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Your check engine light is on, you've read the code, and now you're staring at something like P0420 wondering whether it means a $20 fuel cap or a $2,000 catalytic converter. This library exists to answer that question honestly, for Australian cars, in Australian dollars, with the rego and roadworthy implications no American site will ever tell you about.
Every car sold here since about 2006 speaks the same diagnostic language through a standard port, usually under the dash near your right knee. Codes follow one format: a letter, then four digits. The letter names the system: P for powertrain (engine and transmission, the vast majority of what you'll see), B for body, C for chassis, and U for network communication faults. The first digit tells you whether the code is generic (0, shared across every manufacturer) or manufacturer-specific (1). That's why P0420 means the same thing on a Camry and a Commodore, while a code like C1201 needs a Toyota-specific explanation.
One concept worth thirty seconds because it decides inspections: obd2 readiness monitors. The computer continually re-runs self-tests on its systems, and each test reports ready once it has completed. Clearing codes with an obd scanner resets every monitor to not-ready, and some, like the EVAP and catalyst monitors, take days of specific driving to complete again. An examiner with a scan tool reads a freshly cleared computer as exactly what it is, which is why the fix-it-properly-two-weeks-out approach beats the clear-it-the-night-before one every time.
You can clear it; the computer re-tests and relights the lamp if the fault is real. Clearing without reading first also throws away the freeze-frame data that makes diagnosis faster, so read, note, then clear.
The code itself isn't checked, but the illuminated lamp fails a NSW pink slip on cars over five years old and Victorian or Queensland certificates at sale or transfer. Recently cleared codes are detectable through not-ready monitors, so fix faults weeks before inspection rather than hours.
Generic codes starting with a 0 in the second position, yes: P0420 is P0420 everywhere. Manufacturer codes, with a 1 in that position or in the B/C/U ranges, need brand-specific interpretation, which is why our C1201 entry only makes sense on a Toyota.
