OBD2 Codes Chart: Printable Check Engine Code Reference
When the check engine light clicks on, the car is speaking in code, and most drivers have no translator handy. Every car sold in the US since 1996 carries OBD-II diagnostics: plug in a scanner and you get a five-character code where P means powertrain, the next digits name the system, and the last digits name the exact fault. Without a reference, a code like P0455 is just alphabet soup. This printable chart lists the 12 most common OBD2 trouble codes with their meaning, the affected system, the usual cause, and the first check to make before paying for a repair. Consult the live reference table below next time the light comes on, or print the sheet below and keep it in the glove box for calm, informed decisions.
Quick-Reference Table
The full data from this cheat sheet, right on the page.
| Code | Meaning | System | Common Cause | First Check |
|---|---|---|---|---|
| P0300 | Random / Multiple Cylinder Misfire | Ignition / Powertrain | Bad spark plugs, faulty ignition coils, vacuum leaks | Flashing light means active misfire; swap coils to isolate |
| P0301 | Cylinder 1 Misfire Detected | Ignition / Powertrain | Failing coil or spark plug on cylinder 1 | Move the cylinder-1 coil; if the misfire moves, replace the coil |
| P0171 | System Too Lean (Bank 1) | Fuel & Air Metering | Vacuum leak, faulty MAF sensor, clogged injector | Listen for a vacuum hiss and inspect the MAF sensor |
| P0174 | System Too Lean (Bank 2) | Fuel & Air Metering | Same as P0171, on the engine's second bank | Check intake boots and fuel trim on V6/V8 engines |
| P0420 | Catalyst Efficiency Below Threshold (Bank 1) | Emissions | Failing catalytic converter, bad O2 sensor, exhaust leak | Rule out exhaust leaks, then check the O2 sensor waveform |
| P0430 | Catalyst Efficiency Below Threshold (Bank 2) | Emissions | Failing catalytic converter on bank 2 | Check both cats if P0420 and P0430 appear together |
| P0442 | EVAP System Small Leak Detected | Emissions / EVAP | Loose gas cap or cracked EVAP hose | Tighten or replace the gas cap first |
| P0455 | EVAP System Large Leak Detected | Emissions / EVAP | Missing or loose gas cap, damaged EVAP hose | Check the gas cap, then inspect hoses and purge valve |
| P0128 | Coolant Temp Below Thermostat Temp | Cooling System | Thermostat stuck open, low coolant | Watch the gauge on cold start; a weak thermostat is the usual fix |
| P0401 | EGR Flow Insufficient | Emissions | Clogged EGR valve or carbon-blocked passages | Clean or replace the EGR valve |
| P0101 | MAF Circuit Range / Performance | Fuel & Air Metering | Dirty or failing mass airflow sensor | Clean the MAF sensor with proper cleaner |
| P0135 | O2 Sensor Heater Circuit (Bank 1, Sensor 1) | Emissions | Failed heating element inside the O2 sensor | Replace the sensor; a cold sensor reads slowly |
How to Use This Chart
Pin this chart where you work: it packs 12 entries and 5 columns (Code, Meaning, System, Common Cause, First Check) into one printable page, built for quick lookups.
Scan down the Code column to your item, then read across for Meaning, System, Common Cause, First Check.
Example: for P0300, the chart gives Meaning of Random / Multiple Cylinder Misfire.
- Compare values down a single column to spot the pattern before you commit to a choice.
- When two rows look close, the last column usually breaks the tie.
- Print it once and keep it where you work — that's the whole point of a cheat sheet.
💡 Quick Tips
Check the gas cap first: Tighten or replace the cap before anything else: it fixes most P0442 and P0455 EVAP codes.
Read all codes together: One vacuum leak can trigger P0171, P0300, and P0420 at once, so fix the shared root cause first.
Clear and retest after repairs: A code that comes back after clearing is real, while one that stays gone was a fluke.
Frequently Asked Questions
Can I drive with the check engine light on?
A steady light means the fault needs attention but is usually safe to drive briefly, so schedule a check soon. A flashing check engine light signals an active misfire that can damage the catalytic converter, so reduce speed and get it checked promptly.
What does the first letter of an OBD2 code mean?
The first letter names the system: P is powertrain (engine, transmission, emissions), B is body (airbags, locks), C is chassis (ABS, steering), and U is network (module communication). A second digit of 0 means a generic code every brand shares.
Will an OBD2 code clear itself?
Some codes clear after several drive cycles if the fault does not return, but most stay stored until cleared with a scanner. Clearing a code without fixing the fault just restarts the cycle, so always verify the fix with a test drive.
Printable Sheet Preview
High-Contrast Layout
Download Your Free Printable Sheet
Download the printable PDF below. It is the same sheet shown in the preview above, ready to print on US Letter or A4 — open it and choose Print, with Scale set to Fit to Page.
💡 Pro Printing Tip: In your printer settings, set Scale to “Fit to Printable Area” and enable “Background Graphics” for color headers.