OBD2 check engine codes
OBD2 (On-Board Diagnostics II) codes are standardized fault codes your car's computer stores when it detects a problem. Any vehicle sold in the US after 1996 uses them. A code reader or scanner retrieves them—most auto parts stores will read them free. Select a code below to see what it means, what causes it, and what it typically costs to fix.
Misfires
A misfire means one or more cylinders isn't firing correctly. Can damage the catalytic converter quickly—especially if the check engine light is flashing.
Ignition Coils
Individual coil-on-plug failures cause single-cylinder misfires. The check engine light often flashes. Replace the coil and spark plug on the affected cylinder together.
Fuel & Air Mixture
These codes mean the engine is running too lean (too much air) or too rich (too much fuel). Left unfixed, they cause engine wear and catalytic converter damage.
Catalytic Converter & O2 Sensors
Catalyst efficiency codes indicate the catalytic converter isn't cleaning exhaust as well as required. O2 sensor codes affect fuel control accuracy and emissions.
EVAP (Fuel Vapor) System
EVAP codes mean the fuel vapor containment system has a leak or fault. Usually not a driving safety issue, but will fail an emissions test.
Fuel Injectors
Injector circuit faults mean a cylinder may not be receiving fuel, causing a hard misfire. Can cause unburned fuel to damage the catalytic converter.
Air & Temperature Sensors
Faults in the MAF, throttle position, or temperature sensors cause the PCM to miscalculate fuel delivery, resulting in rough running and poor economy.
Cooling System
Cooling codes indicate the engine isn't reaching or maintaining normal operating temperature—usually a thermostat, coolant sensor, or water pump issue.
Variable Valve Timing (VVT) & Cam Timing
VVT codes indicate the camshaft timing system isn't responding as commanded—usually low oil pressure, dirty oil, or a worn timing chain. Engine knocking or rough idle are common symptoms.
Crankshaft & Camshaft Position
The crankshaft and camshaft position sensors tell the PCM exactly where each piston is in its cycle. A fault can cause a no-start or sudden stall.
Idle & Throttle Control
These codes affect how the engine maintains idle speed and responds to throttle input. Symptoms include high idle, stalling at stops, or limp mode.
Engine Management
EGR, secondary air injection, knock sensor, and other systems that keep the engine running cleanly and within emissions limits.
Transmission
Transmission codes indicate electronic or hydraulic faults inside the gearbox. Many cause limp mode. Address them quickly to avoid expensive internal damage.
Other Sensors & Systems
Vehicle speed sensor, oil pressure sensor, voltage, and ECM faults that affect various systems.
Frequently asked questions
What is the difference between generic and manufacturer-specific OBD2 codes?
Generic codes start with P0, C0, B0, or U0 and are identical across every OBD2-equipped vehicle. Manufacturer-specific codes start with P1, P2, C1, B1, or U1 and vary by brand—they require a make-specific scan tool or database to interpret. This site covers the most common generic P0xxx powertrain codes.
How do I find out which OBD2 code my car has?
You need an OBD2 scanner or code reader. Basic Bluetooth scanners cost $20–$40 and pair with free smartphone apps like Torque or OBD Fusion. Most auto parts stores (AutoZone, O'Reilly, Advance Auto, NAPA) will read your codes for free in the parking lot. Once you have the code, look it up here.
Can I clear a check engine code myself?
Yes—any OBD2 scanner can clear stored codes. However, clearing the code does not fix the underlying problem. The light will return within 1–3 drive cycles if the fault is still present. Only clear codes after you have diagnosed and repaired the cause.
Do OBD2 codes expire or go away on their own?
Some codes self-clear after the fault condition goes away and the PCM completes a set number of drive cycles without seeing the fault again (typically 40–80 warm drive cycles). Intermittent faults—like a loose gas cap—may clear themselves. Persistent hardware failures like a bad sensor or catalytic converter will not self-clear.
Not sure if it's safe to keep driving?
Some codes are fine to drive with for a few days; others mean pull over now. Our driving safety guides give you a straight answer for the most common warning lights and codes.
View driving safety guides →