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 contains cylinder 1. Bank 2 is the opposite bank. On a typical front-wheel-drive transverse V6 (like a Honda Accord V6 or Toyota Camry V6), Bank 1 is typically the front row and Bank 2 is the rear row. On rear-wheel-drive or longitudinal V8s, Bank 1 is usually the driver's side. Check your specific vehicle's service manual or search online for your make and model.

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.

P0174moderate severityGenerally driveable

P0174 Code: System Too Lean (Bank 2)

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 2 is the opposite bank from Bank 1 (the side that does NOT contain cylinder 1). On a 4-cylinder engine there's only one bank, so P0174 only appears on V6, V8, or other multi-bank engines.

Most likely causes — ranked

#1 Dirty or failing mass airflow (MAF) sensor (especially if P0171 is also present)

most likely

Fix: 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.

DIY ~$10Shop ~$280

mass air flow sensor repair guide

#2 Vacuum leak affecting the intake plenum or common intake components

likely

Fix: 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.

DIY ~$25Shop ~$250

#3 Weak fuel pump causing insufficient pressure across all cylinders

possible

Fix: 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.

DIY ~$85Shop ~$450

#4 Faulty Bank 2 upstream oxygen sensor providing inaccurate feedback

possible

Fix: 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.

DIY ~$50Shop ~$200

oxygen sensor repair guide

Get a FREE OBD2 scan first — no purchase required

AutoZone, O'Reilly Auto Parts, and Advance Auto Parts all scan your car's computer for free. Walk in, they plug in a scanner, you get a code in under 2 minutes. Then come back here and look up that code at eli5cars.com/obd2 for the plain-English explanation.

Pro tip: Take a photo of the code before they clear it.

P0174 Code: System Too Lean (Bank 2)

Most likely causes — ranked

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Frequently asked questions