This month has seen a huge increase in MOT tests, due to March being new registration month for new cars, continuing into April. This means that three years down the line these vehicles are due for an annual inspection, and generally most owners also have their car serviced at the same time. This is a wise decision because, without quality maintenance, it can be very difficult for an engine management system to keep the engine running to it’s optimum conditions, which will inevitably lead to emissions faults which can cause an MOT test failure.
On several occasions we were faced with the difficult diagnostic job on analysing MOT emission failures. Difficult because, unless the warning light is on on the dash, the management system assumes everything is running as it should be. This is when we have to break-out the tools and knowledge that allows us to trace this fault.
A Vauxhall Meriva was brought in for service and MOT, which it subsequently failed due to its high emissions levels. The CO volume percentage below, of 6.266% is over double the legal limit!
We analysed the emissions data and performed some component test, to find the reason for this failure. Testing the data seen by the management system and cross-referencing against the actual value at the sensor we found no electrical fault! It had to be mechanical.
I traced the fault to a blocked and swollen engine breather hose, this transfers gases created from the crankshaft rotating in oil and delivers them to the top of the engine where they are burnt during combustion. Sorry, I don’t have a picture of the breather hose, I seemed to have deleted it! But we ran the emissions test after replacing the breather hose and replacing the old contaminated engine oil, the change in the emissions created was immediate, as you can see by the image below.
We can’t stress highly enough the importance of regular servicing and maintenance.