P0040 fault code: O2 Sensor Signals Swapped Bank 1 Sensor 1/ Bank 2 Sensor 1

P0040 means “O2 Sensor Signals Swapped Bank 1 Sensor 1/ Bank 2 Sensor 1” — a fuel and air metering fault in the powertrain.

Can I DIY?

Professional-only boundary. Scan P0040 only; do not unplug or cross oxygen-sensor connectors.

Do not DIYReviewed page boundary
ToolsSpecialist equipment

This is the conservative boundary from the reviewed code record. This page has no frozen site-wide safety grade, so it does not publish a repair route.

What to tell the garage about P0040

Do not drive until checked. The computer thinks the two upstream oxygen sensors (bank 1 sensor 1 and bank 2 sensor 1) have their signals crossed. Fuelling on a V-layout can look mirrored. This is identity/wiring, not heater resistance.

Take these reviewed, code-specific details with you. They help the garage confirm the fault without turning this page into a repair procedure.

  • O2 Sensor Signals Swapped Bank 1 Sensor 1/ Bank 2 Sensor 1: Registration number
  • O2 Sensor Signals Swapped Bank 1 Sensor 1/ Bank 2 Sensor 1: Exact code P0040 and any companion codes
  • O2 Sensor Signals Swapped Bank 1 Sensor 1/ Bank 2 Sensor 1: Whether the engine management light is steady or flashing
  • O2 Sensor Signals Swapped Bank 1 Sensor 1/ Bank 2 Sensor 1: Engine layout if known (V6/V8 versus inline)
P00xxFuel and air metering
MOT fail riskif the warning light stays on

What does P0040 mean?

P0040 is a generic (SAE standard) powertrain code reported by your car’s engine control unit. It sits in the P00xx block, which covers fuel and air metering. The stored description is “O2 Sensor Signals Swapped Bank 1 Sensor 1/ Bank 2 Sensor 1”. The code itself tells you which circuit or system tripped the fault — not always the exact part that failed.

Will P0040 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. P0040 does not change that lamp rule.

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.

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.
P0001Fuel Volume Regulator Control Circuit/Open P0002Fuel Volume Regulator Control Circuit Range/Performance P0003Fuel Volume Regulator Control Circuit Low P0004Fuel Volume Regulator Control Circuit High P0005Fuel Shutoff Valve 'A' Control Circuit/Open P0006Fuel Shutoff Valve 'A' Control Circuit Low P0007Fuel Shutoff Valve 'A' Control Circuit High P0008Engine Position System Performance

P0040 — quick answers

Is it safe to drive with P0040?

Do not drive until checked. We have no verified driving guidance for P0040. Escalated: do not drive until the swapped upstream pair is confirmed. Stop if the engine runs very rough or the lamp flashes.

Will P0040 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. P0040 does not change that lamp rule.

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.