P0174: System Too Lean (Bank 2)

What does P0174 mean?

Your engine control module (ECM or PCM) aims for an air to fuel ratio of about 14.7 to 1. It reads the upstream oxygen sensor and adds or removes fuel many times per second. When it has added fuel up to its limit on bank 2 and the exhaust still shows too much oxygen, it stores P0174.

Bank 2 only exists on V6, V8 and other engines with two cylinder banks. It is the side that does not contain cylinder 1. If you also have P0171 (bank 1 lean), look first for a cause both sides share, such as a large intake leak, a MAF sensor reading low or low fuel pressure.

The oxygen sensor is usually reporting a real lean condition rather than failing itself, so replacing it first rarely fixes P0174.

Common symptoms

• Check engine light on
• Often no obvious drivability change
• Lack of power or hesitation on acceleration
• Rough idle, especially when warm or stopped
• Higher fuel consumption

Common causes

The causes below are the ones commonly seen with P0174. The actual cause can vary by vehicle, engine and mileage, so treat this list as a starting point, not a diagnosis.

• Vacuum or intake air leaks after the MAF sensor, such as intake manifold gaskets, vacuum hoses or PCV hoses
• A dirty or faulty mass air flow (MAF) sensor
• Low fuel delivery from a clogged fuel filter or weak fuel pump
• Dirty or restricted fuel injectors

What to check with a scanner

• In Live data, compare short term plus long term fuel trim for bank 1 and bank 2 at idle and at about 2,500 rpm. High positive trim on both banks points to a shared cause.
• If the trim is high at idle but improves at higher rpm, an intake or vacuum leak is a common cause.
• Watch the MAF reading. A sensor that reads low for the engine size and rpm can make both banks lean.
• Check freeze frame data to see the load, rpm and temperature when the code set.
• Look for other codes such as P0171, P0300 or a cylinder misfire code and note them before clearing.

How to read and clear P0174 with OBD Master Pro

1. Plug in your adapter. Connect a generic ELM327 (or ELM329) OBD2 adapter to the diagnostic port, usually under the dashboard on the driver's side. Bluetooth Classic, Bluetooth LE, Wi-Fi and USB OTG adapters are supported.
2. Switch the ignition on. Turn the key to ON or start the engine, depending on your car.
3. Connect in OBD Master Pro. Open the app, go to Connection and pick your adapter. Classic Bluetooth adapters connect from the Bluetooth tab, Bluetooth LE adapters from the BLE tab with no pairing needed. Use the generic OBD2 / EOBD profile or your car's brand profile.
4. Read the codes. Open the fault codes screen to see stored, pending and permanent codes. Confirm P0174 is listed and note any other codes, since they often explain the cause.
5. Check the details. Look at freeze frame data, Live data and, where your car supports it, Mode 06 test results (see the checks above).
6. Fix, then clear. After the repair, clear the codes to turn off the check engine light. Clearing also resets the emissions readiness monitors, so drive normally and check the readiness screen before any inspection. Permanent codes cannot be erased with a scan tool; they clear on their own once the car's self test passes.

Tip: you can also look up P0174 and other codes in the app's DTC search without connecting to a car. If your adapter will not connect, use Connection > Share connection log and email it to support. Many cheap ELM327 clones only handle basic engine OBD2, which is all you need for P0174.

When to see a mechanic

Get professional help if any of these apply:

• The engine misfires or the check engine light flashes
• You smell fuel or see a leak (stop driving)
• You cannot find a leak and need a smoke test
• Fuel pressure needs to be measured with a gauge

If you are unsure, a qualified technician can confirm the cause with tests such as smoke testing, pressure testing or scope readings. Clearing a code does not repair the fault.

Brand notes

Patterns below come from published manufacturer service information. They do not mean every vehicle of that brand has this problem, and a bulletin only applies to the vehicles it lists.

Related trouble codes

See all OBD2 trouble code guides

Related guides

Check engine light on: what to do · How to read an OBD2 code (P, B, C, U) · Stored, pending and permanent codes and freeze frame · OBD2 readiness monitors explained · OBD2 glossary · OBD2 by car brand