Understanding how to read “check engine” codes can save you time and money when it comes time to diagnose car problems. When things go wrong with modern cars, they store error codes in onboard memory. Reading these codes allows you to quickly pinpoint the source of trouble.

Understanding Check Engine Codes

In the early 1980s the introduction of computers as part of a vehicle’s systems became commonplace. One capability of these engine control modules (ECM) was the ability to store engine status information for later retrieval. As ECMs became more sophisticated, the amount of information they could record increased. The proprietary schemes used by individual carmakers eventually became standardized as the On-Board Diagnostics (OBD) codes. The current standard, OBD-II was put in place in 1996 and is used by all cars manufactured after that date. If your check engine light comes on, here’s how to get at and understand the check engine codes:

• The tool for the job. A diagnostic code reader (also known as a scan tool) is used to read codes from the OBD II system. You can purchase a scan tool with prices starting at around $75. If you only need to scan codes occasionally, most auto parts stores can read your system for free or for a small fee.
• Get connected. The scan tool connects to a diagnostic connector typically located on the lower driver’s side of the care. The connector is rectangular, with a number of female receptacles that fit the scan tool’s cable. You owner’s manual can give you the precise location of your diagnostic connector.
• Error code formatting. An ODB-II code is made up of five characters. This first character is a letter and the other four characters are numbers.
• System code. The letter in an ODB-II check engine code refers to the system that is affected. The letter “P” designates a powertrain issue. The letter “B” signifies a body issue. The letter “C” means the issue involves the chassis. The letter “U” is a catchall designation for other vehicle issues. Note that not all cars have sensors to report problems with all systems.
• Code type. The first digit in a check engine code tells you whether the code is generic (a “0”) or specific to the car’s manufacturer (a “1”). Generic codes can be generated and read by all cars and scan tools.
• Subsystem type. The next digit identifies the subsystem that generated the error code. The individual systems are identified as follows:
  1 — Emission Management (Fuel or Air)
  2 — Injector Circuit (Fuel or Air
  3 — Ignition or Misfire
  4 — Emission Control
  5 — Vehicle Speed & Idle Control
  6 — Computer & Output Circuit
  7 — Transmission
  8 — Transmission
  9 — Reserved
  0 — Reserved

• Code ID. The final two digits are used to identify the specific issue within the subsystem.

• The full list. Several online resources provide the full generic ODB-II code list, including. Individual carmakers also maintain lists of vehicle-specific codes on their respective websites.