Fault codeP0174

MODERATE

System Too Lean (Bank 2)

What this code means

System Too Lean (Bank 2).

“Bank 1” is the side of the engine that holds cylinder number 1; “Bank 2” only exists on V and flat engines. An in-line engine has a single bank.

P0174 is a generic code: it means the same thing on every make. P0174 belongs to family P01xx — Fuel and air metering. Its page explains how to read a code and groups all its codes by part.

Mixture regulation (fuel trim)

The control unit constantly adjusts the amount of fuel injected according to the oxygen sensors: these corrections are the fuel trims, short term and long term. When it reaches its correction limit without getting the right mixture, it records one of these codes.

Symptoms

Rough or high idle, hesitation under acceleration, lack of power, sometimes stalling when cold. The control unit sees too much oxygen in the exhaust, enriches the mixture up to its correction limit, and gives up. Fuel consumption may rise and misfires (P0300) may follow.

Common causes

Air leak after the airflow sensor: split hose, loose clamp, intake gasket, leaking crankcase ventilation
Dirty mass airflow sensor, under-reading the air coming in
Fuel pressure too low: clogged filter, tired pump, regulator
Dirty injectors
Ageing upstream oxygen sensor
Exhaust leak before the sensor, letting air in

How to diagnose it

In live data, read the fuel trims. If long-term trim is high at idle but drops at high revs, it is an air leak: the leak matters less as flow increases. If it is high at all revs, look at fuel delivery or the airflow sensor instead.

Find the leak by ear — a hiss at idle — or by spraying a little brake cleaner on the hoses with the engine running: the revs change when the product is drawn in. Cleaning the airflow sensor with a dedicated product costs a few euros and fixes a share of cases.

Watch it live in Klaro

With an OBD2 adapter plugged in, these live readings show the part at work, when your car reports them:

Short term fuel trim bank 1
Long term fuel trim bank 1
Short term fuel trim bank 2
Long term fuel trim bank 2
Mass air flow

What to do

Have it checked this week. The car remains usable, but the fault will not clear itself and will fail the roadworthiness test.

Clearing a code switches the light off but repairs nothing: if the fault remains, the code comes back. Clearing a fault code yourself →

Repair cost

A hose or gasket: €5 to €50. A new airflow sensor: €60 to €250. A fuel pump: €150 to €500 fitted. Start with the air leak — by far the most common and cheapest cause.

How to read this code

PPowertrain
0standardised code
1Fuel and air metering
74P0174