P0171 and P0174 at the Same Time? Smoke the Intake First

P0171 and P0174 setting together is not a sensor problem. It's air getting into the engine where the MAF can't count it. I've watched a guy throw an O2 sensor, then a MAF, then a fuel pump at a lean-running truck, and the actual leak was a split PCV elbow that cost four dollars.

Two Banks Lean Means Unmetered Air

Long term fuel trim is your first clue. If LTFT on both banks sits +15% or higher, the PCM is adding fuel to cover air it never measured. A lazy O2 sensor usually skews one bank and brings a response or heater code with it. A dirty MAF under-reports everywhere, but it doesn't get dramatically worse at idle. Vacuum leaks do.

The idle versus 2500 rpm trick splits it in one minute. Hold 2500 rpm for 30 seconds and watch trims. If it's unmetered air, trims fall toward zero because the leak is a smaller slice of total airflow. If trims stay lean under load, you're chasing fuel delivery instead — pressure, volume, or a restricted filter. Two completely different paths, same scan tool.

Look Before You Smoke

Check the easy ones first. Cracked intake tube or a clamp you left loose after a filter change. PCV hose or elbow split at the valve cover. Brake booster hose off its fitting, or a booster that hisses when you push the pedal. Manifold gaskets on a high-mile engine. Dipstick or oil fill cap not seated on a sealed system. Any of those will set both banks lean and none of them look like anything but fuel trim on live data.

Smoke Test Order That Saves Time

Deadhead the intake at the throttle body so smoke can't just pour out the front. Feed smoke into a vacuum port or the brake booster hose. Then chase it in order: intake runners, injector seals, throttle body gasket, EVAP purge line, a purge valve stuck open. No smoker in the shop? Spray carb cleaner around the runners while you watch short term trim. When STFT dives negative, you're standing on the leak.

What We Use

Smoke machines run on 12V, and the battery cart is always on the far side of the shop. I run mine off an M12 power adapter clipped to a 3.0Ah pack. Same rig holds radio presets and learned values when I pull a battery, and it powers a relay or solenoid on the bench without dragging out a jump box. PA12-CF housing, built in the USA, lives in my cart.