Common OBD2 Codes Chart

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Generic OBD2 diagnostic trouble codes follow SAE J2012: P0xxx codes are the most common, with P0300-P0399 covering misfire and P0420 (catalyst efficiency below threshold) among the most frequently seen. A loose fuel cap typically sets P0455 (large leak), not P0442 (small leak). Clearing a code does not fix the fault.

The table lists widely encountered generic P0xxx codes with their standard meaning and likely cause. Read the code from a scan tool, match it in column one, then confirm with live data before replacing parts.

Severity Rating of Common Generic OBD2 Codes

Misfire and sensor-circuit codes rate most serious; EVAP leak codes are usually minor and easy to resolve.

Common Generic OBD2 Codes Reference Chart
Code	Standard meaning	Typical cause	System group	Severity
P0101	Mass air flow circuit range/performance	Dirty MAF sensor or intake leak	Fuel and air metering	warn
P0113	Intake air temperature circuit high input	Open IAT sensor circuit or wiring	Fuel and air metering	ok
P0128	Coolant thermostat below regulating temperature	Stuck-open thermostat	Fuel and air metering	warn
P0171	System too lean (bank 1)	Vacuum leak or dirty MAF sensor	Fuel and air metering	warn
P0172	System too rich (bank 1)	Leaking injector or high fuel pressure	Fuel and air metering	warn
P0201	Injector circuit open, cylinder 1	Faulty injector or wiring	Fuel and air metering	bad
P0300	Random or multiple cylinder misfire	Worn plugs, coils or vacuum leak	Ignition and misfire	bad
P0301	Cylinder 1 misfire detected	Faulty plug, coil or injector	Ignition and misfire	bad
P0325	Knock sensor 1 circuit malfunction	Faulty knock sensor or wiring	Ignition and misfire	warn
P0335	Crankshaft position sensor A circuit	Faulty crank sensor or reluctor	Ignition and misfire	bad
P0340	Camshaft position sensor A circuit	Faulty cam sensor or timing	Ignition and misfire	bad
P0401	Exhaust gas recirculation flow insufficient	Carbon-clogged EGR passages	Emissions control	warn
P0420	Catalyst system efficiency below threshold (bank 1)	Aged catalytic converter or O2 sensor	Emissions control	warn
P0430	Catalyst system efficiency below threshold (bank 2)	Aged catalytic converter or O2 sensor	Emissions control	warn
P0442	Evaporative emission system small leak detected	Cracked EVAP hose or seal	Emissions control	ok
P0455	Evaporative emission system large leak detected	Loose or missing fuel cap	Emissions control	ok
P0500	Vehicle speed sensor A malfunction	Faulty VSS or wiring	Speed and idle control	warn
P0505	Idle control system malfunction	Carboned idle air control valve	Speed and idle control	warn
P0700	Transmission control system malfunction	Stored transmission fault, read TCM codes	Transmission	warn
P0128b	Placeholder removed	Placeholder	Placeholder	neutral

Not used
Often minor or easily resolved
Needs diagnosis before parts replacement
Stop or diagnose promptly; drivability or damage risk
Informational

Severity is a practical guide only; the same code can range from minor to serious depending on the vehicle.

Key Facts

Generic codes use the P0xxx format defined by SAE J2012; manufacturer-specific codes use P1xxx.
P0300-P0399 is the misfire group; P0420-P0439 covers catalyst efficiency.
P0455 indicates a large EVAP leak, typically a loose or missing fuel cap.
P0442 indicates a small EVAP leak, usually a cracked hose or seal, not the cap.
Clearing a code resets readiness monitors and does not repair the underlying fault.

How Do You Read a Common OBD2 Codes Chart?

A common OBD2 codes chart maps each stored code to a standard meaning, a typical cause, a system group, and a severity rating. Reading it in a fixed order keeps you from jumping to a part before you understand the fault.

Step-by-Step Reading

Start with the code and its standard meaning. The P0xxx format is defined by SAE J2012, so the definition is the same on every OBD2 vehicle from model year 1996 onward in the US.
Check the system group. The group tells you where to look: fuel and air metering, ignition and misfire, emissions control, speed and idle control, or transmission.
Read the typical cause as a starting point, not a diagnosis. A cause listed for P0171, such as a vacuum leak or dirty MAF sensor, narrows the inspection but still requires live-data confirmation.
Use the severity tone to set urgency. In the chart, “bad” marks codes such as P0300 and P0335 that warrant prompt attention, “warn” marks moderate faults, and “ok” marks minor ones such as P0455.

Three values matter most. P0300 signals a random or multiple cylinder misfire and sits in the P0300–P0399 misfire group. P0420 means catalyst efficiency below threshold on bank 1, within the P0420–P0439 range.

P0455 indicates a large EVAP leak, typically a loose or missing fuel cap, whereas P0442 is a small leak from a cracked hose or seal. Clearing a code resets readiness monitors and does not repair the fault.

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What Factors Change the Values You See on a Common OBD2 Codes Chart?

A common OBD2 codes chart lists the standard definition of each code, but the code that actually sets on a given vehicle depends on operating conditions, component age and how the monitor runs. The chart tells you what a code means, not why it appeared on your specific car.

Operating Conditions and Load

Misfire and fuel-trim codes often surface only under specific load or temperature conditions. A P0300 may set during hard acceleration or cold starts while the engine runs cleanly at idle, so the same fault can be intermittent on the chart’s listed cause.

Component Age and Wear

Catalyst efficiency codes such as P0420 become more likely as the converter ages and its oxygen storage capacity drops. An older vehicle therefore reaches the threshold for the same code sooner than a newer one with an identical fault pattern.

Measurement Method and Monitor Readiness

Clearing a code resets readiness monitors, so some codes cannot reappear until the relevant monitor completes its drive cycle. This delays when a fault shows up again and can make a chart entry seem absent after a reset.

Temperature at Startup

Codes like P0128 depend on how quickly coolant reaches regulating temperature, which varies with ambient conditions. In cold weather a marginal thermostat may set the code, while the same part passes in mild conditions.

What Mistakes Should You Avoid When Using a Common OBD2 Codes Chart?

Most errors with a common OBD2 codes chart come from treating a stored code as a diagnosis rather than a starting point. The chart tells you which circuit or system logged the fault, not which part has failed.

Common pitfalls and how to avoid them

Assuming a code names the failed part. A code such as P0171 points to a lean condition, not a specific component, so confirm the cause with live data before replacing anything.
Clearing the code instead of fixing the fault. Clearing a code resets readiness monitors and does not repair the underlying fault, so diagnose first and clear only after the repair.
Confusing P0455 with P0442. P0455 indicates a large EVAP leak, typically a loose or missing fuel cap, while P0442 indicates a small leak from a cracked hose or seal.
Mixing generic and manufacturer codes. Generic codes use the P0xxx format defined by SAE J2012, while P1xxx codes are manufacturer-specific and need the correct chart.
Ignoring misfire codes. P0300-P0399 is the misfire group, and driving with an active misfire can damage the catalytic converter.

Notes & Assumptions

Code definitions follow the SAE J2012 generic (P0xxx) structure used by all OBD2 vehicles from model year 1996 in the US.
Severity scores are an editorial guide (1 minor, 2 moderate, 3 serious), not a published standard.
Typical causes are the most commonly reported ones; actual cause varies by vehicle and requires live-data diagnosis.
EVAP code behaviour assumes a sealed fuel system; a loose or missing cap normally sets P0455, not P0442.

What Is the Bottom Line on a Common OBD2 Codes Chart?

The most useful rule is that the P0xxx codes on a common OBD2 codes chart are generic SAE J2012 definitions shared across all 1996-and-newer vehicles, while P1xxx codes are manufacturer-specific. Clearing a code resets readiness monitors and does not repair the fault, so the chart points you toward a system, not a fix. Treat P0300-P0399 misfire codes and P0420 catalyst codes as priorities for diagnosis.

Frequently Asked Questions about Common Obd2 Codes Chart

How do I read a common OBD2 codes chart?

Read each code by its first character and digits: P0xxx codes are generic SAE J2012 codes, while P1xxx codes are manufacturer-specific. The chart's meaning column tells you the system, and the typical cause column points to the first thing to check before replacing parts.

What do the P0300 to P0399 codes mean on the chart?

P0300-P0399 is the misfire group, with P0300 meaning random or multiple cylinder misfire and P0301 meaning cylinder 1 misfire. Misfires are serious because unburned fuel can damage the catalytic converter, so diagnose plugs, coils and vacuum leaks promptly.

Is P0455 or P0442 the code for a loose fuel cap?

P0455 is the code for a large EVAP leak, typically a loose or missing fuel cap. P0442 indicates a small leak, usually a cracked hose or seal, so replacing the cap will not fix a P0442 fault.

Which common OBD2 codes are the most serious?

Misfire and sensor circuit codes such as P0300, P0301, P0335 and P0340 are the most serious on the chart because they can cause stalling or converter damage. Codes like P0442 and P0455 are minor emissions faults that rarely affect driveability.

Does clearing a code fix the problem shown on the chart?

No, clearing a code does not repair the underlying fault and also resets readiness monitors. The code will usually return once the monitor runs again, so use the chart to find the cause and confirm the repair before clearing.

Why does my code differ from the common OBD2 codes chart?

Codes starting with P1xxx are manufacturer-specific rather than generic SAE J2012 codes, so they will not match a generic chart. Always confirm the exact code with a scan tool and check the vehicle-specific definition before diagnosing.