P0042 fault code: HO2S Heater Control Circuit

P0042 means “HO2S Heater Control Circuit” — a fuel and air metering fault in the powertrain.

Can I DIY?

Professional-only boundary. Read P0042 only; bank-1 sensor-3 heater work is professional.

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 P0042

Do not drive until checked. The heater circuit for bank 1 sensor 3 is flagged. That extra probe on bank 1 is used on some layouts after a second catalyst. It is not bank 2, not sensor 2 (P0036), and not a boost solenoid.

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

  • HO2S Heater Control Circuit: Registration number
  • HO2S Heater Control Circuit: Exact code P0042 and any companion codes
  • HO2S Heater Control Circuit: Whether the engine management light is steady or flashing
  • HO2S Heater Control Circuit: Whether the tool named sensor 3 or only 'heater circuit'
P00xxFuel and air metering
MOT fail riskif the warning light stays on

What does P0042 mean?

P0042 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 “HO2S Heater Control Circuit”. The code itself tells you which circuit or system tripped the fault — not always the exact part that failed.

Will P0042 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. P0042 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

P0042 — quick answers

Is it safe to drive with P0042?

Do not drive until checked. We have no verified driving guidance for P0042. Escalated stop-until-checked for that third-sensor heater. Stop if exhaust smell is in the cabin.

Will P0042 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. P0042 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.