UK oil refiners have renewed a voluntary agreement to test diesel fuel to ensure it won’t block filters in car engines.

The test was introduced in response to a wave of breakdowns across the country caused by filters becoming blocked by a gel-like substance in diesel.

The voluntary agreement was introduced by UKPIA, which represents the UK refining industry and also has Greenergy as an associate. It said its members had introduced a voluntary Filter Blocking Tendency (FBT) limit of 2.52 for diesel fuel to protect consumers.

The agreement was due to run until April 15, but UKPIA said it was being extended and it was continuing to fully participate in the British Standards Institute’s Task Group, which is investigating the issue.

The problem has been more prevalent in eastern parts of England and Scotland, with the greatest concentration of breakdowns occurring in the North East, and can arise from diesel purchased from any type of fuel retailer. The problem is not specific to any particular vehicle age or make.

It affected a small minority of motorists in the winter of 2012/13, with the largest number in March, but the problem disappeared over the summer. However, last November the RAC alone attended almost 600 such incidents.

The problem is not “waxing”, which used to be an issue in very cold weather, as modern diesel produced for UK winter use is formulated with additives to prevent waxing. The symptoms of the current problem are similar to waxing but the cause appears to be different. And, unlike waxing, when the weather warms up the gel does not dissolve back into the fuel which means a new filter has to be fitted.

UKPIA pointed out its members do not supply all of the fuel in the North East, where the problem has been greatest.

In a statement it said it was working with the British Standards Institute’s Task Group, which is investigating the matter. It added: “The parameters of investigative tests, at this stage, are all encompassing, including quality of biodiesel (FAME), base hydrocarbon fuel, how the product is blended and any other additives used.”