Code
P0174
GWM
P — Powertrain
- The mixture is poor (bank 2)
AI status
Completed
Completed
100%
Causes
- Intake manifold or vacuum leak (bank 2)
- Faulty or contaminated mass air flow (MAF) sensor
- Faulty or slow/weak upstream oxygen sensor on bank 2
- Low fuel pressure or weak fuel pump/fuel filter
- Clogged or leaking fuel injector(s) on bank 2
- Wiring or connector faults to fuel/air sensors (MAF, O2, MAP, injectors)
Symptoms
- Check Engine light (MIL) illuminated
- Hesitation, stumbling or lack of power under acceleration
- Rough idle or surging at idle
- Increased fuel economy sometimes (due to lean) or drivability complaints
- Possible misfire codes if the lean condition is severe
What to check
- Retrieve freeze-frame data and all stored codes; note related codes and freeze frame conditions
- Check long-term and short-term fuel trim for bank 2 (using live data)
- Inspect intake tract for vacuum leaks, loose clamps, cracked hoses, and intake manifold gasket leaks on bank 2
- Visually inspect and test MAF sensor and wiring; check for contamination or debris
- Measure fuel rail pressure against manufacturer spec with a fuel pressure gauge
- Perform a smoke test of intake and vacuum system if a leak is suspected
Signal parameters
- Long Term Fuel Trim (LTFT) bank 2: significant positive value (commonly > +10%) indicates sustained lean condition
- Short Term Fuel Trim (STFT) bank 2: frequent positive corrections (e.g. +10% or more)
- Upstream O2 (bank 2 sensor 1) voltage: when closed loop should oscillate ~0.1–0.9 V; a persistently low voltage (~0.1–0.3 V) suggests lean
- MAF sensor: mass airflow vs rpm/load should match expected values; sudden low MAF reading for load indicates underreported air or dirty sensor
- Fuel pressure: lower than spec (varies by vehicle; gasoline systems often ~40–70 psi) suggests inadequate fuel delivery
- MAP/intake vacuum: unusually high vacuum or abnormal MAP values at given rpm can indicate restrictions or leaks
Diagnostic algorithm
- Read and record all DTCs and freeze frame data; do not replace parts before diagnosis.
- Using a scan tool, monitor LTFT/STFT for bank 2, upstream O2 sensor voltage, MAF reading, and fuel pressure while at idle and under load.
- Visually inspect intake boots, vacuum lines, PCV and intake manifold gaskets on bank 2; fix obvious leaks. If uncertain, perform a smoke test of intake/vacuum system.
- Clean MAF sensor with approved MAF cleaner and re-test. If MAF readings remain inconsistent with load, test/replace MAF.
- Measure fuel rail pressure with a gauge under key on and running conditions; compare to spec. Repair fuel delivery (pump, filter, regulator) if low.
- Inspect injectors on bank 2: check electrical continuity, perform balance/flow test or swap injector with known good to compare behavior.
- Check upstream O2 sensor (bank 2 sensor 1): verify response time and voltage swing. Replace only if sensor is proven faulty and wiring/connectors are good.
- Inspect exhaust manifold/header for leaks upstream of the bank 2 O2 sensor (can cause false lean readings).
- Repair confirmed faults and clear codes. Perform road test and recheck live fuel trim to confirm problem corrected.
- If issue persists, escalate to advanced diagnostics (oscilloscope on sensors, cylinder contribution test, ECU/fuel injector driver checks).
Likely causes
- Intake vacuum hose or intake manifold gasket leak on bank 2
- Dirty/failed MAF causing incorrect airflow reading
- Low fuel pressure (pressure regulator, pump or clogged filter)
- Faulty upstream O2 sensor on bank 2 or contaminated sensor reading lean
Fault status
Status
The engine control module detected a sustained lean condition on bank 2 (excess air or insufficient fuel). MIL is set; fuel trims and sensor data will typically show positive trims and low upstream O2 voltage.
Repair difficulty: Medium
Diagnostic time: 1.5 - 3.0 hours
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