Reference
OBD fault codes: what each one means
Look up the code your car gave you (for example, P0420) and find out what is wrong, what usually causes it and how urgent it is.
Reviewed by Daniel Morales, founder of Reprorace SL, on
How to read a fault code
Every code has a letter and four characters. This glossary covers the generic codes people look up most, the ones that mean the same on every make.
The letter tells you the system
- PPowertrain
- engine, gearbox and emissions
- CChassis
- brakes, steering and suspension
- BBody
- airbags, climate and comfort
- UNetwork and communication
- communication between control units
What each character tells you
- The letter: the system (P, C, B or U).
- The first digit: 0 or 2 for a generic code (the same on every make); 1, and almost every 3, for a manufacturer's own code.
- The second character: the subsystem. In P0 codes, for example, 3 is ignition and 4 emission control.
- The last two: the specific fault.
Example: P0420 = P (engine and emissions) · 0 (generic) · 4 (emission control) · 20 (catalytic converter efficiency, bank 1).
Manufacturer-specific codes are not in this glossary: their meaning depends on the make and model, and only the manufacturer's service information gives it.
The most looked-up codes
Start here if your code is one of the common ones.
- P0300Random or multiple cylinder misfire detected
- P0301Cylinder 1: misfire detected
- P0171Mixture too lean (bank 1)
- P0172Mixture too rich (bank 1)
- P0420Catalyst system (bank 1): efficiency below threshold
- P0401EGR A flow: insufficient flow detected
- P0442EVAP system: small leak detected
- P0455EVAP system: large leak detected
- P0128Coolant thermostat: coolant temperature below the thermostat regulating temperature
- P0101Mass or volume air flow sensor A, circuit: range/performance
- P0113Intake air temperature sensor 1 (bank 1), circuit: signal high
- P0335Crankshaft position sensor A, circuit: malfunction
- P0340Camshaft position sensor A (bank 1 or single sensor), circuit: malfunction
- P0011A camshaft position: timing over-advanced or system performance (bank 1)
- P0016Correlation fault between crankshaft position and camshaft position (bank 1, sensor A)
- P0299Turbocharger/supercharger A: underboost
- P0087Fuel rail/system pressure: too low (bank 1)
- P0380Glow plug/heater, circuit A: malfunction
- P0700Transmission control system: MIL illumination requested
- P2002Particulate filter (bank 1): efficiency below threshold
- P2463Particulate filter: restriction - soot accumulation (bank 1)
- P20EESCR NOx catalyst (bank 1, catalyst 1): efficiency below threshold
- U0100Lost communication with ECM/PCM A
- U0121Lost communication with anti-lock brake system (ABS) control module A
- C0035Right front wheel speed sensor supply: malfunction
- B0001Driver frontal stage 1 deployment control: malfunction
Every code, by block
The first three characters group the codes of the same system. Pick your code's block: P04xx holds P0400 to P0499.
Engine and transmission (P0): the classic generic codes
- P00xxfuel and air metering and auxiliary emission controls191 codes
- P01xxfuel and air metering135 codes
- P02xxfuel and air metering (injector circuit)167 codes
- P03xxignition system or misfire91 codes
- P04xxauxiliary emission controls163 codes
- P05xxvehicle speed, idle control and auxiliary inputs125 codes
- P06xxcomputer and output circuits168 codes
- P07xxtransmission157 codes
- P08xxtransmission114 codes
- P09xxtransmission90 codes
- P0Axxhybrid propulsion12 codes
Engine and transmission (P2): SAE's extension, with modern diesels
- P20xxparticulate filter, AdBlue and NOx174 codes
- P21xxthrottle, accelerator pedal and EGR78 codes
- P22xxNOx sensors, turbo and fuel pressure98 codes
- P23xxignition coils and knock10 codes
- P24xxEGR and particulate filter65 codes
- P25xxturbo control87 codes
- P26xxoutput signals, intake heater and fuel pump81 codes
- P27xxautomatic transmission76 codes
- P28xxtransmission range sensor5 codes
- P2Axxoxygen sensors4 codes
- P2BxxNOx limits2 codes
Engine (P3): cylinder deactivation and camshaft actuators
Communication network (U0)
Network and control module power (U3)
Chassis (C0): brakes, ABS, steering and four-wheel drive
Body (B0): airbags and seat belts
Before you start
Check engine light on: what to do
Your engine warning light has come on: what it means, when to stop, when you can drive to a workshop and how to find out which code the car has stored.
How to read and clear your car's fault codes
Where the OBD socket is, how to read your car's fault codes, how to interpret them and when to clear them (and when not to), in plain English.
Will your car pass its MOT or inspection with the engine light on?
What the MOT or roadworthiness test checks on the engine light and the OBD socket, why clearing the codes just before does not help and how to arrive ready.
Wi-Fi OBD2 reader: which one to choose
How to choose a Wi-Fi OBD2 reader: protocols (CAN FD, DoIP, K-Line), 12 or 24 V, app or no app, remote diagnostics, security and consent.
Reading fault codes by car make
How to read the fault codes of a Volkswagen, SEAT, Renault, Peugeot, Toyota, BMW and other makes: what is the same, what changes and the official sources.
Remote OBD diagnostics: what it is, how it works and what it can't do
What remote OBD diagnostics is, how it works with ECO-LITE, ECO-LITE TOOL and the platform, what can and can't be done remotely, and how the driver approves it.
Stuck on a term? These are explained in the diagnostics glossary:
Read your car's fault codes on your phone
With an ECO-LITE in the OBD socket you see the codes in your account, with the data captured when the fault happened, and your workshop can check them remotely, always with your permission.