KNOWLEDGE
How a diesel particulate filter works, why it has to clean itself and what is behind the numbers in Deep DPF Inspect – from soot mass to the NOx sensor. Including why a measured value can sometimes be negative.
THE BASICS
Diesel combustion produces fine soot particles. The diesel particulate filter (DPF) traps them before they leave the tailpipe. It is usually a ceramic honeycomb made of silicon carbide or cordierite with thousands of fine channels. These are plugged alternately, so the exhaust has to pass through the porous walls and the soot stays behind. An intact filter retains well over 95 % of the particles.
Carbon from combustion. It can be burned off at high temperature – that is the regeneration.
Mineral residue, mostly from engine oil. It does not burn and stays in the filter permanently.
Formed at high combustion temperatures. They are reduced by EGR and the SCR catalyst, not by the DPF.
REGENERATION
A filter that only collected soot would be clogged in no time. That is why the soot is burned off regularly. This happens in two ways:
On longer drives under load the exhaust is warm enough for nitrogen dioxide from the oxidation catalyst to slowly oxidise the soot at around 250–450 °C. You won’t notice anything – but you can see it as a calculated soot mass that barely rises on the motorway.
Once the load reaches the trigger threshold, the control unit raises the exhaust temperature to around 550–650 °C using post-injection. The soot then burns off within roughly 10–20 minutes. Typical signs: higher idle speed, slightly higher consumption, the fan running on, start-stop inactive.
If the filter is too full for a regeneration while driving, a workshop can start a stationary regeneration with a diagnostic tool. Deep DPF Inspect only reads and does not start a regeneration.
IN THE APP
Every reading appears as a tile in the app. The name is at the top, the value with its unit in the middle and – where available – the matching setpoint or limit below it.
The app has not received anything for this tile yet, e.g. right after connecting or while the vehicle has not been fully identified.
The control unit has replied but does not yet flag the value as valid. Typical for NOx sensors while warming up – see below.
Your vehicle does not provide this value, or not with this equipment. That is not a fault.
READINGS
The first tab of the app: filter load, regeneration and the most important engine readings.
A model value from the engine control unit. From injected fuel, load, speed, temperatures and driving profile it estimates how much soot has reached the filter – minus what was burned off passively along the way. It rises steadily in everyday driving and drops to almost zero after a complete regeneration.
Derived from the differential pressure across the filter and the exhaust flow: the more soot in the channels, the higher the flow resistance. It reflects the filter’s real condition but fluctuates more and is only meaningful with sufficient exhaust flow. It can be clearly lower than the calculated soot mass – that is normal. Shortly after a regeneration slightly negative values are possible too – more on that below.
Shows whether a regeneration is running – for example “Normal operation” or an active regeneration. During an active regeneration exhaust temperatures and post-injection quantities rise, often the idle speed too, and the radiator fan keeps running afterwards.
How close the filter is to the next regeneration. At 100 % the trigger threshold is reached and the vehicle starts a regeneration as soon as conditions allow.
Small amounts of fuel injected after the main injection. They do hardly any work in the cylinder; instead they burn in the oxidation catalyst and heat the exhaust for regeneration. Usually 0 in normal operation – clearly above 0 is a reliable sign that a regeneration is running. Some fuel can end up in the engine oil, which is one reason why regenerations should not be interrupted repeatedly.
Exhaust temperature right in front of the filter. Roughly 150–400 °C in normal driving; during an active regeneration it is raised to around 550–650 °C on purpose.
Exhaust temperature behind the filter. During a regeneration it is often higher than the inlet temperature because the burning soot releases extra heat – a good sign that soot really is being burned off.
Distance driven since the end of the last successful regeneration. Depending on engine and driving profile, a few hundred kilometres between regenerations is typical. If the intervals become much shorter, take a closer look at your driving profile and soot readings.
Engine operating time since the last successful regeneration.
Fuel consumed since the last regeneration. This value can also trigger a regeneration.
Pressure difference between filter inlet and outlet. It depends strongly on exhaust flow: low at idle, many times higher under load. Only compare it at similar operating points, e.g. at a steady speed.
Engine temperature. The control unit only starts an active regeneration once the engine is warm.
Temperature of the engine oil. It rises more slowly than the coolant temperature – after a cold start the oil takes considerably longer to warm up.
Exhaust gas recirculation (EGR) routes part of the exhaust back into the intake. This lowers the combustion temperature and therefore nitrogen oxides. The large value is the current valve position, “Target:” below it is the control unit’s setpoint. Small differences during load changes are normal.
Engine oil level. The second line shows the mileage at which it was last measured. If the oil level rises without topping up, this can indicate fuel in the oil caused by frequently interrupted regenerations.
Pressure in the engine’s oil circuit, with the setpoint below. Modern engines regulate oil pressure according to speed and load – it is lower at idle than at high engine speed.
The vehicle’s operating point. Regenerations work best at a steady speed with medium load, e.g. on country roads or the motorway.
Incombustible residue, mainly from engine oil additives. Ash cannot be burned off and accumulates over the filter’s whole life. The second line “of max.” shows the limit – once reached, the filter must be cleaned or replaced. An approved low-ash (low-SAPS) oil slows the increase.
Total mileage of the vehicle and fuel left in the tank.
READINGS
The second stage of exhaust aftertreatment: in the SCR catalyst nitrogen oxides are converted into nitrogen and water using AdBlue. DEF is the international name for AdBlue.
Fill level of the AdBlue tank. AdBlue is a 32.5 % urea solution.
Estimated range of the remaining AdBlue. Refill in good time: once the tank is empty, the engine cannot be restarted after switching it off.
Urea content of the AdBlue. The target is about 32.5 %. Clear deviations point to diluted or wrong fluid.
Temperature of the SCR catalyst. It only works effectively above a certain temperature – with a cold catalyst little or no AdBlue is injected.
The amount of AdBlue the control unit is currently injecting. It depends mainly on the nitrogen oxides upstream of the catalyst and on the SCR temperature. The value also appears in the NOx/PM tab.
AdBlue injected during the current driving cycle.
Average AdBlue consumption the control unit expects, and the actual one. If both differ widely over a long period, this may point to dosing or fluid-quality problems.
Temperature of the AdBlue in the tank. AdBlue freezes at about −11 °C, so tank and lines are heated. Negative values in winter are normal. The value also appears in the Temperatures tab.
Total amount of AdBlue injected, summed up over the vehicle’s life.
READINGS
All temperatures at a glance. Coolant and oil temperature are explained above under “DPF values”, DEF temperature under “SCR / AdBlue”.
Outside air temperature. In cold weather it takes longer for the engine, exhaust system and sensors to warm up.
Temperature of the air in the intake. It is usually close to the outside temperature but can rise clearly above it when driving slowly or standing still.
Control of the electric radiator fans. If the vehicle has only one fan, the second tile shows N/A. If the fan keeps running after switching off, a regeneration was often just active.
READINGS
Sensors in the exhaust that measure directly what leaves the tailpipe. The tab also shows the AdBlue dosing rate target because it is directly linked to the NOx readings.
Nitrogen oxide concentration upstream of the SCR catalyst – i.e. what the engine produces. It varies strongly with load and EGR.
Nitrogen oxide concentration downstream of the SCR catalyst. When the system is warm and working properly, it is clearly lower than “NOx before SCR”. The ratio of the two shows how well the catalyst is converting right now.
Newer vehicles have a particle sensor behind the DPF that checks whether the filter is intact. Low values are good. The sensor collects soot itself and burns itself clean at intervals – so readings are not always continuous. The B2S1 tile only applies to vehicles with a second exhaust bank; otherwise it shows N/A.
READINGS
Tapping the car icon in the top right opens the overview of the connected vehicle and adapter.
The detected vehicle brand and the vehicle identification number. The VIN is partly masked with asterisks on screen.
Software and hardware part numbers of the engine control unit plus the engine code. They identify the engine unambiguously.
The model year derived from the VIN and a plain-text description of the engine, e.g. “R4 2.0l TDI”.
How the app is connected to the vehicle: Bluetooth Low Energy, the OBD adapter in use and the diagnostic protocol the vehicle speaks.
COMMON QUESTION
Negative values often look like an error but usually have a physical explanation. The most important cases:
Measured soot mass is not weighed but calculated: the control unit compares the measured differential pressure with the pressure a soot-free filter (including ash) would have at the same exhaust flow. The excess is converted into grams of soot.
When the real soot content is close to zero – especially right after a regeneration – small tolerances are enough to push the result below zero: sensor zero point and accuracy, pressure pulsations, low exhaust flow at idle or on overrun, condensate in the sensor hoses and an ash share that differs slightly from the model.
In short: A few grams below zero is normal and not a fault. If the value stays clearly negative even under higher load and with a loaded filter, the differential pressure sensor and its hoses should be checked – e.g. for leaks or swapped connections.
At idle or on overrun the exhaust flow is so low that the pressure difference is almost zero. Sensor tolerance and pulsation then produce small negative values too.
Outside air, AdBlue or a cold engine before starting can of course be below zero in winter.
NOX SENSORS
NOx sensors are heated ceramic sensors that only measure at an operating temperature of several hundred degrees. After a cold start, though, their heater must not switch on fully straight away: there is still condensate in the exhaust, and a droplet on the hot ceramic could destroy it. The control unit therefore waits until the exhaust is dry (“dew point end”), then heats the sensor and only releases the value once it measures stably.
Depending on outside temperature, driving style and vehicle, this takes from under a minute to several minutes. During this time the control unit does supply data but marks it as not yet valid – the app then shows “wait”. As soon as the sensor is released, the ppm values appear automatically. After longer idling or overrun a sensor can briefly return to this state.
Condensate in the exhaust, sensor unheated. Display: -- or wait
Exhaust dry, sensor heater starts. Display: wait
Sensor hot and stable, value released. Display: ppm value
IMPORTANT
Which values are available and which ranges are normal depends on make, engine, equipment and software version. The figures given are typical orders of magnitude. Readings may be incomplete or incorrect; if warning lights or problems occur, have the vehicle checked by a professional.
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