AdBlue: The Diesel Driver’s Friend… or Foe

History lesson incoming, not the most riveting part of this article, I’m aware, but stay with me, we’re just setting the scene…
The pursuit of cleaner automotive emissions in Europe began in earnest in the 1970s; however, it wasn’t until the 90s, with the introduction of the first Euro standard,  Euro 1, in 1992, that the initiative took off. This first standard mandated the use of catalytic converters for petrol cars to reduce carbon monoxide (CO) emissions. Subsequent standards, Euro 2 (1996) and Euro 3 (2000), progressively tightened limits for both petrol and diesel vehicles.

The focus intensified on diesel with Euro 4 (2005) and  Euro 5 (2009), specifically targeting particulate matter  (PM) and nitrogen oxides (NOx). A key development  under Euro 5 was the mandatory inclusion of Diesel  Particulate Filters (DPFs) for all new diesel cars from January 2011. This continuous tightening of emissions requirements, particularly for NOx, drove the development of sophisticated exhaust after-treatment systems like AdBlue (Selective Catalytic Reduction or SCR).

• • As of 2024, there were 11.6 million diesel passenger vehicles on UK roads.

• • This represents roughly a third of the total UK car parc.

• • The estimated Number of AdBlue-equipped diesel vehicles in the UK is approximately 5.6 million.

• • This equates to approximately 48.5% of the total 11.6 million diesel vehicles currently on UK roads.

• • The average age of a licensed vehicle in the UK at the end of 2024 was 10 years.

• • AdBlue systems became a fundamental requirement for new diesel cars sold from September 2015.

• • This means the earliest AdBlue-equipped vehicles are now  8-9 years old.

• • The average annual mileage for diesel cars in 2023 was 8,300 miles.

• • Meaning, as of today, a typical AdBlue-equipped diesel vehicle at the average age would have accumulated approximately 79,000 to 83,000 miles.

• • Some manufacturers have issued warranties on heating elements in the AdBlue system for 10 years or 100,000 miles.

• • However, some vehicles start to experience AdBlue system problems under 40,000 miles.

• • For certain models, particularly popular commercial vehicles like the Citroen Berlingo, Jumpy, Jumper, and Peugeot Partner, Expert, and Boxer, the AdBlue tank is a sealed, non-repairable unit. A replacement can cost between £2,000 and £3,000.

• • AdBlue Crystallisation: This is arguably the most prevalent issue. AdBlue is a urea-water solution, and when exposed to air or extreme temperatures (below -11°C or above 30°C), the water evaporates, leaving behind solid urea crystals. This crystalline build-up can clog filters, block injectors, and significantly reduce the efficiency of the SCR system. Frequent short journeys, where the SCR system may not reach its optimal operating temperature, can exacerbate this problem.  

• • Fluid Contamination: The introduction of low-quality, expired, or contaminated AdBlue (for example, mixed with dirt, fuel, oil, or water) can severely damage the SCR system components. This contamination reduces the fluid’s effectiveness, leading to increased NOx emissions and potential damage to catalysts and sensors.  

• • Sensor Failures: The AdBlue system relies on various sensors, including level sensors within the tank and NOx sensors in the exhaust. These electronic components can fail, leading to inaccurate readings or false warning lights, even if the AdBlue fluid level is correct. Replacing a NOx sensor alone can range from approximately £167 to £869.  

• • Pump and Injector Malfunctions: The AdBlue pump is responsible for delivering the fluid to the injector, which then sprays it into the exhaust stream. Both the pump and injector are susceptible to mechanical wear, electrical failure, or blockages caused by crystallisation. A faulty pump may not provide sufficient pressure, hindering the system’s operation. Injector blockages, in particular, lead to poor spray patterns and reduced efficiency.  

• • Software Glitches: In some instances, the physical components of the AdBlue system may be functioning correctly, but the vehicle’s Engine Control Unit (ECU) can develop a software glitch. This can result in persistent warning messages that require a manual reset or a software update by a qualified technician.   

• • AdBlue Tank Heater Failure: AdBlue freezes at temperatures below -11°C. Modern vehicles are equipped with heating systems to prevent this, but these heaters themselves can fail. A malfunctioning heater can lead to the AdBlue freezing and subsequent crystallisation, causing system malfunction.