P0174moderate severityGenerally driveableP0174 Code: System Too Lean (Bank 2)
Written in plain English and reviewed by the eli5cars editorial team · Last reviewed June 2026
What it means in plain English
Same problem as P0171 — the engine is running with too much air and not enough fuel — but this time it's Bank 2 of the engine. Bank 1 is always the bank that contains cylinder #1; Bank 2 is the opposite bank. On a 4-cylinder engine there's only one bank, so P0174 only appears on V6, V8, or other multi-bank engines. Which physical side of the engine is Bank 1 varies by manufacturer and engine layout — do not assume front = Bank 1 or driver's side = Bank 1 without checking your vehicle's service data.
Most likely causes — ranked
#1 Dirty or failing mass airflow (MAF) sensor (especially if P0171 is also present)
most likelyFix: When both P0171 and P0174 are set together on a V6 or V8, the MAF sensor is the overwhelmingly likely cause — it's the only single component that affects both banks simultaneously. Clean the MAF sensor element with CRC MAF Sensor Cleaner, let dry, reinstall, and clear codes. If both codes return, replace the MAF sensor.
#2 Vacuum leak affecting the intake plenum or common intake components
likelyFix: A large vacuum leak at the intake manifold gasket, throttle body, or main intake tract affects all cylinders and both banks. Spray carburetor cleaner around the intake manifold base gasket and all vacuum connections while the engine idles — a change in RPM reveals the leak. A smoke test at a shop is more thorough.
#3 Weak fuel pump causing insufficient pressure across all cylinders
possibleFix: A failing fuel pump that can't maintain adequate rail pressure will cause lean conditions on all cylinders, setting codes on both banks. Have fuel pressure tested under load; compare to the factory spec for your vehicle at idle and during a snap throttle test.
#4 Faulty Bank 2 upstream oxygen sensor providing inaccurate feedback
possibleFix: If only P0174 is set (not P0171), the Bank 2 upstream O2 sensor may be the culprit rather than a component shared between banks. Test the Bank 2 Sensor 1 O2 sensor with live data — it should switch between rich and lean 2–5 times per second at operating temperature. A lazy or dead sensor reports incorrectly and causes the ECM to over-lean Bank 2.
Check engine light on? Get a free code scan
If your check engine light is on, most auto-parts stores (AutoZone, O'Reilly, Advance) will read the code for FREE — no purchase required, just walk in. Bring the code back here and look it up at eli5cars.com/obd2 for the plain-English explanation.
Note: a scan only helps when there's an active fault code. For purely mechanical symptoms (noises, vibrations, leaks), a scan may show nothing — the suspects listed above are your starting point.
Want to read the codes yourself?
First, the free option: AutoZone, O’Reilly, and Advance Auto Parts read your codes for free — no purchase needed. That’s the cheapest way to get the code to look up here. If you’d rather have your own so you can check anytime (and clear a code after a fix), the Veepeak OBDCheck BLE is an inexpensive, reliable plug-in reader that pairs with a free phone app and works on any car sold since 1996.
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Frequently asked questions
I have P0171 and P0174 together. Where do I start?
Start with the MAF sensor — clean it first ($10 and 10 minutes). Both banks running lean at the same time almost always points to a single upstream component affecting all airflow measurement or fuel delivery, not two separate bank-specific failures.
Which cylinder is Bank 2 on my car?
Bank 1 is the bank that contains cylinder #1. Bank 2 is the opposite bank. The physical location of Bank 1 depends entirely on your engine layout — it is NOT always the front or driver's side. For example, on Honda's transverse J-series V6 (used in the Accord V6, Pilot, and Odyssey), Bank 1 is the REAR bank (firewall side) and Bank 2 is the front bank — the opposite of what many people expect. On most rear-wheel-drive longitudinal V8s (Chevy, Ford), Bank 1 is typically the driver's side. Always verify your specific engine's bank layout in the factory service manual or a model-specific forum before replacing any O2 sensor or chasing a bank-specific fault.
Can P0174 cause engine damage?
A lean-running engine can cause increased cylinder temperatures and pre-ignition (pinging) under load, which over time causes accelerated piston and ring wear. Address it within a few weeks — it's not a pull-over-now emergency, but it's not something to ignore for months either.
Does P0174 affect fuel economy?
Yes — your ECM is actively trying to compensate by injecting more fuel, which means it's working outside its efficient operating range. You may notice slightly worse fuel economy and/or a hesitation during acceleration.