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PRE MOT CHECKS

Avoiding MOT Failure
Petrol EmissionsDiesel EmissionsCatalytic ConvertersDiesel Particulate Filters
EMISSIONS

Most vehicles have their exhaust emissions tested by meter as part of the MOT.

Presence of Catalysts and Particulate Filters on Diesel Powered Vehicles
On 16 February 2014 Section 7.1 of the inspection manual will be amended to include a check for the presence of catalysts and particulate filters on diesel powered vehicles. Any vehicle where a catalytic converter or particulate filter is missing where one was fitted as standard will fail the test

Petrol vehicles used before August 1975 and diesel vehicles used before August 1979 are tested by sight.
The only vehicles that don't have an emissions test as part of the MOT are:
• vehicles with less than 4 wheels
• those with 2-stroke engines
• hybrid vehicles
• quadricycles

Emissions 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 is a reason for failure. Diesel emissions are checked by using a smoke meter.

A catalytic convertor fitted as original equipment but missing will be a reason for failure.

Fuel & Exhaust >>>

Exhaust System

Have your assistant place a rag over the exhaust while the engine is running. (Note they are not trying to block the exhaust just impede it)

Check system for leaks.
This will be a lot easier with the car jacked up but if you cannot jack the car up, a leak should still be more obvious when the pipe is partially blocked with a rag.

Joints are the normal places for leaks to occur and can sometimes be fixed by simply tightening up the bolts that hold the joints together. I say simply but the bolts can sometimes be badly corroded and seized. Applying some oil and allowing time for penetration will help.
Exhaust System IN SERVICE VEHICLE EMISSIONS STANDARDS

INTRODUCTION

This page explains the test procedures used to check vehicle exhaust emissions which Testers and Vehicle Examiners will follow whether at the roadside or during the MOT test. It contains, in the Annex, emissions limits for vehicles fitted with advanced emissions control systems such as three way catalytic converters, and offers general advice to motorists and vehicle operators on submitting vehicles for test. This information deals separately with petrol-engined vehicles and those with diesel engines.

The test procedures described on this page are included solely as a guide; they do not form part of the Regulations. Detailed test procedures are described in the: • MOT Inspection Manual, Private Passenger and Light Commercial Vehicle Testing
• HGV Inspection Manual and PSV Inspection Manual
WARNING TO VEHICLE OWNERS
It is important for owners of vehicles fitted with catalytic converters to understand that the catalyst is only one in a number of components, which collectively make up the emissions control system. Experience has shown that there are a number of reasons why vehicles fail the emissions test and, in many cases, defects which lead to excessively high emissions are often rectified by a simple adjustment. These defects can usually be rectified at little cost to the owner.

Where replacement parts are necessary, costs can increase significantly and it is important, therefore, that defects are accurately diagnosed. Incorrect diagnosis may lead to high cost items (such as fuel injection pumps, fuel injectors, catalytic converters, etc) being replaced unnecessarily. For day-to-day use of the vehicle, and care of the emissions control system, owners are advised to consult the vehicle handbook or seek advice from the manufacturer?s agent/dealer.

Accurate diagnosis of engine management systems often requires the use of diagnostic equipment and appropriately trained vehicle technicians. Motorists should, therefore, seek advice from reputable vehicle repairers before agreeing to repairs.

1.2 Checks on the vehicle before the test

As a general precaution motorists are advised to check that the vehicle has enough engine oil, the coolant level is correctly topped-up and there is enough fuel to carry out the test. If defects are known or suspected it is advisable to seek advice from a qualified service engineer and have the defects rectified in advance of the test appointment.

Wherever possible vehicles should arrive at the test station with the engine at its normal operating temperature (e.g. after a drive of approximately 5 miles). Before carrying out the test, the Tester will confirm that the engine is at its normal operating temperature which, in some cases, will require the use of an oil temperature probe inserted into the dipstick tube. Before proceeding, the tester will also check that the engine has sufficient oil and fuel to complete the test.

For vehicles with manual transmission the test will be carried out with the gear lever in the 'neutral' position and with the clutch engaged. For vehicles with automatic transmission the gear selector will be in either the 'neutral' or 'park' position.

1.3 Test Procedure

Visual inspection (all vehicles)
Once the preliminary checks have been completed, the tester will raise the engine speed to around 2500rpm or half the maximum engine speed if this is lower. The engine speed will be held steady for approximately 20 seconds after which the engine will be allowed to return to its natural idle speed. Once the emissions have stabilised the tester will assess the smoke emitted from the tailpipe. If the exhaust is emitting dense blue or clearly visible black smoke then the vehicle will fail the test. In exceptional cases, especially on certain vehicles manufactured before 1960 where emissions of smoke are unavoidable due to the engine design, these vehicles will not fail the test.

Standard emissions test
For vehicles first used on or after 1 August 1975, emissions will also be checked using an approved analyser. After completing the visual test, the tester will use the analyser to assess the concentration of carbon monoxide (CO) and hydrocarbons (HC) in the exhaust gases by inserting a sample probe into the exhaust tailpipe. The test is carried out with the engine at its normal idling speed and the analyser displays the results continuously. Once a stabilised figure is achieved the tester will record the result.

The vehicle must comply with the appropriate emissions limits. This test (the so-called “non-catalyst” test) is used for petrolfuelled cars and light goods vehicles which are not equipped with advance emission control systems such as catalytic converters.

Basic Emissions Test procedure (BET)
The basic emissions test is applied to all petrol fuelled vehicles first used on or after 1 August 1992. Despite its name this test is primarily aimed at identifying, and assessing emissions from, vehicles with advanced emissions control systems such as three-way catalytic converters. The test uses EC minimum in service emission limits for modern cars.

For cars subject to this procedure emissions are assessed during two separate tests. As the procedure is more complicated than the non-catalyst test, the emissions analysers include computer software aimed at guiding the Tester through the test sequence. The primary reason for the introduction of these meters is to ensure that the car is tested accurately.

The first test consists of checking the emissions at .fast-idle speed. which involves running the engine at a speed of 2500 - 3000 rpm. During this test the emissions of CO and HC will be checked and a further check will be made on the lambda (1) value. The second test consists of a check of emissions at .normal idle speed. in which the engine will idle between 450 and 1500rpm, but in this case only the CO emissions are assessed.

The emissions limits to be met are specified for both the fast and normal idle tests. At fast idle, CO must be at or less than 0.2%, HC at or less than 200 parts per million (ppm), and the lambda value(1) must be between 0.97 and 1.03. At normal idle, CO must be at or less than 0.3%.

Where these limits are not met, a check will be made to identify vehicles (e.g. those without a catalyst) which should only be required to meet the non-catalyst emissions check requirements.

Extended emissions test
As indicated above, an extended emissions test will be applied where the limits are not met under the basic emissions test and where the vehicle has been identified as needing to meet the "catalyst" test requirements. These will include petrol-fuelled cars fitted with advanced emissions control systems (e.g. vehicles with catalytic converters) first used on or after 1 August 1995, petrol fuelled cars first used on or after 1 August 1992 which are large passenger cars and light goods vehicles first used on or after 1 August 1994.

As in the basic test procedure, emissions are assessed during two separate tests. The "fast-idle" test in this case involves running the engine at the speed specified in the Annex to this publication (typically 2500 - 3000 rpm), and the emissions of CO, HC and Lambda values are checked. A check of CO emissions at .normal idle. speed is also carried out.

The analyser will require the tester to complete each stage of the test before proceeding to the next. In addition, where a failure is recorded during the fast idle test, the analyser will automatically schedule a repeat test.

This is to ensure, as far as practicable, that the catalytic converter has reached its normal operating temperature.

(1) Lambda is the ratio of the mass of air/mass of fuel divided by 14.7.
(2) Passenger cars first used before 1 August 1995 and not mentioned in the Annex to this publication are tested to non-catalyst standards.


Before starting the emissions test, the Tester will check the engine oil temperature to ensure that the engine is at the manufacturer.s stated operating temperature. This will require an oil temperature probe to be inserted into the dipstick tube.

Once the preliminary checks have been carried out and the Tester has identified the vehicle model (stored on the emissions analyser database) the Tester will begin the fast idle emissions test.

This test requires the engine speed to be increased to that specified in the Annex. Once the engine speed has stabilised between the upper and lower limits, the emissions will be measured for a continuous 30-second period. At the end of this period, the analyser will automatically compare the results of the test with those specified in the emissions analyser database. Provided the results are equal to or below the specified limits, the vehicle will have passed this section of the test and the analyser will automatically proceed to the normal idle test. As indicated above where a failure is recorded during the fast idle test, the analyser will automatically schedule a repeat test.

The "normal idle" test is the same as that described above for the basic emissions test.

The vehicle must pass both the fast and the normal idle emissions tests to secure a pass result.

Test procedures may be amended at any time. You are advised to refer to the relevant VOSA Inspection Manual for details of the test procedure which will be applied to a vehicle undergoing examination.

More about Emissions - Petrol >>>

DIESEL ENGINED VEHICLES

Vehicles to be tested
This inspection applies to all diesel engined vehicles with four or more wheels.

2.1 Checks on the vehicle before the test

As a general precaution motorists and vehicle operators are advised to check that the vehicle has enough engine oil, the coolant level is correctly topped-up and there is sufficient fuel to carry out the test. If defects are known or suspected it is advisable to seek advice from a qualified service engineer and have the defects rectified in advance of the test appointment.

Wherever possible vehicles should arrive at the test station with the engine at its normal operating temperature (e.g. after a drive of approximately 5 miles).

Before carrying out the test for cars and light goods vehicles the Tester must confirm that the engine is at its normal operating temperature. In most cases this will require the use of an oil temperature probe inserted into the dipstick tube.

The Tester will also check that the engine has adequate oil and fuel to complete the test before proceeding. In addition the Tester will check the condition of the camshaft drive belt (where visible) and the fuel injection pump (governor) anti-tampering seals.

For vehicles with manual transmission the test will be carried out with the gear lever in the 'neutral' position and with the clutch engaged. For vehicles with automatic transmission the gear selector will be in either the 'neutral' or 'park' position.

Emissions - Diesel >>>
Diesel Particulate Filters >>>
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