Trouble codes
OBD-II trouble codes explained
When your check engine light comes on, the car stores a code that says which system reported a problem. Below are 16 of the most common ones, in plain English — what each means, what usually causes it, and roughly what the repair costs.
A code is a symptom, not a diagnosis
A trouble code tells you which system reported something out of range. It does not tell you which part failed. P0420 says the catalytic converter is not cleaning exhaust as well as expected — it does not say whether the converter is worn out or an oxygen sensor is reporting badly, and those are very different bills.
That gap is why live sensor data matters. Reading the code is the first step; working out which cause is actually yours takes data from the running engine. How an AI car diagnostic app does that.
All codes
Catalytic converter
Fuel trim and air/fuel mixture
Evaporative emissions (EVAP)
Sensors and circuits
Not finding your code?
This list covers common powertrain and emissions codes. Codes for brakes, ABS, steering and airbags are not here — those systems are safety-critical, and a web page is the wrong place to start on them. If you have one of those, get a professional inspection rather than a DIY walkthrough. The app reads any code your car reports, including the ones not listed here.
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than guessing.
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