Engine MIL Inoperative or Indicates a Malfunction (8.2.1.2 (h)) and MOT in the UK

The engine MIL (Malfunction Indicator Lamp), also known as the engine management or check engine light, is a dashboard warning light that signals a problem with your vehicle’s engine or emissions system. According to UK MOT guidelines (sections 8.2.1.2 (h) and 8.2.2.2 (g)), if the engine MIL is inoperative or indicates a malfunction, your car can fail its MOT test. This is because the light is a key indicator of your vehicle’s compliance with emissions and safety standards.

Main Reasons the Engine MIL Comes On

Several issues can cause the engine MIL to illuminate or become inoperative. Here are the most common:

Faulty Oxygen (O2) Sensor

The car oxygen sensor monitors exhaust gases to optimise fuel combustion. A faulty sensor affects emissions and fuel efficiency, triggering the MIL.

Mass Air Flow (MAF) Sensor Failure

The car mass air flow sensor measures the amount of air entering the engine. If it fails, the engine may run poorly or emit excess pollutants, causing the MIL to light up.

Loose or Damaged Fuel Cap

A loose, cracked, or missing fuel cap can allow fuel vapours to escape, which the system detects as an emissions fault.

Catalytic Converter Problems

The car catalytic converter reduces harmful emissions. If it fails, your car will pollute more and the MIL will come on.

Ignition System Issues (Spark Plugs/Ignition Coils)

Misfires caused by worn spark plugs or faulty ignition coils will trigger the MIL and can damage the catalytic converter if left unresolved.

Vacuum Leaks

Leaks in hoses or gaskets can disrupt the air-fuel mixture, causing rough running and MIL activation.

ECU or Wiring Faults

Problems with the Engine Control Unit (ECU) or wiring can prevent the MIL from working or cause it to display incorrect warnings.

Key Engine MIL-Related Car Parts and Price Ranges

The following table gives an overview of the most common engine MIL-related parts and their typical UK price ranges (parts only):

Part	Typical Price Range (GBP)
Oxygen (O2) Sensor	£40 – £150
Mass Air Flow (MAF) Sensor	£50 – £200
Fuel Cap	£10 – £30
Catalytic Converter	£200 – £1,000+
Ignition Coil	£30 – £100
Spark Plug	£10 – £40 (per plug)
Engine Control Unit (ECU)	£200 – £600+
Wiring Harness Repair	£50 – £300+

How Does Engine MIL Inoperative or Malfunction Affect Your MOT?

If your engine MIL is illuminated or not working at all, your vehicle will almost certainly fail its MOT. The MOT tester will check that all dashboard warning lights function correctly and are not showing faults. According to DVSA regulations, an illuminated or inoperative engine management light is a major defect and leads to automatic MOT failure.

Frequently Asked Questions (FAQ)

Can you pass MOT with engine light on?

No, you cannot pass the MOT with the engine light on. The fault must be diagnosed and repaired, and the light reset before the vehicle can pass.

Is an engine light always an MOT failure?

If the engine light is related to emissions or engine management, it is considered a major defect and will result in MOT failure.

What should I do if my engine light is on before an MOT?

Book a diagnostic check with a qualified mechanic. Address the underlying issue, repair or replace the faulty part, and ensure the light is off before your MOT test.