Save money, avoid unnecessary repairs with a complete car diagnosis

Contrary to popular belief, diagnostic trouble codes (DTCs) are not an actual diagnosis. They’re only clues, which serve as a starting point for further testing.To fix your vehicle right the first time, and avoid purchasing unnecessary parts, a thorough inspection should be performed.

Have you ever retrieved a diagnostic trouble code to fix a problem, then replaced the corresponding part, only to have the issue persist?

At first, you may think the replacement component is defective, or perhaps, the code reader is out to lunch. But that’s rarely the case. Almost always, the real problem is the diagnostic procedure being used. Or, to be more precise, the lack thereof.

diagnosis bmw

Diagnostics: it’s more than just pulling codes

One thing many people (including some professional technicians) fail to understand, is diagnostic trouble codes (DTCs) are not an actual diagnosis. Instead, they’re merely clues, which serve as a starting point for further testing.

For example, if you retrieve an idle air control valve (IAC) oder air flow sensor code, that doesn’t necessarily mean the sensor itself has failed. It just indicates the engine control module has detected a problem within that system. The real cause of the concern could be anything from a vacuum leak to a problem with the IAC circuit.

Relying on DTCs alone leads to lost time and wasted money. Before ordering parts or doing any repairs, a thorough inspection of the problem system should be performed.

There are two main ways professionals go about this:

Trouble Code MAF

1. Some follow a diagnostic “trouble tree”, while others use knowledge to create their own test procedure. A “trouble tree” is a type of diagnostic flowchart. The automaker supplies one of these step-by-step test procedures for each DTC a vehicle supports.
TIP: DIYers who wish to follow a trouble tree can get the information by purchasing a single vehicle subscription from ALLDATA (

There are also repair guides for specific German brands like Audi, BMW, Mercedes, and VW:

Some OEMs also provide short-term subscriptions to factory repair information on their website. The issue with trouble trees is they’re often unnecessarily time-consuming. Plus, engineers design them for vehicles that are brand new, not taking into account issues that occur over time. In other words, with factory flowcharts, you may end up chasing your tail.

2. Seasoned technicians often forgo trouble trees altogether and develop their own test procedures.

For this strategy to be effective, however, an in-depth understanding of the problem system is required. Let’s take our IAC code as an example. Instead of jumping right in and replacing the valve, a professional will first perform a sequential system test.

– The process usually starts with listening to the vehicle and performing a visual inspection.
– If nothing is found, the next step is to test the IAC and its circuit. In most cases, this is either done with an OEM-level scanner and/or a digital multimeter.

Get the information to diagnose your vehicle right

You’re probably wondering where you, the consumer, can get the information needed to perform professional-grade diagnostics. A single-vehicle subscription to ALLDATA is a great place to start. There, you can retrieve factory repair data, such as component descriptions and operation, test procedures and wiring diagrams.

Aftermarket repair manuals, such as those from Chilton, can be helpful as well.

No matter what, just remember, codes are not a diagnosis. Perform a thorough system inspection every time to eliminate frustration and save money.