OBD-II code lookup
Check-engine light on? Enter the code and get a plain-language explanation, how serious it is, and where to start looking.
Covers the most common generic OBD-II codes. Definitions are general guidance, not a diagnosis — actual causes vary by vehicle.
Reading a trouble code like a mechanic
Every OBD-II code has the same anatomy. Take P0420: the P means powertrain, the 0means it’s a generic code with the same meaning on every brand, the 4 points at auxiliary emissions controls, and 20 is the specific fault — catalyst efficiency. A code starting P1 is manufacturer-specific and needs a brand reference.
For a fleet, the expensive part isn’t the code — it’s the days between the light coming on and anyone telling the office. Always ON Tracking reads codes remotely from every vehicle’s OBD port, so a misfire on a Tuesday morning shows up in the dashboard Tuesday morning — not at the next oil change.
Check-engine questions, answered
When a vehicle detects a fault, its computer stores a Diagnostic Trouble Code (DTC) and usually turns on the check-engine light. Codes follow a standard format: a letter (P = powertrain, B = body, C = chassis, U = network) followed by four digits identifying the subsystem and specific fault.
A steady light with a minor code (like a loose gas cap) is usually safe to drive and fix soon. A FLASHING check-engine light means an active misfire that can destroy the catalytic converter — pull over and get it diagnosed. When in doubt, check the code before deciding.
It can clear the light, but the underlying fault stays and the code returns once the vehicle re-runs its checks. It also erases readiness monitors, which can fail an emissions inspection. Fix the cause; the light then clears itself or with a scan tool.
OBD-based GPS trackers read diagnostic codes remotely. Always ON Tracking surfaces DTCs, engine data, and fault alerts for every vehicle in the fleet from one dashboard — no plugging in, no waiting for drivers to report a warning light.
See every vehicle’s codes — without plugging in
Our OBD trackers report diagnostics, engine data, and fault alerts for the whole fleet, live.