P0175 on the 2018 GMC Sierra: Higher-Mileage V8 Diagnosis
The 2018 GMC Sierra 1500 — the final year of the K2XX generation — is commonly equipped with the 5.3L L83 V8 (355 hp) or the 6.2L L86 V8 (420 hp). Both feature Active Fuel Management (AFM), direct fuel injection, and variable valve timing. At 6-8 years old with potentially 80,000-140,000 miles, these engines are in the zone where multiple fuel system components may be reaching end-of-life simultaneously.
Bank 2 on the GM V8 is the passenger side (cylinders 2, 4, 6, 8), and P0175 indicates that bank is running too rich for the ECM to compensate.
Symptoms at Higher Mileage
- Check engine light — may have been intermittent before becoming steady
- Fuel economy well below the expected 16-22 MPG
- Pronounced rough idle, especially in gear at a stop
- Strong fuel smell from the exhaust
- Visible black smoke under acceleration
- AFM operation may feel rough or cause shuddering
- Reduced towing power
- Spark plugs fouling earlier than expected
Root Causes at Higher Mileage
1. Worn MAF Sensor — High Likelihood
After 80,000+ miles of truck duty, the MAF sensor has been thoroughly contaminated by dust, PCV oil vapors, and road debris. At this mileage, replacement rather than cleaning is the right approach. The Gen V direct injection engines produce more crankcase blowby than the older Gen IV port-injected engines, which accelerates MAF degradation. An AC Delco replacement sensor costs $60-100 for the part.
2. Failed O2 Sensors — High Likelihood
At 80,000-140,000 miles, O2 sensors are at or past their expected service life. The upstream wideband sensor on Bank 2 has endured years of extreme heat from the exhaust manifold. A sluggish or failed sensor causes constant fuel trim miscalculation. At this mileage, replacing all four O2 sensors (two upstream, two downstream) is cost-effective and addresses the most common failure point.
3. Fuel Injector Wear — Medium Likelihood
GM's direct injection fuel injectors have been firing at extreme pressures for 80,000+ miles. Carbon buildup on injector tips is inevitable with GDI engines, and seal wear allows injector drip. The rich condition from a leaking Bank 2 injector is often worse at cold startup. GM dealers can perform an injector balance test to identify specific offenders.
4. EVAP Purge Valve Failure — Medium Likelihood
The EVAP purge solenoid is a common failure item on GM trucks across multiple generations. At this mileage, the solenoid diaphragm can wear and stick open. The continuous fuel vapor introduction enriches the mixture, particularly noticeable at idle. This is one of the cheapest and easiest fixes for P0175 on GM trucks.
Diagnostic Approach
- Scan for all stored codes — check for multiple fuel system and emissions codes
- Inspect spark plugs on Bank 2 — black, sooty plugs confirm rich running
- Compare Bank 1 vs Bank 2 fuel trims
- Test MAF sensor at idle and under load
- Monitor all four O2 sensor waveforms
- Test EVAP purge valve — disconnect at idle to check
- Run fuel pressure and injector balance tests
Repair Costs
- MAF sensor: $70–$190
- O2 sensor (each): $80–$220
- Fuel injector (each): $120–$280
- EVAP purge valve: $40–$120
- Spark plugs (set of 8): $60–$120
DIY Accessibility
The 2018 Sierra is very DIY-friendly. The engine bay is spacious, parts are widely available and affordable, and repair procedures are extensively documented in forums and YouTube. MAF sensor is a 5-minute job. Spark plugs on the Gen V require a 5/8" socket with extensions. O2 sensors may need penetrating oil for heat-seized threads. EVAP purge valve is typically accessible near the intake. Even injector work is manageable for experienced DIYers.