P3400 fault code: Cylinder Deactivation System
Driveability guidance is not available for this code — treat as do not drive until checked. Park the car; a skip-cylinder system fault needs a workshop, not a DIY solenoid swap.
Can I DIY?
Professional-only boundary. Confirm P3400, then book a garage. We do not publish cylinder-deactivation repair steps.
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 P3400
Do not drive. The computer has a system-wide fault on the skip-cylinder feature that shuts some cylinders to save fuel. That is not a single-wire open on cylinder 1, and it is not an exhaust-cleaning code.
Take these reviewed, code-specific details with you. They help the garage confirm the fault without turning this page into a repair procedure.
- Cylinder Deactivation System: Registration number
- Cylinder Deactivation System: Exact code P3400 and whether any P34xx cylinder-circuit codes are also stored
- Cylinder Deactivation System: Whether the engine management light is steady or flashing
- Cylinder Deactivation System: Whether the engine shook, misfired, or refused to idle smoothly
What does P3400 mean?
P3400 is a powertrain code from the extended SAE range reported by your car’s engine control unit. It sits in the P34xx block, which covers cylinder deactivation. The stored description is “Cylinder Deactivation System”. The code itself tells you which circuit or system tripped the fault — not always the exact part that failed.
Will P3400 fail an MOT?
An illuminated engine management light is a Major defect at MOT until the fault is repaired and the light stays off. P3400 is a cylinder-deactivation system flag, not a measured exhaust-flow or oxygen-storage reading.
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
Related fault codes
P3400 — quick answers
Is it safe to drive with P3400?
Do not drive. Driveability guidance is not available for this code — treat as do not drive until checked. Park the car; a skip-cylinder system fault needs a workshop, not a DIY solenoid swap.
Will P3400 fail my MOT?
An illuminated engine management light is a Major defect at MOT until the fault is repaired and the light stays off. P3400 is a cylinder-deactivation system flag, not a measured exhaust-flow or oxygen-storage reading.
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.