Why your car needs a spring break – Part 4  

Posted on April 9th, 2021 by Rob Marshall

Why your car needs a spring break - Part 4   

In our fourth instalment, looking at how cars suffer from infrequent use and what can be done at service time, Rob Marshall focuses on exhaust emissions that can hit your wallet hard, if they cause a post-lockdown MOT Test failure.


The topic of exhaust emissions is immensely complex. While you can read much on the Internet about how to reduce them as much as possible, we hope that this blog gives a simple explanation about why a car can fail an MOT emissions test, if it has spent the last year being driven regularly on short trips.


The most toxic exhaust gases are treated primarily by catalytic converters (petrol), or diesel particulate filters, positioned within the exhaust system. Unless alerted to a problem by a dashboard warning lamp/message, you should not be too worried.


Catalytic converters and diesel particulate filters are considered by mechanics to be ‘after-treatment’ systems. Therefore, if the engine is not working efficiently, these systems must work harder. An emissions-based MOT failure may not be an after-treatment fault but may indicate a problem with the engine.


One reason why driving with a cold engine repeatedly causes engine efficiency to drop is that oily deposits build on specific components. These can restrict the air intake (effectively strangling the engine), deteriorate the fuel injector spray patterns (reducing engine efficiency), or even gum-up other systems. Soot can also build within the after-treatment systems, so embarking on an occasional longer trip, especially at speeds over 40mph, is essential to help burn-off these deposits.



Especially if a car was unused for long periods over lockdown, corrosion can also strike at electrical connections, of which there are many in a modern car. On petrol engines especially, this can cause a weak spark (or misfires) that, again, result in inefficient running and high emissions.


Therefore, it would be worth asking your garage at service time for advice if there is any extra work that they would recommend to clean-up existing issues and what your technician recommends as a preventative measure.