P0674 fault code: Cylinder 4 Glow Plug Circuit Medium severity

P0674 means “Cylinder 4 Glow Plug Circuit” — a computer & output circuits fault in the powertrain. Depends on symptoms — if the car runs normally, short journeys are fine while you investigate. Typical UK repair cost: £0–£800 depending on the underlying cause.
MediumSeverity
P06xxComputer & Output Circuits
£0–800Typical UK repair cost
MOT fail riskif the warning light stays on

What does P0674 mean?

P0674 is a generic (SAE standard) powertrain code reported by your car’s engine control unit. It sits in the P06xx block, which covers computer & output circuits. The stored description is “Cylinder 4 Glow Plug Circuit”. The code itself tells you which circuit or system tripped the fault — not always the exact part that failed, which is why a short diagnosis pays for itself before replacing components.

Symptoms you may notice

  • Multiple unrelated warning lights at once
  • Intermittent electrical gremlins
  • Car cranks but will not start

Most common causes

  1. Low battery voltage corrupting module communication (very common)
  2. Corroded earth straps or connectors
  3. Failed relay or output driver
  4. Genuine ECU internal fault (rare — always rule out power supply first)

How to diagnose P0674 yourself

  1. Test battery health and charging voltage first — weak batteries trigger phantom ECU codes
  2. Inspect and clean main earth points and battery terminals
  3. Clear codes and re-scan after a full drive cycle
  4. Only consider ECU repair or replacement once supply voltage is proven good
Tools that read and clear P0674

A basic code reader confirms the fault; a full-system scanner shows the live data you need for the steps above.

Will P0674 fail an MOT?

Warning lights tied to these faults (engine, ABS, airbag) are checked at MOT — an illuminated MIL, ABS or airbag lamp is a Major defect.

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.

Typical UK repair costs

OutcomeTypical cost (parts + labour)
DIY: read code + basic checks£0 (scanner assumed)
Independent garage diagnosis£30–£60
Typical repair range for this fault£0–£800
Main dealer equivalent£0–£1440

Costs are indicative UK ranges for independent garages in 2026. Main dealers typically charge 40–80% more.

Diagnose the symptom, then price the repair

Car diagnostic test cost UK (2026): scan vs actual diagnosis£40–£100 is typical for an independent first assessment — but a code read is not the same as finding the cause.
P0600Serial Communication Link P0601Internal Control Module Memory Check Sum Error P0602Control Module Programming Error P0603Internal Control Module Keep Alive Memory (KAM) Error P0604Internal Control Module Random Access Memory (RAM) Error P0605Internal Control Module Read Only Memory (ROM) Error P0606ECM/PCM Processor P0607Control Module Performance

P0674 — quick answers

Is it safe to drive with P0674?

Depends on symptoms — if the car runs normally, short journeys are fine while you investigate.

Will P0674 clear itself?

If the underlying fault stops occurring, most ECUs extinguish the light after around 40 healthy warm-up cycles — but the code stays stored as history. Fix the cause, clear it with a scanner, then confirm it stays away over a full drive cycle.

How much does it cost to fix P0674 in the UK?

Typically £0–£800 at an independent garage depending on which cause you confirm. Diagnosing it yourself with a scanner avoids a £30–£60 diagnostic fee and stops parts being replaced on guesswork.

Will P0674 fail my MOT?

Warning lights tied to these faults (engine, ABS, airbag) are checked at MOT — an illuminated MIL, ABS or airbag lamp is a Major defect.

Code definitions follow the SAE J2012 standard. 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.