Catalytic Converter vs DPF vs FAP – Differences + Lambda Sensors

Catalytic converter, DPF, FAP filter – three exhaust parts that are often mixed up and very expensive to replace. Add two or three lambda sensors, temperature sensors and a pressure sensor, and it gets confusing. This guide explains, with a 3D model of a petrol and a diesel exhaust system, what each part does, how they were developed, where the sensors are and how everything works together.

Video: Catalytic Converter vs DPF vs FAP – What's the Difference? + Lambda Sensors (3D Animation), 6:19 min, by Auto Oprema. The video has English and Serbian audio (choose the audio track in the ⚙ settings) and subtitles.

The video is published on 15 November 2026. Until then the player shows that it is not yet available; the guide below is already complete.

Quick answer: what is the difference?

Part	What it does	Where
Catalytic converter (three-way, petrol)	Cleans the gases: CO and HC into CO₂ and water, NOx into nitrogen.	Petrol engines
Oxidation catalyst (DOC, diesel)	Burns CO and HC and heats the exhaust gas for DPF regeneration.	Diesel engines, before the DPF
DPF (diesel particulate filter)	Traps soot like a sieve and burns it during regeneration.	Diesel engines
FAP (filtre à particules)	The French name used by Peugeot and Citroën; the classic FAP uses an additive to burn soot at a lower temperature.	PSA diesel engines

A short history

1975 – the first catalytic converters on American cars; they needed unleaded petrol.
1976 – Volvo introduced the three-way catalyst with a lambda sensor and cut emissions by more than 90%.
1992 – Euro 1 made catalytic converters mandatory for petrol cars in Europe.
2000 – Peugeot launched the first series particulate filter, the FAP, on the 607 2.2 HDi.
Euro 5 made particulate filters standard on diesels; Euro 6 brought AdBlue, SCR and NOx sensors.

The catalytic converter

Inside is a ceramic honeycomb with thousands of thin channels, coated with platinum, palladium and rhodium. On a petrol engine it is a three-way catalyst: it turns carbon monoxide and hydrocarbons into carbon dioxide and water, and nitrogen oxides into nitrogen – but only if the mixture is exactly right (lambda 1) and only when it is hot, above about 300 °C.

A diesel always runs with excess air, so a three-way catalyst can't work. Instead it has an oxidation catalyst (DOC). It burns carbon monoxide and hydrocarbons and, during regeneration, burns the extra fuel injected by the computer to heat the exhaust gas for the particulate filter behind it.

The DPF and regeneration

The DPF looks like a catalytic converter but works like a sieve. The channels are closed alternately at the inlet and the outlet, so the exhaust gas must pass through the porous walls and the soot stays inside. When the filter fills up it must be cleaned by burning – regeneration:

On the highway the soot burns slowly by itself.
If that is not enough, the computer adds post-injections, the oxidation catalyst heats the filter to around 600 °C, and the soot burns into carbon dioxide.

DPF vs FAP: what is the difference?

FAP is the French name, filtre à particules, used by Peugeot and Citroën. The classic FAP uses an additive from a separate tank: the computer doses it into the fuel, and the cerium in the additive lowers the soot burning temperature to around 450 °C. Most other makers use a catalysed DPF without additive – the precious-metal coating helps the soot burn. If the additive runs out on a FAP system, regeneration fails and the filter clogs.

Lambda sensors and other exhaust sensors

Petrol engine

Front lambda sensor (before the catalyst) – regulating sensor. A classic zirconia sensor switches between about 0.1 and 0.9 V, and the computer corrects the mixture all the time.
Rear lambda sensor (after the catalyst) – diagnostic sensor. A working catalyst stores oxygen, so the rear signal stays almost flat. If the rear signal copies the front one, the catalyst is worn and you get code P0420.

Diesel engine

A wideband lambda sensor, usually after the turbo and before the oxidation catalyst and the DPF, measures excess oxygen for EGR control, smoke limit and regeneration.
Temperature sensors before the catalyst and before the DPF control the heat.
A differential pressure sensor measures the pressure before and after the DPF; from that the computer calculates how much soot is inside.
Euro 6 diesels add an AdBlue injector, an SCR catalyst and NOx sensors before and after it.

How it all works together – and what kills the DPF

Everything starts with the lambda sensor. A dirty or old sensor sends a wrong signal, often falsely lean. The computer adds fuel, consumption goes up, and the extra soot and heat end up in the catalyst and the DPF. On a diesel, short trips interrupt regeneration and the filter fills faster. These also kill the DPF:

a faulty pressure sensor,
a leaking EGR valve,
worn injectors,
the wrong engine oil – use low-ash oil approved for particulate filters.

Forced DPF regeneration with a diagnostic tool

If the DPF is full, a forced regeneration with a diagnostic tool can help. Example from an Opel Antara 2.0 CDTI:

The engine must be fully warm (coolant at least 72 °C), with enough fuel and no other fault codes.
The exhaust temperature climbs to around 600 °C and the differential pressure drops from 5–6 kPa to about 1 kPa.
It takes several minutes.
Safety: do it outside, never leave the car and keep a fire extinguisher nearby.

Summary

The catalytic converter cleans the gases, the DPF and the FAP trap the soot, and the FAP uses an additive to burn it at a lower temperature. The front lambda sensor regulates the mixture, the rear one checks the catalyst, and on a diesel temperature and pressure sensors control regeneration. Check the sensors first, then the filter.

Frequently asked questions

What is the difference between a DPF and a FAP?

FAP is the French name (filtre à particules) used by Peugeot and Citroën. The classic FAP uses an additive with cerium that lowers the soot burning temperature to about 450 °C; most other makers use a catalysed DPF without additive.

Does a diesel have a catalytic converter?

Yes, but not a three-way catalyst. A diesel has an oxidation catalyst (DOC) that burns CO and HC and heats the exhaust gas for DPF regeneration.

What does code P0420 mean?

The rear lambda sensor signal copies the front one, which means the catalytic converter no longer stores oxygen and is worn.

What is DPF regeneration?

Burning the trapped soot. On the highway it burns slowly by itself; otherwise the computer adds post-injections and the oxidation catalyst heats the filter to around 600 °C.

What damages a DPF?

A faulty lambda or pressure sensor, a leaking EGR, worn injectors, short trips that interrupt regeneration and engine oil that is not low-ash.

How does the computer know the DPF is full?

A differential pressure sensor measures the pressure before and after the DPF, and the computer calculates how much soot is inside.

Related 3D diagnostics guides

How to Check a Turbo – oil leaks, shaft play, blade damage and when to repair or replace the turbo.
Intercooler Cleaning – where the oil comes from and how to clean the intercooler the right way.
False Air / Vacuum Leaks – symptoms of unmetered air and 4 ways to find the leak.
Crankshaft Position Sensor Test – no start, stalling when hot, limp mode and 4 tests for the CKP sensor.
How to Test a MAF Sensor – power loss, high fuel consumption and 4 tests before you buy a new sensor.
Clogged EGR Valve – black smoke, rough idle, how to test the EGR and clean it properly.

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