Most Common Check Engine Light Codes and What They Mean
Your check engine light is on. Here are the 10 most common OBD2 codes, what they mean in plain English, and what to do about each one.
Still stuck after this guide? You can ask a mechanic online — see pricing, expert credentials and common questions first.
How to Read the Code Yourself
Buy a basic OBD2 scanner for 15 to 25 dollars at any auto parts store or Amazon. Plug it into the OBD2 port under your dashboard on the driver side (usually below the steering wheel). Turn the ignition to On without starting the engine. The scanner reads the diagnostic trouble code. You can also get the code read for free at most AutoZone, O’Reilly, and Advance Auto Parts locations — just drive in and ask.
P0420: Catalytic Converter Efficiency Below Threshold
This is the single most common check engine code. It means the catalytic converter is not cleaning exhaust gases as efficiently as required. The fix ranges from cheap to expensive: it could be a faulty oxygen sensor (100 to 250 dollars) or a failing catalytic converter (500 to 2,500 dollars). Always replace the downstream oxygen sensor first — it is the cheaper test and resolves P0420 about 30 percent of the time.
P0171 and P0174: System Too Lean
These codes mean the engine is getting too much air and not enough fuel. The most common causes are a vacuum leak (a cracked hose or gasket letting extra air in), a dirty or failing mass airflow sensor, or clogged fuel injectors. Clean the mass airflow sensor first with MAF sensor cleaner spray (about 8 dollars) — this fixes it about 40 percent of the time and takes 10 minutes.
P0300 to P0308: Engine Misfire
P0300 means random misfires across multiple cylinders. P0301 through P0308 indicate a misfire in a specific cylinder (the last digit is the cylinder number). Common causes are worn spark plugs (replace all for 50 to 150 dollars), a failing ignition coil (40 to 100 dollars each), or a clogged fuel injector. Start by replacing spark plugs if they have not been changed in more than 60,000 miles.
P0442: Small EVAP System Leak
This code is often triggered by a loose or cracked gas cap. Tighten or replace the gas cap (5 to 15 dollars) and drive for a day or two. The light often turns off on its own. If it does not, there may be a small leak in the evaporative emission system hoses. This is not a drivability issue and the car is safe to drive — it only affects emissions.
P0455: Large EVAP System Leak
Similar to P0442 but indicates a larger leak. Check the gas cap first. If the cap is tight and not cracked, the purge valve or vent valve may be stuck open, or a hose in the EVAP system is disconnected. These repairs cost 50 to 200 dollars. Like P0442, this is an emissions code and does not affect drivability.
