P0174 — System Too Lean (Bank 2)
P0174 is the bank 2 twin of P0171. On its own it points at that bank; together with P0171 it points at something both banks share, which narrows the search considerably.
The engine usually runs, perhaps with a rough idle or hesitation. A sustained lean mixture raises combustion temperature and damages the converter and pistons over time. This code will cause an emissions test failure.
What P0174 means
The module trims fuel per bank using that bank’s own upstream oxygen sensor. When bank 2 needs more added fuel than the module has authority to supply, it sets P0174. The single most useful diagnostic fact about this code is whether P0171 is stored with it: one bank lean means the cause is on that bank, while both banks lean means the cause is shared — the airflow sensor, fuel pressure, or a leak before the intake splits.
| System | Powertrain — fuel and air metering |
|---|
Symptoms you may notice
What causes P0174
Vacuum leak on the bank 2 side
A cracked hose, a failed intake gasket or a leaking PCV connection downstream of where the intake splits admits unmetered air to one bank only. This is the most common cause when P0174 is stored alone.
Contaminated mass airflow sensor
A sensor under-reporting airflow starves both banks equally. If P0171 is stored alongside P0174, this becomes the leading suspect rather than a bank-specific leak.
Common misdiagnosis
Only clean the sensor with dedicated MAF cleaner. Solvents and physical contact destroy the sensing element.
Injectors on bank 2 restricted
Deposits reduce flow on one bank while the other runs correctly. Less common than a leak, and worth checking only once air has been ruled out.
Fuel delivery falling short
A tiring pump or clogged filter leans both banks, most obviously under load. Expect P0171 to be stored alongside if this is the cause.
How to diagnose it
Check whether P0171 is stored alongside
Scan for all codes. Whether one bank or both are lean determines the entire direction of the diagnosis.
Record fuel trim on both banks
With the engine warm, read short and long term trim for each bank at idle and at a steady 2500 rpm. Comparing the banks against each other is more informative than either value alone.
Inspect the bank 2 intake for leaks
Examine the hoses, gaskets and connections feeding bank 2. Flex each hose with the engine idling and listen for the idle to change.
Smoke test the intake
Introduce low-pressure smoke into the intake with the engine off and watch for escapes around the manifold, injector seals and vacuum connections.
Related codes
Common questions
What does bank 2 mean?
The group of cylinders that does not contain cylinder 1. Inline engines have only one bank, so P0174 on a four-cylinder usually indicates a wiring or module fault.
P0171 and P0174 together — what does that mean?
Both banks are lean, so the cause is something they share: the mass airflow sensor, fuel pressure, or a leak upstream of where the intake splits. A bank-specific gasket cannot cause both.
Is a lean condition harmful?
Over time, yes. Lean mixtures burn hotter, which damages the catalytic converter and, in severe cases, pistons and valves.
Last verified 2026-07-27 · Sources: SAE J2012 — Diagnostic Trouble Code Definitions and Failure Type Byte (SAE International)
This information is for general guidance and is not a substitute for professional diagnosis. Cost figures are estimates, not quotes.