P0174 fault code: System Too Lean

Last updated · OBDCode UK editorial

Can I drive with P0174? — Drive with care

You may keep driving if the car feels normal. Stop if the engine management light flashes, the engine runs roughly, overheats, or you smell petrol or exhaust in the cabin.

P0174 means “System Too Lean” — a fuel and air metering fault in the powertrain. Drive with care. The engine computer has measured a lean mixture on Bank 2: too much air, or not enough fuel, on that bank. This is a fuelling-trim fault, not a catalytic-converter diagnosis on its own.

Can I DIY?

Caution — conditions apply. We do not publish DIY repair or strip-down steps for this code on this page. Read codes and observe only; leave the repair to a qualified garage.

Caution — conditions applyDIY boundary
ToolsBasic tools only — for example a household socket set, tyre gauge, or a consumer OBD reader.

Some checks may be possible for a careful DIY driver with the right tools and a safe place to work. Stop if anything feels beyond your skill or kit.

P01xxFuel and air metering
MOT fail riskif the warning light stays on

What does P0174 mean?

P0174 is a generic (SAE standard) powertrain code reported by your car’s engine control unit. It sits in the P01xx block, which covers fuel and air metering. The stored description is “System Too Lean”. The code itself tells you which circuit or system tripped the fault — not always the exact part that failed.

How to diagnose P0174 yourself

  1. Confirm P0174 and freeze-frame: Scan with an OBD-II reader, engine idling outdoors. Note whether P0174 is current or pending, freeze-frame, and whether P0171 is also stored (both banks lean). Stop if: The engine management light is flashing, the engine will not idle, or exhaust fumes collect around you.
    Generic driver footwell showing a broad OBD-II socket search zone and the usual 16-pin socket shape.
    Park safely and switch off before locating the socket. Its position varies, so use the vehicle handbook and never force or dismantle trim.
  2. Inspect Bank 2 intake vacuum hoses: Engine off, bonnet open, engine cold enough to touch. Look along Bank 2 intake plumbing for cracked, split or disconnected vacuum hoses. Do not remove an EGR valve or other bolted parts. Stop if: You see a fuel leak, damaged wiring, or you would need to crawl under the car.
  3. Read Bank 2 fuel trims and O2 data: Outdoors, with a scanner that shows live data, note short-term and long-term fuel trim for Bank 2 and whether the Bank 2 oxygen sensor is switching. Do not idle in a closed garage. Stop if: The light starts flashing, the engine overheats, or you feel unwell from exhaust fumes.

Will P0174 fail an MOT?

An illuminated engine management light is a Major defect at MOT (automatic fail) until the fault is repaired and the light stays off.

Before booking your test, check what usually fails on your car: see our MOT pass rates by make and model, built from 33.1 million DVSA tests.

What the MOT data says

No MOT certificate item corresponds to this code; MOT outcome data cannot be shown.

Related guides

Car diagnostic test cost: compare the scopeUK booking marketplaces currently publish diagnostic averages of about £52–£65. Engine management light on: steady or flashing?A steady engine management light means the computer has stored a fault. Drive with caution and get the code read. A flashing light is different — stop when it is safe.
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P0174 — quick answers

Is it safe to drive with P0174?

Drive with care. You may keep driving if the car feels normal. Stop if the engine management light flashes, the engine runs roughly, overheats, or you smell petrol or exhaust in the cabin.

Can I fix P0174 myself?

Caution — conditions apply. We do not publish DIY repair or strip-down steps for this code on this page. Read codes and observe only; leave the repair to a qualified garage. Basic tools only — for example a household socket set, tyre gauge, or a consumer OBD reader.

Will P0174 fail my MOT?

An illuminated engine management light is a Major defect at MOT (automatic fail) until the fault is repaired and the light stays off.

Fault-code identifiers are standard diagnostic facts. Their explanations and diagnostic guidance on this site are original editorial summaries, not reproduced SAE wording. MOT defect rules reflect the DVSA MOT inspection manual (2026). This page is general guidance for UK drivers, not a substitute for a professional diagnosis.