The EVAP system traps gasoline vapor from the fuel tank in a charcoal canister and later lets the engine draw that vapor in and burn it, a process called purging.
Variable valve timing lets the engine computer shift camshaft timing while driving, using engine oil pressure routed through a control solenoid to a phaser on the camshaft.
The thermostat keeps the engine cold-running time short by holding coolant in the engine until it warms up, then opening to keep it at a steady temperature.
The engine computer times spark and fuel from the crankshaft and camshaft position sensors, protects the engine from knock using knock sensors, and drives the ignition coils.
These codes cover inputs and outputs the engine computer depends on but that are not part of a single emissions system: the vehicle speed sensor, idle speed control, the engine oil pressure sensor and the system voltage the computer runs on.
Turbocharged and direct-injection engines rely on boost pressure and high fuel pressure, and many engines recirculate some exhaust (EGR) to lower combustion temperatures.
Anti-lock brakes stop the wheels locking under hard braking, and the same control module usually runs traction control and electronic stability control, which brake single wheels and can cut engine power to keep the car pointed where the driver steers.
Modern vehicles have many control modules (engine, transmission, ABS, airbags, body) that share information over a wired network, usually a two-wire CAN bus.
The engine computer (ECM or PCM) runs fuel, spark and emissions control and also checks itself: its memory, its processor, its programming and the regulated voltage it supplies to sensors.
The fuel pump in the tank supplies fuel, often through a separate pump control module, and the injectors spray it into the engine on command from the engine computer.
Hybrids and electric vehicles add a high-voltage traction battery, an inverter that turns its direct current into alternating current for the drive motor, the motor-generators themselves and a DC/DC converter that charges the ordinary 12-volt system.
Most new vehicles use electric power steering: an electric motor on the column or rack adds force based on a torque sensor that measures how hard the driver turns the wheel.
Body electrical covers the modules and circuits outside the engine and chassis: the body control module, keys and the immobilizer, exterior and interior lighting, door locks, the instrument cluster and the power feeds to these modules.
Modern diesels clean their exhaust with a particulate filter (DPF) that traps soot and burns it off, and a selective catalytic reduction (SCR) system that sprays diesel exhaust fluid (DEF) to cut nitrogen oxides.
A secondary air system uses an electric air pump to blow fresh air into the exhaust for the first minute or two after a cold start, which helps the catalytic converter warm up quickly and burn off extra fuel.
The alternator (generator) keeps the battery charged and powers the car while the engine runs, and on most newer cars the engine computer controls its output.
Four-wheel-drive and all-wheel-drive systems send engine power to both axles through a transfer case, a power transfer unit or an electronically controlled coupling.
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Severity scale
How we decide severity
Every code gets one of four labels. Stop and Inspect soon calls cite a source, and a technician reviews the rules. When we can't verify severity, we say so instead of guessing low.
Severity: StopDon't drive. Driving on risks damage or danger.9 codes
Severity: Inspect soonUsually drivable for a short time with care.592 codes
Severity: MonitorLow immediate risk.156 codes
Severity: Severity not verifiedNo source yet, so we never guess low.292 codes
Check engine light 
Check engine light 
Check engine light