If your car’s sensor sends information to the Electronic Control Unit (ECU) that falls outside the normal range, the ECU saves the information in what is known as a Diagnostic Trouble Code (DTC). The DTC is a series of numbers and letters that locates the source and nature of the problem. The OBD-II codes, on the other hand, is a term that includes the entirety of the DTC system (body codes, chassis codes, powertrain codes, and network codes).

The ECU then sends the information through the indicator light that an issue has been found. If the Malfunction Indicator Light (MIL), or as we know it, the check engine light, turns on and stays on, the issue is typically related to emissions systems or some safety-related faults. However, if the check engine light turns on and is flashing, this indicates an issue severe enough to cause damage to critical systems like the catalytic converter, internal engine fuel, or air induction systems.

This is an indication of a critical DTC – a code relating to a vehicle issue that is significant and can immediately affect vehicle performance. In contrast, a non-critical DTC could be issues related to window washer fluid levels or cabin air filters — essentially, issues that don’t need to be addressed in a hurry.

OBD-II codes alert you when your car is having issues – everything from airbags to brakes. The most common place to access them is through the OBD-II port under your vehicle’s steering column. They’re generally standard in issue but can be manufacturer-specific. This is why advanced OBD-II scanners are typically needed to properly access your vehicle’s OBD-II codes.

Your mechanic accesses the codes through the use of an OBD-II scanner and uses that as a starting point to further diagnose the vehicle. The standard five-digit OBD-II codes are found across all makes and models, but some manufacturers may use OEM (Original Equipment Manufacturer) codes for systems that are specifically unique to their vehicles. You’ll need higher quality scanners to properly access these codes.

There are four different types of OBD-II codes – P, B, C, and U. The P stands for powertrain (engine and transmission) systems, B for body, C for chassis, and U for network and wiring systems. If the ECU commands the check engine light to illuminate, there will always be at least one P0 code stored. However, sometimes there are “pending codes”, which are OBD codes that don’t command the check engine light to illuminate and are caused by intermittent faults. The ECU will clear these codes if it doesn’t find the code after a period of 40 warm-up cycles (the time it takes for a motor to heat up to operating temperature).

The Society of Automotive Engineers (SAE) is the governing body that regulates the standardization of OBD codes. OBD-I codes were the original diagnostic codes issued and focused strictly on the emissions controls of the vehicle. OBD-I codes were also around much longer than OBD-II codes, which came into use in the mid-1990s. OBD-II was a vast improvement over OBD-I and can even be remotely accessed through some Bluetooth scanners. OBD-I codes were very manufacturer-specific, while OBD-II codes have become more standardized across manufacturers.

OBD-III codes are being discussed to solve the issue of minimizing the time between the detection of an emissions issue and the repair of said issue. This includes the reading of stored OBD information from in-use vehicles and the direction to owners of vehicles with trouble codes to make immediate repairs. California is leading this discussion as its emissions standards are by far the strictest in the United States. But for now, OBD-II codes are the latest standard. There is no concrete update in the development of these discussions.

OBD-II codes with the letter P indicate powertrain codes, while B is used for body, C for chassis, and U for network codes. For all four sets of these codes, the third character indicates where in the vehicle the system occurred. One and two are for air and fuel metering problems, three for ignition issues, four for auxiliary emissions control, five relates to idle control problems, six for computer or output circuit faults, and seven and eight relate to vehicle transmission issues.

While it’s smart to call a technician when an OBD code appears, it isn’t impossible to access your vehicle’s OBD codes without a scanner. By using the vehicle’s ignition key and sometimes the trip odometer, you can often access your vehicle’s settings menu to access the OBD code. This isn’t guaranteed and the process of accessing the information can be different between manufacturers. You can also take your vehicle to an AutoZone or O’Reilly Auto Parts store to have their workers plug in their code reader for free.

To clear the OBD codes found in you vehicle using a scanner, you need to take a few specific steps:

1. Hook up the scanner to the OBD female port located under your vehicle’s steering column.
2. Find the Clear option on the scanner and press it to clear the code from the ECU’s memory.

To clear the OBD codes without a scanner you need to:

1. Disconnect your vehicle’s battery cables from their ports.
2. Cycle the ignition switch from ON to OFF about three to five times.
3. Press on your vehicle’s horn for 30 seconds or until it stops honking.
4. Wait 10-15 minutes before connecting your battery cables again and turning the vehicle back on to ensure the check engine light is no longer there.