OBD-II (On‑Board Diagnostics II) is the system built into modern vehicles that monitors engines, emissions, and key subsystems. When a fault is detected, your car stores a Diagnostic Trouble Code (DTC) such as P0420 and turns on the Check Engine Light.

DTCs follow a standardized format, so a P0XXX code has the same base meaning across makes and models, with some manufacturer‑specific variations.

Your Engine Control Unit (ECU) constantly checks sensor inputs (O2 sensors, MAF, MAP, coolant temp, etc.) and actuator behavior. When readings fall outside expected ranges for a period of time, the ECU stores a code and may illuminate the MIL (Malfunction Indicator Lamp).

Use an OBD-II scanner to read codes, pending codes, and permanent codes. You can often clear codes, but do so only after you’ve addressed the underlying issue—otherwise the light may return quickly.

Readiness monitors indicate whether emissions tests have completed. After a battery disconnect or code clear, monitors reset to “Not Ready.” A specific drive cycle is often needed to set them to “Ready.”

Not every vehicle fault sets a code immediately, and some problems are intermittent. Treat OBD-II as a powerful guide, not the sole source of truth. For brakes/airbags/steering, consult a professional.