Codes drivers look up most
Widely reported generic codes. Severity is our rating from each code’s flags; costs are labor at an assumed rate.
- P0420Catalyst System Efficiency Below Threshold (Bank 1)Often: catalyst aged or contaminatedModerate$190–$625 labor est.
- P0300Random/Multiple Cylinder Misfire DetectedOften: vacuum leak affecting all cylindersUrgent$65–$750 labor est.
- P0171System Too Lean (Bank 1)Often: MAF sensor contaminated, under-readingModerate$65–$500 labor est.
- P0455Evaporative Emission System Leak Detected (Large Leak)Often: EVAP hose disconnectedLow$15–$315 labor est.
- P0442Evaporative Emission System Leak Detected (Small Leak)Often: fuel cap loose, missing, or worn sealLow$15–$250 labor est.
- P0128Coolant Thermostat Below Regulating TemperatureOften: thermostat stuck openModerate$90–$375 labor est.
- P0401Exhaust Gas Recirculation Flow Insufficient DetectedOften: intake EGR passages blocked with carbonModerate$125–$625 labor est.
- P0174System Too Lean (Bank 2)Often: MAF sensor contaminatedModerate$65–$500 labor est.
- P0440Evaporative Emission Control System MalfunctionOften: fuel filler cap loose or wornLow$25–$315 labor est.
- P0446Evaporative Emission Vent Control Circuit MalfunctionOften: vent valve seized or contaminated by water/dirtLow$40–$190 labor est.
- P0301Cylinder 1 Misfire DetectedOften: spark plug for cylinder 1 worn or fouledUrgent$40–$500 labor est.
- P0302Cylinder 2 Misfire DetectedOften: spark plug for cylinder 2 worn or fouledUrgent$40–$500 labor est.
Every code, by system
Codes are grouped by what they affect, so you can see related faults side by side.
PPowertrain codes
All powertrain codesHigh-voltage battery, drive motors, inverters, chargers and the cooling loops that serve them. Most need a shop with high-voltage training.
Automatic, dual-clutch and manual transmission controls: shift and pressure solenoids, speed sensors, range sensors, clutches and fluid temperature.
Cylinder misfires, ignition coils, spark and glow plugs, knock sensors and ion-sense circuits.
Upstream and downstream oxygen sensors, wideband air-fuel sensors and their heater circuits.
Camshaft and crankshaft position sensors, variable valve timing, valve lift and cylinder deactivation.
Catalytic converters, the EVAP fuel-vapor system, EGR, secondary air, diesel aftertreatment (DPF, SCR/DEF, NOx) and crankcase ventilation.
Coolant temperature sensors, thermostats, cooling fans, coolant pumps and over-temperature protection.
Lean and rich conditions, fuel trim, injectors, fuel pressure, MAF and MAP sensors, throttle, intake air temperature and turbo/supercharger boost.
Vehicle speed sensors, idle air control, cruise control inputs, brake switches and torque-management signals.
ECM/PCM internal faults, sensor reference voltages, charging system, starter and power relays.
Powertrain codes outside the groups above, including newer SAE blocks that mix several subsystems.
BBody codes
All body codesAirbag deployment loops, crash and impact sensors, seat-belt pretensioners and occupant detection. A lit airbag light can mean an airbag may not deploy.
Headlamps, brake lamps, turn signals, marker and interior lights.
Blend and mode doors, blowers, A/C sensors, heated seats and rear defrost.
Power windows, door switches and locks, power seats, mirrors, sunroof and liftgate.
Instrument cluster, displays, audio, Bluetooth, antennas, pedestrian alert speakers and camera/sensor cleaning.
Wipers, washers, horn and other body electrical circuits.
CChassis codes
All chassis codesTire pressure sensors, air and adaptive suspension, ride-height sensors and dampers.
Electric power steering motors and torque sensors, steering-angle sensors and rear-wheel steering.
Yaw-rate and acceleration sensors, traction control and electronic stability control.
Wheel-speed sensors, ABS inlet/outlet valves and pump, brake pressure sensors, electronic parking brake and brake boosters.
Transfer case, trailer, adaptive cruise and other chassis controls.
UNetwork codes
All network codesA module stopped hearing another module on the data bus. Usually power, ground, wiring or a failed module.
A module received implausible data, or two modules run incompatible software.
CAN/LIN bus circuit faults: shorts, opens and bus-off conditions.
Module supply voltage, ignition inputs, improper shutdown and module cooling.
Network codes outside the groups above.
Codes, severity and cost — how we do it
Reading a code
P = powertrain, B = body, C = chassis, U = network. A second digit of 0 or 2 means a generic SAE code with the same meaning on every car.
Severity rating
- Urgent Limp mode or a safety system at risk
- Moderate Check engine light; fix soon
- Low Rarely affects driving
A published rule over each code’s OBDex flags and the system it affects. It is not an inspection of your car.
Cost estimates
Typical labor hours for the code × an assumed $125/hr rate that you can change on every code page. Parts are extra and vary by vehicle, so we link to parts searches instead of guessing prices.
Need a shop? Get quotes on findmechanic.ai
Check engine light questions
Frequently asked
What is an OBD-II code?
Every car and light truck sold in the U.S. since the 1996 model year has an OBD-II system. When a control module detects a fault, it stores a five-character diagnostic trouble code (DTC) such as P0420 and often turns on the check engine light. The code tells you which system and which kind of fault, not exactly which part has failed.
How do I read the code from my car?
Plug an OBD-II scan tool into the diagnostic port under the dashboard, usually near the steering column, and read the stored codes. Inexpensive Bluetooth readers work with phone apps, and many auto parts stores will read codes for free.
Is it safe to drive with the check engine light on?
A steady light usually means you can drive to get it checked soon. A flashing check engine light means an active misfire that can damage the catalytic converter: reduce speed and stop driving as soon as it is safe. Each code page here rates severity and explains what to watch for.
Should I just clear the code?
Clearing a code turns the light off but does not fix the fault, and it resets the readiness monitors an emissions inspection checks. Note the code and its freeze-frame data first, fix the cause, then clear it.
What is the difference between generic and manufacturer codes?
Generic codes (P0xxx, P2xxx, and most B0, C0 and U0 codes) mean the same thing on every vehicle. Manufacturer codes (for example P1xxx) are defined by each automaker, so their meaning depends on the make. This library covers generic codes.