OBD-II codes: what they mean and how serious they are

How an OBD-II code is built

Each character narrows the fault down. The structure comes from SAE J2012, the standard that federal rules name for emission-related codes; we describe it in our own words. Reading the first three characters already tells you a lot: P0420 is a powertrain code (P), generic (0), in the emission controls subsystem (4).

Generic codes have a single standard description across makes, and the rules require them for faults that affect emissions. Manufacturer codes, P1xxx for example, mean whatever each maker has defined, and the same number can mean different things on two brands. The lookup decodes their structure and says plainly that their meaning is not ours to guess; the service information for your vehicle or a dealer is the place to find it. Our list holds 72 engine timing, air, fuel and ignition codes, 48 emission, vapor and oxygen sensor codes, and the most common transmission and network codes.

Steady light, flashing light

The check engine light is the malfunction indicator of the emission rules. Under 40 CFR 86.1806-05 it must read "Check Engine" or "Service Engine Soon", or show an engine symbol, and come on when a fault could push emissions past the limits. A steady light means a code is stored and you have time to have it read. A flashing light means misfire is damaging the catalytic converter now: the same rule requires the light to blink at least once a second in that case. Slow down and stop when it is safe. The check engine light page and the flashing light page go through each case.

How serious is the code?

Each code in our list carries one of four levels of severity, from a fault that only affects emissions to one where you should stop. They are our estimates for a typical car, written from what each fault does to the engine and to safety, not a rule from any agency. A flashing check engine light, a red oil pressure or temperature warning and a red brake light with a soft pedal are always at the top, whatever the code.

Most codes sit in the two lower levels. Fuel vapor leaks, oxygen sensor heaters and a slow thermostat do not change how the car drives. Lean or rich running, the catalytic converter and the air flow sensor are moderate: the car is drivable, but the fault costs fuel and can damage parts if left for months. Misfires, timing chain correlation and transmission faults are high: limit driving until they are diagnosed.

The codes drivers look up most

A small number of codes make up a large share of check engine lights. Each of these has a page with its causes in order, its repairs, the labor hours we estimate and the cost at the starting rate for each state.

Three families stand out. Fuel vapor (EVAP) codes such as P0455, P0442 and P0456 often come from a gas cap that is loose or worn, and they cost little to fix when that is the case. Catalytic converter codes, P0420 and P0430, are feared for the price of a converter, but a sensor or an exhaust leak is sometimes the real cause, and the converter often failed because of something else. Misfire codes, P0300 and P0301 to P0304, are the ones to act on quickly.

When the light goes off by itself

Fixing the fault does not turn the light off at once. The federal rule lets the light go off after 3 consecutive trips in which the self-test runs and passes, and lets the code itself be erased after 40 engine warm-up cycles without it, a warm-up being a drive in which the coolant rises at least 40 °F and reaches at least 160 °F (40 CFR 86.1803-01). Some tests, the fuel vapor test in particular, run only in certain conditions, so a few days of normal driving may pass before they do. The counter below turns those rules into days for your driving.

A shop can clear a code at once with a scanner, and so can you. That also resets the readiness monitors, the record of which self-tests have run since the last clearing. If the fault is still there, the light comes back as soon as the test runs again.

Emissions inspections

Many states inspect cars from 1996 onward by plugging into the OBD port rather than measuring the exhaust. The California Air Resources Board fact sheet describes the logic used in California's Smog Check, which other programs share: a car fails if the check engine light is on, and fails if too many readiness monitors are incomplete, which is what happens after a code has been cleared or the battery disconnected. About a week of mixed driving usually lets the monitors complete. Independent shops, not only dealers, can repair emission faults, and in California a replacement catalytic converter must be approved by CARB for the vehicle.

Emission warranties

A fault that sets the light on a recent car may be repaired at no charge. Federal rules require makers to warrant emission parts against defects for 2 years or 24,000 miles, and specified major components, including the catalytic converter and the engine's emission control computer, for 8 years or 80,000 miles, whichever comes first (40 CFR 85.2103). California's rules add 3 years or 50,000 miles for any fault the OBD system detects, 7 years or 70,000 miles for high-priced parts, and 15 years or 150,000 miles on PZEV and TZEV models. The warranty check on the service engine soon page applies those limits to your car's age and mileage. Your warranty booklet lists the parts and prevails.

What a repair costs in labor, state by state

Shops set their own labor rates, and no public agency publishes them. To give a starting figure, we take the mean hourly wage of automotive service technicians and mechanics in each state from the Bureau of Labor Statistics (occupation 49-3023, May 2025) and multiply it by 4 to cover overhead and margin, an assumption explained on the method page. The national mean wage is $27.13, which gives a starting rate of $110 an hour. Wages are highest in District of Columbia and lowest in West Virginia.

Each code page multiplies our hours for each common repair by the rate for the state you choose, and lets you type your shop's posted rate instead. Parts are not priced: they vary too much by vehicle and brand, and no public, dated source covers them. Ask for the parts and the labor separately on any estimate.

Dashboard warning lights

The check engine light is one of several warnings, and the others are set by vehicle safety standards rather than emission rules. Federal Motor Vehicle Safety Standard 101 lists the symbols and, for some, the colors (FMVSS 101): the brake warning must be red; the antilock, stability control and tire pressure warnings must be yellow. The tire pressure light must come on when a tire is 25 percent below its placard pressure (FMVSS 138), and it flashes for a minute or so at start-up when the system itself is faulty. The symbol finder identifies a symbol by its shape, and each light has a page: tire pressure, traction control, ABS, battery, oil pressure and airbag.

B, C and U codes

Codes from the body (B), chassis (C) and network (U) are stored by modules other than the engine computer: the airbag module, the ABS and stability control module, the body controller. A basic code reader that only talks to the engine computer does not see them. Their generic ranges are short and most of the codes drivers meet are manufacturer-defined. The network codes are the exception: U0100, lost communication with the engine computer, and U0101, with the transmission module, are generic and fairly common after battery or wiring problems. The lookup explains the structure of any B, C or U code and gives the meaning of the generic network codes in our list.

What this site does not do

It does not diagnose your car. A code tells you which self-test failed; the causes in order tell you where diagnosis usually starts. A technician with the maker's service information, live data and test equipment will know more after an hour with the car than any page can. The site does not interpret manufacturer codes, does not reproduce the SAE standard or any maker's data, and does not sell parts, tools or repairs. Severity levels, the order of causes and the labor hours are our estimates, labeled as such; the rules and wages link to their sources.

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