How to Diagnose a BMW

1. Open the BMW's driver-side door. Look into the leg-space beneath the dashboard. There, you will find 16 pin-receiving Data Link Connector. This DLC outlet is featured in different locations, depending on the model of BMW you are working with. Some may readily seen, but others may be concealed behind an access panel. Should you have trouble locating your BMW's DLC outlet, there is an online application that could assist (see Resources).
2. Connect your diagnostic handheld to the DLC outlet. For most devices, this is a similar process. A scanner or a code reader typically comes with a diagnostic cable with a 16-prong plug. Connect the cable to the device, and plug the cable into the DLC outlet.
3. Switch your device on. Then slide your BMW's key into the vehicle's ignition. Switch to 'On.' This will activate the BMW's electrical array. Depending on the device and the system you are working with, you may also need an active engine. If this is the case, start the automobile's engine at this time.
4. Wait a few seconds to a minute. Your device and the vehicle's diagnostic system will establish a connection. Some devices, once an incoming data stream is detected, will automatically retrieve active trouble codes. If you do not possess a device preprogrammed to do this, consult your device's handbook. Button configurations and general layout differ by brand and type of diagnostic handheld. Your device's handbook will detail the exact instructions on how to enter a code-retrieving command.
5. Look through the codes on your device's display screen. How the codes are displayed also depends on the device you are using. OBD-II codes are alphanumeric, and the first letter of the trouble code will specify where the problem is located. Anti-locking brake codes tend to be purely numerical. Many devices will also differentiate a code's status. Any code listed as 'pending' are infrequent malfunctions; these problems have not activated one of your BMW's warning lights.
6. Copy the active trouble codes onto a separate sheet of paper. If you are using more than one scanner and troubleshooting more than one diagnostic system, make sure you categorize this list. Columns are useful ways to organize trouble codes. This is also true for OBD-II diagnostics, which acts as an umbrella for many types of codes. For instance, P-codes cover the BMW's powertrain, while B, C, and U codes cover the body, chassis and network communications system.
7. Look up each code pulled from the BMW. Some device handbooks will contain coding definitions. OBD-II scanners, for example, offer generic coding lists good for all post-1996 vehicles. However, you will have to go online to find BMW's supplemental OBD-II codes. As a general rule, a vehicle's owner's manual will not contain coding definitions for any diagnostic system. However, trouble codes can easily be found online (see Resources).
8. Copy the appropriate definitions to their corresponding trouble codes. Read through all of the definitions and make a decision. If you have the mechanical experience, you can attempt DIY fixes on some of the problems. However, some diagnostic systems, like ABS brakes, are very complex and should be deferred to a trained mechanic.
9. Take your code list with you, if you decide to consult a BMW-certified mechanic. This list will save you money in diagnostic fees.