal; software update charge
  • Labor: 0.3–1.0 hours
  • Typical total: $50–$180 (dealer or certified shop)

• Miscellaneous and diagnostic fees

  • If the shop charges for diagnostic time, expect $50–$130 initial diagnostic fee in many shops, sometimes waived with major repair.

DIY VS PROFESSIONAL

• DIY considerations
  • If you have basic tool access and comfort with engine electrical and vacuum systems, you can tackle vacuum hose inspection, MAF cleaning, and basic data checks with an OBD-II scanner and manual.
  • Important cautions: working with the fuel system (pressurized lines) and high-voltage O2 sensor circuits requires care. Do not work on live fuel lines without proper procedures. If you are not confident with fuel pressure testing, let a professional handle it.
• Professional considerations
  • If LTFT remains high and you have eliminated obvious vacuum leaks and sensor issues, a professional can perform controlled fuel pressure tests, smoke testing for leaks, and precise sensor diagnosis with strain on multi-parameter data.
  • A shop can perform a more thorough teardown if required (intake manifold, gaskets, injector testing), and ensure proper calibration if software updates are needed.

PREVENTION

• Regular maintenance
  • Replace or clean air filter at recommended intervals to prevent unmetered air entry.
  • Inspect and replace damaged vacuum hoses and PCV hoses promptly.
• Sensor care
  • Clean MAF sensor when you notice abnormal readings or after long-term exposure to dirty air, oil, or contaminants.
  • Periodically inspect O2 sensor wiring and connections; repair any damaged harnesses.
• Fuel system care
  • Use good quality fuel and avoid frequent low-fuel situations; consider occasional use of a reputable fuel system cleaner as a maintenance aid (not a substitute for proper diagnostics).
• Avoid exhaust leaks
  • Regularly inspect exhaust system for leaks around manifolds, gaskets, and pipes that could affect upstream O2 readings.
• Environmental awareness
  • In areas with heavy humidity or salt exposure, inspect under-hood areas for corrosion on sensor connectors; keep all electrical connections clean and dry.

Helpful diagnostic tips specific to Tacomas

• On 2016–2023 Toyota Tacomas (2.7L I4 or 3.5L V6), Bank 2 lean conditions are often traced to air leaks or sensor issues rather than a pure fuel delivery failure. Start with vacuum/air intake components, MAF sensor health, and fuel pressure.
• When LTFT is persistently positive on Bank 2, but O2 sensor readings are slow to switch, suspects include a lean air source (vacuum leak) or a faulty MAF sensor.
• If both Bank 1 and Bank 2 lean codes are present (P0171 and P0174), look for a common air source like the intake boot or PCV system, or a mass air/MAF or MAP sensor issue causing widespread lean conditions.
• If only Bank 2 lean is present and Bank 1 is normal, target Bank 2 specific components (Bank 2 O2 sensor, injector balance for related cylinder bank, or a Bank 2-specific air leak like a fuel return line or port).

Final considerations

• Use live data and freeze-frame information to guide your diagnosis. Rely on a methodical approach to avoid replacing multiple components unnecessarily.
• If you’re unsure or uncomfortable with fuel system testing, fuel pressure checks, or exhaust leak testing, it’s prudent to work with a qualified technician. A misdiagnosis can lead to unnecessary parts replacement and expense.
• With no NHTSA recalls reported in the provided data for this exact combination (P0174 on 2016–2023 Tacoma), focus on the diagnostic path above rather than relying on recall-based fixes.