"I'd like for you to hook it up to the code machine and see what it says to replace" - anonymous customer

Sound about right? It sure does. Of course, as auto repair professionals, we know better. We know that reading the code is not diagnostics, but rather that is where diagnostics might begin. We know that simply pulling a code and then jumping straight to replacing a part, is a recipe for disaster.

So how do we get from reading a code to accurately replacing a part? To start with, let's look at what the fault code is truly telling us. We're looking for more than just the official description that might look like "EGR low flow detected" or "Bank 1 lean condition" and so on, we are looking deeper to see what the code is truly saying. That information can usually be found by reading the code set criteria. Since the true meaning of the code is shaped by how the system works, then the true meaning of a fault code can be different from one make to another. Let's pick one and look at it.

Let's look at a P0402 on a Ford, then compare it to a Nissan to see what happens between the two makes, even though the generic description remains the same: "EGR excessive flow detected."

Picking a common Ford product, if you were to read the code set criteria for a P0402 on a 1997 Explorer 4.0L, you would find that the PCM is looking for the DPFE to register a voltage that is higher than a threshold limit (typically just ov...