DPF regeneration: how it works, how to force it and when it fails

The DPF regeneration system is designed to look after itself in most situations. It works on three levels, from the most natural to the most technical, each one matching a different degree of soot loading in the filter.

Which type of regeneration takes place depends on how much soot is loaded in the DPF. This is measured continuously by the differential pressure sensor and the temperature sensors fitted before and after the filter. The engine control unit (ECU) analyses this data in real time to decide on the right regeneration strategy.

Passive regeneration is the ideal way for a DPF to work. It happens naturally, without any intervention from the ECU, whenever your driving produces enough heat in the exhaust system.

For the soot to start burning on its own, exhaust gas temperature needs to reach 550 to 650°C. That is typically achieved with sustained driving: motorway or dual carriageway speeds, with engine revs held above 2,500 to 3,000 rpm for at least 20 to 30 minutes in one go.

Under these conditions, the soot particles (mostly carbon) react with the nitrogen dioxide (NO₂) in the exhaust gas and are oxidised into CO₂. The process is continuous and gradual: soot is burnt off as it builds up, keeping the filter in optimum condition.

On cars fitted with an additive-assisted DPF (the system used by PSA, now Stellantis, which injects a cerium-based fluid), the additive acts as a catalyst and lowers the soot combustion temperature to around 450°C. This makes passive regeneration easier, even in less favourable driving conditions. The additive reservoir needs refilling roughly every 75,000 miles.

When passive regeneration is not enough to keep soot levels under control, the ECU automatically triggers an active regeneration. This kicks in when the filter reaches roughly 40 to 45% of its soot capacity, which on average means every 200 to 300 miles depending on how the car is driven.

The principle is simple: the ECU commands a post-injection of fuel, an extra, late injection in the engine cycle. This fuel does not burn in the combustion chamber but ignites when it reaches the oxidation catalyst ahead of the DPF, raising exhaust gas temperature to 600 to 650°C for 10 to 20 minutes.

The ECU may refuse to start an active regeneration in certain conditions: fuel level too low (usually below a quarter of a tank), abnormal coolant temperature, active fault codes in the engine management system, or a faulty differential pressure sensor. In that case, the soot keeps building up until the conditions are met, or until a trip to the workshop becomes necessary.

A forced regeneration is the last resort before professional DPF cleaning or replacement. It is carried out in a workshop by a qualified technician using a professional diagnostic tool (Delphi, Autel MaxiSYS, VCDS for VAG group cars, and so on).

The technician plugs the tool into the car's OBD2 port and accesses the special functions of the engine ECU. From there, a regeneration cycle can be commanded manually while the critical parameters are monitored live: exhaust temperature before and after the DPF, differential pressure, engine speed and injection quantity.

The procedure usually takes between 20 and 40 minutes. The engine is held at idle or slightly raised revs (1,500 to 2,000 rpm) while the ECU controls the post-injection to reach 600 to 700°C inside the DPF.

Before carrying out a forced regeneration, the technician must check that no blocking fault code will stop the process (failed pressure sensor, faulty temperature sensor, sticking injector). They also need to make sure soot loading has not gone beyond the point where regenerating could damage the filter (generally 80 to 85% loading). Beyond that, a professional chemical or ultrasonic DPF clean is the better option.

Prevention is still the best strategy for avoiding DPF problems. A few simple habits can significantly extend the life of your filter and save you the cost of cleaning or replacement.