Fuel and air
P0174System too lean, bank 2
System Too Lean Bank 2
Get it diagnosed soon
Generic OBD-II code
What the computer is measuring
P0174 is the same lean condition test as P0171, applied to the second cylinder bank. The computer has been adding fuel to bank 2 to correct what the oxygen sensor reports, and the correction has reached the top of its range.
Whether this code arrives alone or in company is the whole diagnosis. The two banks share the air filter, the mass air flow sensor, the throttle body, the fuel pump, the filter and the pressure regulator. Anything wrong in that shared path leans out both sides and sets both codes. Anything wrong past the point where the intake splits, or in one bank’s injectors or exhaust, sets one code only.
So the first thing to do is not to inspect anything. It is to read what else is stored, and then look at the two banks’ fuel trims side by side. That single comparison halves the search before you open the bonnet.
Bank 2 is the side of the engine that does not contain cylinder number one, and which physical side that is varies by make. Confirm it for your engine before you start tracing hoses.
What you will notice
- Steady check engine light.
- Rough or hunting idle, better once the engine is revved.
- Hesitation under acceleration.
- Reduced fuel economy.
Likely causes, most common first
Ordered by how often each one actually turns out to be the problem, not alphabetically and not by part price.
A vacuum leak on the bank 2 side of the intake
A manifold gasket, an intake boot or a vacuum line feeding that bank. Still the most common single cause.
A shared cause showing on both banks
If P0171 is stored too, stop looking at bank 2 specifically. A dirty MAF sensor, low fuel pressure or a leak in a shared intake affects both sides equally.
Low fuel pressure
Starves both banks together, which is why this code rarely appears alone when the pump is the problem.
Restricted injectors on bank 2
Less common than a leak, but it produces a genuinely one sided lean condition.
An exhaust leak ahead of the bank 2 upstream oxygen sensor
Fresh air reaching the sensor reads as a lean exhaust regardless of what the cylinder actually burned.
What to check, in order
Check whether P0171 is stored alongside it.
Both codes together means the cause is shared and bank specific inspection is wasted effort. P0174 alone means the difference is physically on bank 2.
Compare the fuel trim values for the two banks directly.
A large gap between them confirms a one sided problem. Trims high and similar on both banks confirms a shared one.
Inspect the bank 2 intake runners, gaskets and vacuum lines with the engine idling.
A leak found on that side explains the split without further testing.
Verify fuel pressure against specification before condemning any injector.
Correct pressure rules out the pump and filter and leaves the bank 2 hardware.
Where this one turns up most
- Transverse V6 engines where the rear bank intake gasket is both hotter and harder to reach.
Codes that often show up alongside it
What a DriveDecode adapter can read
A DriveDecode adapter reads engine and emissions systems: the powertrain and emissions codes the OBD-II standard requires on every US vehicle it covers. It does not read ABS, airbag (SRS), TPMS or transmission-specific manufacturer codes. Those need make-specific decoding that generic OBD-II hardware cannot do.
Reading P0174 on your own car
DriveDecode pulls P0174 straight off the vehicle along with the freeze frame the computer stored when it set: the coolant temperature, engine load and speed at that exact moment. That context is what separates a fault that happens on a cold start from one that happens under load, and it is the first thing the list above asks you to narrow down.
Last reviewed 2026-09-05. Definition sourced from the published standard.