Cars · Lesson 5 of 15Intermediate8 min read

Exhaust and Emissions

Burned gases leave the engine hot, loud and full of pollutants. The exhaust system carries them out, quiets them, and cleans them up, while the engine computer watches every step and tells you when something goes wrong.

The path out

Exhaust leaves each cylinder through the exhaust manifold, which gathers it into one pipe. It flows past an oxygen sensor, through the catalytic converter, past a second oxygen sensor, through the muffler, and out the tailpipe. The upstream sensor helps the computer set the fuel mixture; the downstream one lets it check whether the converter is still doing its job.

Three loops send gases back to be burned or cooled instead of released. Sources: U.S. EPA; OpenStax Chemistry 2e; Wikipedia (secondary) for sensor placement.

The catalytic converter

Inside the converter, the exhaust flows over surfaces coated with catalytic metals, most often platinum and rhodium. A catalyst speeds up a chemical reaction without being used up. In a modern three-way converter, carbon monoxide and unburned hydrocarbons combine with oxygen to make carbon dioxide and water vapor, and nitrogen oxides are split back into nitrogen and oxygen. These reactions work best when the engine runs close to the chemically correct air-fuel ratio, which is one reason the computer works so hard to hold it there.

The first catalytic converters arrived in 1975. Unleaded gasoline was introduced at the same time, because lead in gasoline ruins converters.

Three loops that keep pollution in

EGR (exhaust gas recirculation) sends a little exhaust back into the intake. Exhaust won’t burn again, so it soaks up heat and lowers the peak combustion temperature, and cooler burning makes less nitrogen oxide.
PCV (positive crankcase ventilation) handles blow-by: combustion gases that sneak past the piston rings into the crankcase. Instead of venting them to the air, the PCV system draws them into the intake to be burned.
EVAP (evaporative emissions) catches gasoline vapor from the tank in a canister of activated charcoal. When the engine runs, the canister is purged and the vapor is burned. That’s why a loose or missing gas cap can turn on the check engine light: the system tests itself for leaks.

OBD-II: the car checks itself

Every car and light truck from the 1996 model year on has a second-generation on-board diagnostic system, OBD-II. It monitors emission-related parts for wear and failure. When it finds a problem, it stores a diagnostic trouble code and turns on the check engine light, which is usually yellow.

Steady light: the car may not be running properly. It isn’t an emergency, but get it diagnosed soon. First, check that the gas cap is tight; after a loose cap is fixed, the light should go off within a few days.
Flashing light: a severe misfire is happening. Unburned fuel can overheat and seriously damage the catalytic converter, often the most expensive part to replace. Get it addressed right away, drive as little as possible, and avoid high speeds and heavy loads.

Reading codes with a scan tool

The standard diagnostic connector sits under the dashboard on the driver’s side. A scan tool plugs in and reads the stored codes. Codes that start with P are powertrain codes (engine and transmission); C, B and U codes cover chassis, body and network problems. In a P code, a second digit of 0 marks a standard code that means the same on every car, and 1 marks a manufacturer’s own code. The third digit names the area: for example, 3 means the ignition system or a misfire.

Type a code to see what each part means. The common codes listed come from New Jersey’s emissions inspection program.

NoteA code is a clue, not a diagnosis

A misfire code such as P0301 tells you the engine is misfiring, not why. The cause could be a spark plug, an ignition coil, an injector or a compression problem. Good technicians test before they replace parts.

Clearing codes doesn’t fix anything. It also resets the car’s readiness monitors, the self-tests an emissions inspection checks, and a car can fail inspection as “not ready” until it has been driven enough for them to run again; most cars get there in a few minutes of normal driving, but some need several days of mixed city and highway driving.

Federal rules require emission warranties: for 1995 and newer cars, the on-board computer and the catalytic converter are covered for 8 years or 80,000 miles. Check before you pay for those repairs.

Parts and what they do
Part	What it does	Signs of trouble
Exhaust manifold	Collects exhaust from the cylinders into one pipe.	Exhaust leaks: noise, an exhaust smell. Have leaks fixed promptly.
Oxygen sensors	Report rich or lean; the second one checks the converter.	Check engine light; codes such as P0141 (sensor heater).
Catalytic converter	Turns carbon monoxide, hydrocarbons and nitrogen oxides into less harmful gases.	Rotten-egg smell; code P0420.
Muffler	Quiets the exhaust.	A loud rumble; a clunk from a loose pipe or muffler.
EGR valve	Recirculates some exhaust to cool combustion.	Code P0401 (not enough flow).
EVAP canister and purge valve	Store fuel vapor and feed it to the engine.	Codes such as P0455 (vapor leak).
Gas cap	Seals the fuel system.	Check engine light if loose or missing.

StopExhaust leaks and carbon monoxide

Exhaust contains carbon monoxide, an odorless, colorless gas that kills without warning. A small leak in the exhaust system can let it build up inside the car, so the CDC advises having a mechanic check the exhaust system every year. Never run a car or truck inside a garage attached to a house, even with the garage door open. A rotten-egg smell usually points to the catalytic converter or another emission control part.

Check yourself

Tap an answer to check it, or open “Show the answer.”

What does a flashing check engine light usually mean?
Show the answer
A severe misfire that can damage the catalytic converter. A blinking light means a severe misfire. Get it addressed right away and drive as little as possible.
Where is the OBD-II connector on most cars?
Show the answer
Under the dashboard on the driver’s side. It’s under the dash on the driver’s side.
How does EGR cut nitrogen oxides?
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It lowers the peak combustion temperature. Recirculated exhaust lowers the peak combustion temperature, and less NOx forms.
Why can a loose gas cap set the check engine light?
Show the answer
The EVAP system detects a vapor leak. The evaporative system tests itself for leaks, and an open filler neck is a leak.
In code P0301, what does the 3 tell you?
Show the answer
Ignition system or misfire. For standard P0 codes, a third digit of 3 means the ignition system or a misfire.

Go deeper

U.S. EPA: What to Do with the Check Engine Light (EPA420-F-00-041) EPA’s one-page guide to the check engine light.
New Jersey Motor Vehicle Commission: Your Vehicle's Emissions (owner education brochure) A state inspection program’s guide to common codes and readiness monitors.
OpenStax Chemistry 2e, 12.7 Catalysis The chemistry of catalysts, including catalytic converters.
Clemson University Vehicular Electronics Laboratory: On Board Diagnostic Systems How OBD-II works, from Clemson University.

Sources

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A free course from Teddy Rook Book. It explains how cars work in general and follows U.S. sources; your owner’s manual and your vehicle’s service information are the final word for your car, along with state and local rules.