Car systems
Exhaust and emissions: how they work and what fails
The exhaust and emissions system cleans the gases before they leave: the catalytic converter transforms them, the oxygen sensor watches the mixture and the EGR lowers nitrogen oxides; on diesels, the particulate filter traps soot and DEF (AdBlue) neutralizes the NOx. It is the most common cause of a lit check engine light and of many failed emissions inspections.
Reviewed by Daniel Morales, founder of Reprorace SL, on
How it works
The oxygen sensor ahead of the catalytic converter measures the oxygen in the gases so the computer can adjust the mixture, and the one after it checks that the converter is still working. The EGR valve sends part of the exhaust back into the intake to lower the combustion temperature, and the EVAP system collects fuel vapors from the tank and burns them in the engine.
On diesels, the particulate filter (DPF) fills with soot and empties by burning it off during regenerations, which need trips at a steady speed; the differential pressure sensor tells how full it is. The SCR system injects DEF (AdBlue) ahead of a dedicated catalyst and NOx sensors check the result.
The most common faults
- Oxygen sensor: it ages or gets contaminated: higher consumption, a poorly corrected mixture and codes P0130 to P0167.
- Worn-out catalytic converter: efficiency below the threshold (P0420/P0430), almost always after misfires or oil consumption have damaged it.
- Clogged EGR: carbon leaves it stuck open or closed: jerking, black smoke, a rough idle and limp mode.
- Full particulate filter: short trips that do not let the regeneration finish: the DPF light, less power and very frequent regenerations.
- DEF (AdBlue): fluid quality, pump, injector or NOx sensors; with the countdown on the dashboard, the car may not restart once it reaches zero.
A guide, most likely first: a repair shop confirms the diagnosis of each car with its own data.
Symptoms that can come from this system
Engine warning light on and steady: what it means and what to do
Check without hurry
Rotten egg smell from the car: what it means
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AdBlue warning light on: what it means and what to do
See a garage soon
Diesel particulate filter (DPF) warning light: what to do
See a garage soon
Particulate filter regenerating too often: causes
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Exhaust fumes smell inside the car: why it is dangerous
Stop as soon as it is safe
Exhaust louder than normal: what it means
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Black smoke from the exhaust: causes and what to do
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Car loses power: what limp mode is
See a garage soon
The most relevant trouble codes
- P0420Catalyst system (bank 1): efficiency below threshold
- P0430Catalyst system (bank 2): efficiency below threshold
- P0130O2 sensor (bank 1, sensor 1), circuit: malfunction
- P0133O2 sensor (bank 1, sensor 1), circuit: slow response
- P0135O2 sensor heater (bank 1, sensor 1), circuit: malfunction
- P0136O2 sensor (bank 1, sensor 2), circuit: malfunction
- P0400EGR A flow: malfunction
- P0401EGR A flow: insufficient flow detected
- P0442EVAP system: small leak detected
- P0455EVAP system: large leak detected
- P2002Particulate filter (bank 1): efficiency below threshold
- P2463Particulate filter: restriction - soot accumulation (bank 1)
- P242FParticulate filter: restriction - ash accumulation (bank 1)
- P20EESCR NOx catalyst (bank 1, catalyst 1): efficiency below threshold
- P204FReductant system (bank 1): performance
Generic OBD-II/EOBD codes; each manufacturer adds its own. The same symptom can come from another system.
Code blocks of this system
- P04xxauxiliary emission controls
- P20xxparticulate filter, AdBlue and NOx
- P24xxEGR and particulate filter
- P22xxNOx sensors, turbo and fuel pressure
- P01xxfuel and air metering
- P00xxfuel and air metering and auxiliary emission controls
- P05xxvehicle speed, idle control and auxiliary inputs
- P21xxthrottle, accelerator pedal and EGR
Questions and articles about this system
- What is an oxygen sensor and when does it fail?
- What is a catalytic converter and why does it fail?
- What is EGR and what happens when it fails?
- What is a particulate filter and how long does a regeneration take?
- What is AdBlue (DEF) and what happens if it runs out?
- What are readiness monitors and why do they matter for inspections?
- How can a delivery fleet avoid particulate filter and DEF (AdBlue) faults?
- How do you prepare fleet vehicles for the periodic inspection?
Learn more
See what your car is saying, system by system
The ECO-LITE plugs into the OBD port and shows the codes and live data your car publishes on your phone, explained; when needed, you share the session with your repair shop.
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Frequently asked questions
Will a car pass inspection with the check engine light on?
Often not: emissions inspections check the light and, on many cars, read the onboard diagnostics. Clearing the codes just before leaves the monitors incomplete, which can also be a problem.
Can the EGR or the DPF be removed?
It is not legal on public roads in the EU or the US, and the car would no longer meet its approval. The right thing is to repair or clean them.
How do I know if the DPF is regenerating?
Some cars show it; on others you notice the fan running after you park, a hot smell and a slightly higher idle. With live data you can see the soot load and the filter's differential pressure.