P0078 fault code: Exhaust Valve Control Solenoid Circuit

P0078 means “Exhaust Valve Control Solenoid Circuit” — a fuel and air metering fault in the powertrain.

Can I DIY?

Professional-only boundary. Do not strip an exhaust cam solenoid; book a workshop diagnosis.

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 P0078

Do not drive. The solenoid that varies exhaust-valve timing has a circuit fault. Exhaust cam control can freeze, which on some engines also changes leftover exhaust and turbo response, not just a noisy idle.

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

  • Exhaust Valve Control Solenoid Circuit: Registration number
  • Exhaust Valve Control Solenoid Circuit: Exact code P0078 and any companion codes
  • Exhaust Valve Control Solenoid Circuit: Whether the engine management light is steady or flashing
  • Exhaust Valve Control Solenoid Circuit: Any exhaust smell in the cabin versus only a lamp on the dash
P00xxFuel and air metering
MOT fail riskif the warning light stays on

What does P0078 mean?

P0078 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 “Exhaust Valve Control Solenoid Circuit”. The code itself tells you which circuit or system tripped the fault — not always the exact part that failed.

Will P0078 fail an MOT?

A lit malfunction indicator lamp fails MOT as Major. Exhaust-valve control solenoids are not a named MOT emissions fixture check.

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

P0078 — quick answers

Is it safe to drive with P0078?

Do not drive. We have no verified driving guidance for P0078. Do not drive until checked. Exhaust-cam control is not a driveway timing repair.

Will P0078 fail my MOT?

A lit malfunction indicator lamp fails MOT as Major. Exhaust-valve control solenoids are not a named MOT emissions fixture check.

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.