Most Common Check Engine Codes and What They Usually Mean
Last reviewed May 2026 · Find This Code Editorial Team
Not all check engine codes are created equal. A handful of OBD-II fault codes account for the majority of check engine lights seen in real-world driving. This guide covers the most common ones — what they mean, what typically causes them, and what to do first. Remember: a code identifies the system that triggered the fault, not necessarily the part that needs to be replaced.
P0420/P0430— Catalyst Efficiency Below Threshold
These are two of the most common check engine codes seen in shops. P0420 covers Bank 1 and P0430 covers Bank 2. Both mean the catalytic converter is not reducing emissions as efficiently as the ECM expects — based on comparing the oxygen sensor readings upstream and downstream of the cat.
What to check first: Important: the code points to reduced catalyst efficiency, not necessarily a dead catalytic converter. Causes range from a failed upstream O2 sensor giving false data, to an exhaust leak near the sensor, to a genuinely worn-out catalyst. Always diagnose before replacing an expensive catalytic converter.
P0171/P0174— System Too Lean
P0171 (Bank 1 lean) and P0174 (Bank 2 lean) appear when the engine is running with too much air and not enough fuel — or more precisely, when the ECM has to add more fuel than expected to maintain the correct air/fuel ratio. The ECM knows this because the upstream oxygen sensors are staying lean.
What to check first: Lean codes rarely point directly to a failed fuel injector. Far more often the cause is a vacuum leak, a dirty or failing mass airflow (MAF) sensor, a clogged fuel filter, or low fuel pressure. Start by checking for vacuum leaks and cleaning the MAF sensor before assuming a fuel delivery problem.
P0300/P0301/P0302/P0303/P0304/P0305/P0306— Misfires
P0300 indicates a random or multiple-cylinder misfire. P0301 through P0306 indicate misfires in specific cylinders (cylinder 1 through 6). A misfire means one or more cylinders failed to complete combustion in a given cycle. If the check engine light is flashing rather than solid, a severe active misfire is happening and you should stop driving immediately to prevent catalytic converter damage.
What to check first: Misfires can have many causes: worn spark plugs, a faulty ignition coil, a bad fuel injector, a vacuum leak, low compression, or a timing problem. Always read freeze frame data — it shows the RPM, load, and coolant temp when the misfire occurred, which helps narrow down the cause.
P0440/P0442/P0455/P0456— EVAP Leaks
These codes relate to the evaporative emission control system (EVAP), which prevents fuel vapors from escaping into the atmosphere. P0440 is a general EVAP fault. P0442 and P0456 indicate small leaks. P0455 indicates a large leak. These codes will not cause drivability problems — your car will run fine — but they can cause emissions test failures.
What to check first: Before assuming a major repair, check the gas cap first. A loose, cracked, or worn gas cap is the single most common cause of EVAP codes, particularly P0442 and P0456. Tighten or replace the cap and drive several cycles — the light may clear on its own. If it returns, a professional smoke test is the most reliable way to find EVAP leaks.
P0135/P0141/P0155— Oxygen Sensor Heater Faults
P0135, P0141, and P0155 indicate that the heater circuit inside an oxygen sensor has failed or is not performing within spec. Modern O2 sensors have an internal heater so they reach operating temperature quickly after a cold start — allowing the ECM to enter closed-loop fuel control faster and reduce cold-start emissions.
What to check first: A heater circuit fault often means the O2 sensor itself needs to be replaced, but check the wiring and connector first. A chafed wire, a broken connector pin, or a blown fuse can cause the same code without requiring a sensor replacement.
Codes tell you which system, not which part
One of the most important things to understand about OBD-II codes is that they describe a condition the ECM detected — not a diagnosis. A P0420 doesn't mean "replace your catalytic converter." It means the catalyst efficiency monitor failed. A P0171 doesn't mean "replace a fuel injector." It means the engine ran lean.
The code is the starting point for diagnosis, not the end. Using live data, freeze frame data, and systematic testing will get you to the actual cause far more reliably — and far more cheaply — than replacing parts based on the code alone.