Nuisance (8)
(Previously MOT Test of Exhaust and Emissions)
The MOT tests both the exhaust system and the emissions coming from the exhaust system. The May 2018 revision of the MOT Testing Manual groups exhaust emissions with noise and fluid emissions under the new heading of ‘Nuisance’.
Noise suppression system
The Tester will inspect:
- exhaust silencers
- under-bonnet noise deadening material fitted as original equipment.
Exhaust noise will be assessed during the emissions test for the vehicle or by revving the engine to around 2,500rpm or half the maximum engine speed if this is lower on vehicles not subject to an emissions test
Exhaust noise from the vehicle must not be unreasonably above the noise level normally expected from a similar vehicle with a standard silencer in average condition.
The following categories apply to defects in this section:
| Defect | Category |
|---|---|
| a. Exhaust noise levels in excess of those permitted | Major |
| b. Any part of the noise suppression system: i. insecure | Major |
| ii. likely to become detached | Dangerous |
MOT Test of Exhaust and Emissions
Exhaust
This part of the MOT is considerably simplified, although it now covers exhaust fumes entering the vehicle which was not the case previously.
Spark ignition engine emissions
This inspection is only for vehicles that must have a full catalyst emissions test (disregarding the basic emissions test (BET)). Only components that are visible and identifiable, such as catalytic converters, oxygen sensors and exhaust gas recirculation valves will be checked.
The exhaust system will fail the MOT if:
- Part of the system missing or excessively deteriorated
- Mounting are missing or damaged so that the exhaust system is insecure
- There is a major leak
- The system emits appreciably more noise than a similar vehicle fitted with a standard system in good condition.
Emissions
These are checked using specialised equipment, the details of the check depending on the year that the vehicle was first used on the road. Excessive smoking (checked visually) is a reason for failure.
Diesel smoke emissions are checked by using a smoke meter.
The following categories apply to defects in this section:
| Defect | Category |
|---|---|
| a. Emission control equipment fitted by the manufacturer: missing, obviously modified or obviously defective. | Major |
| b. An induction or exhaust leak that could affect emissions levels. | Major |
Gaseous emissions
All vehicles with spark ignition engines first used on or after 1st August 1975 will be checked.
The following vehicles will not be checked:
- L category vehicles
- hybrid vehicles – with electric and combustion engines
- hydrogen fuel cell vehicles
- two stroke engines – unless they are subject to a catalyst test
If a vehicle first used before 1 September 2002 is fitted with an engine that’s older than the vehicle, it will be Tested to the standards applicable for the engine. The vehicle presenter must have proof of the age of the engine.
If a vehicle first used on or after 1 September 2002 is fitted with a different engine, it will be Tested to the emissions standards for the age of the vehicle.
If an engine has been modified in any way, it still has to meet the exhaust emission requirements according to the age of the vehicle.
A personal import must be Tested according to its date of first use. However, if you show the Tester a letter from the vehicle manufacturer proving that the engine doesn’t meet British emission standards it will be Tested to the next lower emission standard.
For emissions purposes only the following will be treated as first used before 1 August 1975:
- kit cars and amateur built vehicles first used before 1 August 1998
- Wankel rotary engined vehicles first used before 1 August 1987
- Q plated vehicles
Kit cars
Kit cars and amateur built vehicles first used on or after 1 August 1998 must have either Single Vehicle Approval (SVA) or Individual Vehicle Approval (IVA).
Kit cars or amateur built vehicles will be Tested to the limits in the vehicle’s registration document (V5c). If the V5c doesn’t show any limits, it will be tested to the limits based on the date the vehicle was first used.
Vehicles exempt from emission limits
Some vehicles may never have been able to meet the MOT limits for CO or HC emissions. The vehicle owner must provide proof of this, such as a letter from the vehicle manufacturer. If the vehicle owner can’t provide proof of this, the vehicle will fail the MOT test if the vehicle isn’t within the emissions limits.
Specialist conversions
For emissions purposes, specialist conversions will be treated as if they had not been converted.
For example, a motor caravan or ambulance converted from a goods vehicle will still to be treated as not being a passenger car, whereas an ambulance converted to a goods vehicle, or a passenger car with seats added will still be treated as being a passenger car.
Similarly, a vehicle originally built with 6 or more passenger seats, in addition to the driver, which has had seats removed will still be treated as not being a passenger car.
Dual exhaust systems
A dual exhaust system has 2 separate pipes from the engine manifold to the tailpipes.
The emissions from both tailpipes will be averaged – even if the system has a balance tube between the separate pipes.
Leaking exhaust system
If a vehicle has an exhaust holed to the extent that it will fail its MOT, the emissions should be re-checked when the exhaust is repaired even if the vehicle doesn’t leave the Testing Station. The Tester should inform you that any emission readings taken with a leaking exhaust might be incorrect.
Dual fuel systems
Vehicles which run on more than one fuel, such as petrol and LPG, should be Tested on the fuel they are running on when presented.
Emissions limits
The Tester will use various documents and charts to determine the correct emissions test for your vehicle. This will take into account the age of the vehicle (date first used) the class and the fuel type, as well as:
- vehicle make
- vehicle model
- model code
- engine code
- engine size
- VIN
The following categories apply to defects in this section:
| Defect | Category |
|---|---|
| a. Emissions levels exceed the manufacturer’s specified limits | Major |
| b. Emissions levels exceed default limits | Major |
| c. Lambda coefficient outside the default limits or the range specified by the manufacturer | Major |
| d. Emissions test unable to be completed | Major |
| e. Engine is idling clearly above its normal idle speed | Major |
| f. Exhaust emits dense blue or clearly visible black smoke for a continuous period of 5 seconds at idle | Major |
| g. Exhaust emits excessive dense blue or clearly visible black smoke during acceleration which would obscure the view of other road users | Major |
| h. Engine MIL inoperative or indicating a malfunction | Major |
Compression ignition engine emissions (diesel)
Only components that are visible and identifiable, such as diesel oxidation catalysts, diesel particulate filters, exhaust gas recirculation valves and selective catalytic reduction valves will be checked.
If a diesel particulate filter has clearly been cut open and rewelded, it will be rejected unless you can show evidence that there was a valid reason to cut it open, such as for filter cleaning.
The following categories apply to defects in this section:
| Defect | Category |
|---|---|
| a. Emission control equipment fitted by the manufacturer missing, obviously modified or obviously defective | Major |
| b. An induction or exhaust leak that could affect emissions levels | Major |
| c. Evidence that the diesel particulate filter has been tampered with | Major |
Diesel emissions opacity
The Tester will ensure that your vehicle is properly prepared and it is safe to carry out a diesel emissions test. A diesel smoke meter will be used if the vehicle was first used on or after 1st January 1980.
The smoke test will not be carried out if:
- there is insufficient oil in the engine
- the engine oil pressure is too low
- there is abnormal engine noise
- the governor has been tampered with
- the camshaft belt is in an unsatisfactory condition
If the Tester judges it to be unsafe to conduct the smoke test, you will be shown the reason for refusing to carry out the test on the VT30 (see Introduction 4h).
The following categories apply to defects in this section:
| Defect | Category |
|---|---|
| a. Smoke opacity levels exceed the manufacturer’s specified limit | Major |
| b. Smoke opacity levels exceed default limit | Major |
| c. Exhaust emits excessive smoke or vapour of any colour to an extent likely to obscure the vision of other road users | Dangerous |
| d. Exhaust on a vehicle fitted with a diesel particulate filter emits visible smoke of any colour | Major |
| e. Emissions test unable to be completed | Major |
| f. Emissions test aborted because smoke levels are significantly in excess of the specified limit values | Major |
| g. Engine MIL inoperative or indicating a malfunction | Major |
Read MOT Test of Exhaust and Emissions article
Fluid leaks
The Tester will check for fluid leaks on all vehicles other than Class 3. This will be done with the engine idling.
A leak of fluids such as engine coolant, screen wash and fluid required for Selective Catalyst Reduction aren’t reasons for failure. The Tester will fail a vehicle if a fluid leak creates a pool on the floor within 5 minutes that’s more than 75mm in diameter or if there are many leaks which collectively leak fluid at the same rate.
The Tester may refuse to carry out the test if there’s an excessive fluid leak. In these circumstances, if a Test fee has been paid, it will be returned.
The following categories apply to defects in this section:
| Defect | Category |
|---|---|
| a. Fluid: i. leaking excessively and likely to harm the environment or to pose a safety risk to other road users | Major |
| ii. leaking continuously and likely to pose a serious risk to road safety | Dangerous |
Link to all MOT Test procedures and failure items descriptions HERE