Putting the wrong fuel in your vehicle—known as misfuelling—is more common in the UK than many drivers realise. While some motorists deal with the issue immediately, others may drive on unaware or assume no long-term damage has been done. Unfortunately, incorrect fuel use can have lasting effects that may surface later during an MOT test.
Misfuelling typically occurs when petrol is added to a diesel vehicle or, less commonly, diesel is put into a petrol car. This often happens at unfamiliar fuel stations, when drivers are distracted, or when switching between vehicles. Even a small amount of the wrong fuel can affect engine components if the vehicle is started or driven.
In some cases, misfuelling causes immediate breakdowns or warning lights, prompting drivers to seek repairs straight away. However, if the vehicle continues to run, internal damage may still occur. Over time, contaminated fuel can reduce lubrication, clog filters, and damage injectors or fuel pumps—issues that may not become obvious until months later.
One of the most common ways misfuelling impacts MOT results is through emissions failures. Damaged injectors or incomplete combustion can cause higher exhaust emissions, particularly in diesel vehicles. This may result in your car failing the MOT emissions test, even if it previously passed without issues.
Excess smoke, poor fuel burning, or warning lights linked to engine management systems can all be traced back to earlier misfuelling incidents.
Long-term engine damage caused by incorrect fuel use may lead to rough idling, reduced power, or starting problems. While some of these issues may not cause an outright MOT failure, they often appear as advisories. Over time, unresolved advisories can turn into failures, increasing repair costs.
Checking your vehicle’s MOT history using a trusted service like Check MOT can help identify patterns or recurring issues linked to fuel or engine performance.
Misfuelling can damage components such as catalytic converters, diesel particulate filters (DPFs), and oxygen sensors. These parts are directly assessed during an MOT. A faulty DPF or emissions control system caused by fuel contamination may result in an automatic failure, even long after the original misfuelling incident.
If you’ve ever misfuelled your vehicle, it’s wise to inform your mechanic and keep records of any fuel system cleaning or repairs. Regular servicing, early diagnostics, and addressing warning lights promptly can reduce the likelihood of MOT failures related to past fuel mistakes.
Yes. Damage to emissions systems or fuel components may only become apparent during later MOT tests.
Not always, but issues like emissions failures or engine warning lights may be linked to previous misfuelling.
Generally, yes. Petrol reduces lubrication in diesel engines, increasing the risk of serious damage.
While not required, ensuring repairs are properly documented can help if issues arise.