A trouble code tells you what the car noticed, not what to replace.
Use the exact code from your scanner as evidence. Then combine it with the vehicle, symptoms, and testing to narrow down the cause.
Start with the exact code your scanner showed.
The code identifies a detected condition or system. It does not automatically identify the failed part.
What the first letter means
Powertrain
Engine, transmission, emissions, fuel, ignition, and related systems.
Body
Body electronics and systems that can vary significantly by manufacturer.
Chassis
Chassis-related systems such as braking, steering, and stability functions.
Network
Communication faults between control modules and vehicle networks.
Detailed guides for common OBD-II codes
These guides go beyond the definition. Each one explains likely cause categories, useful first checks, common parts swapping mistakes, and when the symptoms change the urgency.
Random / multiple cylinder misfire detected
P0300 is a starting point, not a spark-plug verdict. The useful next step is to determine whether the misfire is tied to ignition, fuel, air, mechanical condition, or a vehicle specific issue.
Read the P0300 guide →Catalyst system efficiency below threshold · Bank 1
A worn catalytic converter is one possibility, but P0420 can also be triggered by problems that damage the converter or make its oxygen sensor signals look inefficient.
Read the P0420 guide →System too lean · Bank 1
The engine is seeing a mixture or sensor pattern that requires more fuel correction than expected. The cause may be extra air, too little fuel, inaccurate airflow information, or another condition that makes the exhaust appear lean.
Read the P0171 guide →Evaporative emission system leak detected · large leak
A loose fuel cap is one common possibility, but a large EVAP leak can also come from disconnected hoses, valves, the canister, filler neck sealing, or other parts of the vapor-control system.
Read the P0455 guide →Coolant temperature below thermostat regulating temperature
A thermostat stuck open is a common cause, but coolant level, temperature-sensor accuracy, cooling-fan operation, ambient conditions, and vehicle specific logic also matter.
Read the P0128 guide →Why the definition is only the beginning
The same code can require a different diagnostic path on different vehicles or when the surrounding symptoms change. Manufacturer-specific codes can also use definitions that vary by make, model, engine, and control module.
Use the code as evidence. Combine it with the exact vehicle, companion codes, symptom timing, recent work, and test results before deciding which part or system actually deserves attention.
Ready to narrow it down?
Start with the vehicle, symptoms, or trouble code you already have. CarCause will guide you through the next useful check.