Car Diagnostic Codes List: A Beginner’s Guide

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a driver checking OBD codes on his car

What are diagnostic trouble codes?

When a problem is detected, your car’s system can register that as its unique code. This diagnostic trouble code causes the Check Engine Light on your dashboard to turn on, and it won't turn off permanently unless you fix the root of the issue.

History of OBD codes

In 1988 The California Air and Resource Board - Commission on Air Quality issued regulations requiring an onboard diagnostics monitor (OBD) to all cars sold in California. 

During that time, OBD1 is the system used to monitor emissions causing electrical component circuit problems. The problem with OBD1 is that they are manufacturer-specific. Thus, professional mechanics have to work on several lists so they can work on several cars.

The National Institute for Automotive Service Excellence proposed to unify the codes, thus the requirement of the OBD2 system in cars manufactured from 1996 onwards. As a result, the diagnostic port also became standardized across models. OBD2 covers the powertrain, body, and chassis.

What are the types of diagnostic trouble codes?

A mechanic checking under a car's hood

Kinds of Diagnostic Trouble Codes

  • Generic DTCs. Apply in standard requirements for OBD 2 and EOBD 2 to all official automakers.
  • Manufacturer-specific DTCs. Exclusive to manufacturers and do not appear in most published DTC lists. Car manufacturers give this list of codes to ASE-certified mechanics so they can work on these issues.

Classification of Diagnostic Trouble Codes

  • Pending Codes. These are codes that the OBD system detects once but does not turn the CEL on because the OBD detects it as random or not serious.
  • Confirmed Codes. When the OBD system detects an error for the second time, the pending code (detected once) becomes a confirmed code. This code will turn on the CEL and needs action to turn the CEL off.
  • Permanent Codes. Usually related to emissions, you cannot clear permanent codes with an OBD scan tool. However, it may be clear on its own after fixing the source of the DTC.
  • Enhanced Codes. Enhanced codes include ABS, SRS, transmission, body control module, airconditioning, stereo, and transfer case. These codes are usually manufacturer-specific.

How do I read trouble codes?

For diagnostic purposes, your car needs a code reader or scanner. They are available as scan tools, mobile apps, or computer software usually used by professional mechanics. Plug the code reader or the wireless adapter (Bluetooth or WiFi) into the OBD2 diagnostic port and let it do its magic.

You should get a five-character alphanumeric code followed by a description or definition of the code. Basic readers, however, give only the code, leaving you to look up the code on lists or databases.

How do I Interpret trouble codes?

engine oil poured into a car's engine oil tank

The first character (Letter)

The codes of OBDII begin with the letter denoting a defective part of the vehicle.

  • P (Powertrain) - Covers the motor, the gearbox, and the related accessories.
  • B (Body) - Covers all codes present in passenger compartments.
  • C (Chassis) - Covers mechanical parts and functions, such as steering suspension and brake
  • U (User Network and Vehicle Integration) - covers other aspects of the car, such as module failures and losses of communication

The second character (number)

The next character on the DTC is a number from 0 or 1. The number 0 indicates generic codes, while 1 indicates manufacturer-specific codes. In rarer cases, the numbers 2 and 3 appear, denoting more manufacturer-specific codes.

The third character (number/letter)

The third character may be a number or another letter and denotes which car system has a malfunction:

  • 1 - Fuel or air metering system codes
  • 2 - Fuel or air metering injection system codes
  • 3 - Ignition system codes
  • 4 - Emissions system codes
  • 5 - Vehicle speed controls and idle control system codes
  • 6 - Computer output circuit
  • 7, 8, and 9 - Transmission or gearbox-related codes
  • A, B, C - Hybrid propulsion codes

The fourth and fifth characters (number)

The fourth and fifth digits (00 - 99) pinpoint the specific source of the malfunction or fault code.

How are trouble codes set?

OBD2 systems look at all functions related to pollution from outside. It also monitors fuel, emissions, catalysts, and evaporative energy systems when the car is in use. The check engine light (CEL) or Malfunction Indicator Light (MIL) turns on when the OBD system detects an error.

Caution: Don't rely on DTCs alone

  • Do not replace parts or components of the car, unless recommended by the manufacturer in the user manual.
  • Some DTCs are not always accurate, especially manufacturer-specific codes, and you're using a basic scan tool only.
  • Always check with the user's manual.
  • When in doubt, enlist the help of a professional mechanic.

List of Car Diagnostic Codes

Common Powertrain Causes Symptoms
P0135: O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)
  • Bad oxygen sensor
  • HO2S ignition circuit is open
  • HO2S heater low control circuit is open
  • HO2S heater element is compromised
  • Damaged HO2S connector (e.g., damage or moisture in pins)
  • HO2S heater low control shorted to voltage or ground
  • Malfunction of the sensor wiring and/or a circuit issue
  • Malfunction of the engine coolant temperature sensor
  • Issues with the PCM (e.g., outdated software)
  • Low fuel pressure
  • Leaks in the intake air system or exhaust system
  • Check engine light comes on
  • Fuel economy becomes less efficient
  • Vehicle may run rough or idle
  • Engine doesn’t operate as smoothly during start-up
  • Exhaust smells bad or emits black smoke
P0141: O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 2)
  • Faulty oxygen sensor heater
  • Damaged wiring, loose connections
  • Issues with the PCM
  • Check engine light
  • Failed emissions test
P0171: System Too Lean (Bank 1)
  • Dirty mass airflow sensor
  • Vacuum leak
  • Bad )2 Sensors
  • Low fuel pressure
  • Low gas level
  • Rough idling
  • High idle
  • Engine misfire
  • Power loss when accelerating Stalling
  • Check Engine Light on your dashboard
P0174: System Too Lean (Bank 2)
  • Vacuum leak due to damaged intake manifold gaskets or punctured vacuum and positive crankcase ventilation (PCV) hoses
  • Malfunctioning mass airflow (MAF) sensor
  • Clogged fuel filter
  • Failing fuel pump
  • Clogged fuel injectors
  • Failing fuel pressure regulator (although this usually causes rich codes rather than lean)
  • PCM issues
  • Illuminated Check Engine Light (most common)
  • Loss of power (may be accompanied by other codes)
  • Misfiring (will be accompanied by other codes)
  • Rough idling (may be accompanied by other codes)
  • Engine stalling (not common)
P0200: Injector Circuit Malfunction
  • Open or shorted fuel injector
  • Faulty or defective fuel injector
  • Open or shorted wiring harness of the fuel injector
  • Clogged fuel injector as a result of dirt accumulation
  • Fuel injector is corroded
  • Low internal resistance of the fuel injector
  • Poor electrical connection
  • Driver circuit shorted to voltage
  • Defective Powertrain Control Module (PCM). Please note that this may give rise to codes other than P0200
  • Illumination of the Check Engine Light (CEL)
  • Misfiring
  • Stalling of the engine
  • Delayed pickup time or failure of the engine to start
  • Acceleration is poor
  • Cylinder misfire codes are recorded
P0218: Transmission Over Temperature Condition
  • Faulty engine coolant thermostat, stuck engine coolant thermostat
  • Low coolant temperature, car takes too long to warm up
P0300: Random/Multiple Cylinder Misfire Detected
  • Faulty spark plugs
  • Damaged spark plug coil
  • Worn or damaged distributor cap
  • Worn or damaged rotor button
  • Defective fuel injectors
  • Clogged exhaust gas recirculation (EGR) valves
  • Ignition timing incorrect
  • Leaky vacuum
  • Low fuel pressure
  • Leaking head gasket
  • Cracked distributor cap
  • Faulty camshaft/crankshaft Sensor
  • Faulty mass airflow sensor
  • Faulty O2 sensor
  • Faulty throttle position sensor
  • Faulty catalytic converter
  • Faulty PCM
  • Flashing Check Engine Light
  • Car takes longer to start
  • Rough idling
  • Jerky acceleration
  • Less power during driving
  • Increased fuel consumption
P0301: Cylinder 1 Misfire Detected
  • Faulty worn spark plug
  • Failed coil pack or coil-on-plug coil
  • Bad spark plug wire on a vehicle that has plug wires
  • Faulty fuel injector
  • Wiring issues with the fuel injector circuit
  • Problems with the PCM
  • Leaking valve
  • Damages piston ring, piston, or cylinder walls
  • Worn camshaft lobe or damaged lifter
  • Leaking head gasket
  • Rough idling
  • Hard starting
  • Car jerks from time to time, either while running or while idling
  • Lack of power at acceleration
  • Decreased fuel economy
  • Check Engine Light
P0302: Cylinder 2 Misfire Detected
  • Defective spark plug
  • Compression issues
  • Damaged caps and rotor buttons
  • Faulty fuel injector
  • Clogged EGR valves
  • Faulty Ignition timing sensors
  • Check engine light
  • Car won't start
  • Issues with acceleration
  • Poor fuel economy
P0303: Cylinder 3 Misfire Detected
  • Defective spark plug
  • Compression issues
  • Damaged caps and rotor buttons
  • Faulty fuel injector
  • Clogged EGR valves
  • Ignition timing sensors
  • Engine may run poorly, stall, misfire, or become difficult to start
  • Poor gas mileage
P0304: Cylinder 4 Misfire Detected
  • Worn out or faulty spark plugs
  • Faulty spark plug coils or wires
  • Distributor failure
  • Faulty fuel injector
  • Leak in the vacuum
  • Low fuel pressure
  • Defective camshaft/crankshaft sensor
  • Engine timing is off
  • Leaky head gasket
  • Low engine compression
  • Poor fuel quality
  • Check engine light on and flashing
  • Vehicle runs shakily
  • Low power from the engine
  • Fuel odors from the exhaust
  • Vehicle is jerky during acceleration
P0325: Knock Sensor 1 Circuit Malfunction (Bank 1 or Single Sensor)
  • Failure in the ECU or knock sensor
  • Shorts in the wiring harness
  • Faulty engine coolant system
  • Lack of power
  • Check engine light
  • No symptoms
P0401: Exhaust Gas Recirculation Flow Insufficient Detected
  • Clogged EGR passages
  • Faulty EGR valve
  • A failed sensor
  • Inadequate vacuum supply to the EGR valve
  • Damaged wiring and loose connections
  • Issues with the PCM
  • Check Engine Light
  • Failed emissions test
  • Engine knocking or pinging during acceleration
P0411: Secondary Air Injection System Incorrect Flow Detected
  • Faulty air injection pump
  • Issues with the air pump relay
  • Faulty air control solenoids or airlines
  • Air injection system has excessive carbon buildup
  • Perforated or spoiled exhaust components
  • Damaged check valve
  • Wiring harness of the injection system is defective
  • Check engine light
  • Poor fuel economy
  • Backfiring during acceleration
  • Poor acceleration
  • Squealing belt
P0420: Catalyst System Efficiency Below Threshold (Bank 1)
  • Faulty catalytic converter
  • Leaking fuel injector
  • Faulty O2 sensor
  • Leak in the exhaust
  • Check Engine Light
  • Engine dips while idling
  • Low engine power
P0440: Evaporative Emission Control System Malfunction
  • Faulty ECM
  • Loose gas cap
  • Leaky fuel vapor system
  • Faulty vapor pressure sensor
  • Check Engine Light
  • Fuel vapor odor
P0500: Vehicle Speed Sensor Malfunction
  • Defective vehicle speed sensor
  • Problem with the PCM or other module
  • Short or open wire leading to VSS or issues with connectors
  • PCM is not configured for the tire size used
  • Damaged drive gear
  • Malfunctioning speedometer
  • Erratic shifting with an automatic transmission
  • Inactive or erratic ABS performance
  • Traction control issues
P0505: Idle Control System Malfunction
  • Dirty idle air control
  • Clogged idle air control
  • Vacuum leak in the exhaust system
  • Check Engine Light
  • Engine idle not compatible with vehicle specifications.
  • Rough idling
  • Engine stalls when it comes to a stop
  • High idle
P0600: Serial Communication Link Malfunction
  • Defective ECU
  • Low battery voltage
  • Short in the ECU wiring harness
  • Check Engine Light
  • Warning light from the ABS and traction system
  • Rought idling
  • Engine stalls out
  • Not starting
  • Transmission fails to shift
P0603: Internal Control Module Keep Alive Memory (KAM) Error
  • Corrosion in the battery terminal
  • Loose battery connections
  • Low battery charge
  • Faulty or defective charging system
  • Open wire in the Keep-Alive Power (KAPWR) circuit
  • Routing of the KAPWR circuit wire
  • Secondary ignition voltage inference
  • Faulty PCM program
  • Open or shorted PCM wiring
  • Check Engine Light
  • Illumination of other warning lights
  • Misfires
  • Stalling of the engine
  • Difficulty in engine startup
  • Erratic or harsh shifting in transmission
  • Rough running
  • Rough idle
P0604: Internal Control Module RAM Error
  • A faulty PCM
  • PCM software needs reprogramming
  • Check Engine Light
  • Failed emissions tests
  • Anti-Brake Lock System/Traction Control Light activates
  • Engine operation and shifting transmission gear issues
  • Vehicle will not start
P0604: Internal Control Module RAM Error
  • A faulty PCM
  • PCM software needs reprogramming
  • Check Engine Light
  • Failed emissions tests
  • Anti-Brake Lock System/Traction Control Light activates
  • Engine operation and shifting transmission gear issues
  • Vehicle will not start
P0700: Transmission Control System Malfunction
  • Faulty transmission control module
  • Malfunction in the transmission valve body
  • Issues in the shift solenoid
  • Wiring harness issues
  • Poor gas mileage
  • Shifting problems
  • Engine stalling
P0705 - Transmission Range Sensor Circuit Malfunction (PRNDL Input)
  • Damaged, worn, or corroded TRS wiring or connectors.
  • Damaged/faulty TRS
  • Damaged TCM or ECM
  • Misadjusted shift linkage
  • Dirty/contaminated transmission fluid
  • Damaged/faulty transmission valve body
  • Transmission won't lock into gear
  • Vehicle won't shift smoothly
  • Check engine light

Common Body DTCs

a mechanic checking a car's chassis

  • B0001: Driver Frontal Stage 1 Deployment Control
  • B0005: Collapsible Steering Column Deployment Control
  • B0020: Left-side Airbag Deployment Control
  • B0028: Right-side Airbag Deployment Control
  • B0050: Driver Seatbelt Sensor
  • B0052: Passenger Seatbelt Sensor
  • B1212: EIC Switch-2 Assembly Circuit Short To Ground
  • B1217: Horn Relay Coil Circuit Failure

Common Chassis DTCs

  • C0020: ABS Pump Motor Control
  • C0021: Brake Booster Performance
  • C0031: Left Front Wheel Speed Sensor
  • C0034: Right Front Wheel Speed Sensor
  • C0042: Brake Pedal Position Sensor A
  • C0043: Brake Pedal Position Sensor B
  • C0077: Low Tire Pressure
  • C0081: ABS Malfunction Indicator

U (User Network and Vehicle Integration) DTCs

These DTCs are more advanced and need a professional mechanic to address them properly since these entail vehicle communication, software, invalid data received, etc.

  • U0001: High-Speed CAN Communication Bus
  • U0002: High-Speed CAN Communication Bus Performance
  • U0020: Low-Speed CAN Communication Bus Performance
  • U0106: Lost Communication With Glow Plug Control Module
  • U0302: Software Incompatibility with Transmission Control Module
  • U0303: Software Incompatibility with Transfer Case Control Module

Car Diagnostic Codes FAQ

1. How do I find my diagnostic code?

To read a code, you will need an OBD II scan tool. Plug the scanner into the 16-pin OBD II diagnostic connection, usually found under the dash near your steering column. The device will display what triggers the car's Check Engine Light.

2. Where can I read car codes for free?

Some auto parts stores provide a free reading of your diagnostic codes. However, these readings are not conclusive and may leave you wondering if your car has these codes.

Instead of getting these free scans, it is better to get a good OBD2 scanner so you can make the scans on your own.

3. What are P codes?

P codes or powertrain codes are fault codes that your OBD2 system picked up from the powertrain (also known as drivetrain). 

The powertrain is likened to the body's muscular system and includes the engine, transmission, driveshaft, differentials, axles, and all other parts that convert the engine's power into movement.

4. What are automotive codes?

More commonly known as diagnostic trouble codes, error codes, or fault codes, an automotive code is an error detected by your car's OBD system. It alerts you through the illumination of the check engine light when a car malfunctions. 


If you're a vehicle owner, it is important to know what these car diagnostic codes mean. We explained the most common dashboard warning lights and provided helpful tips to keep your car in good shape. 

Also, check out our review on car health monitors and see how these can help you with car maintenance. What is your common practice to maintain your vehicle? We would love to hear from you!