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Since the 6.5 was the leader in pioneering the world of electronic diesels, we have come to appreciate, especially on forums like these, the "code." One of the first questions that gets asked is:

"Do you have any codes?"

We tend to take those numbers to heart and expect them to pinpoint the exact problem. Truth is, these numbers only give us a direction to head in. You see, the computer is, well, uh, dumb. Yes, the computer is totally utterly stupid. It knows two things. On & Off (or 1 & 0) It shows no compassion for any other factors that come into play. It is our job as the troubleshooter, to discover what the issue truly is.

The world of the ECM or PCM, whichever term you give "the computer", is governed totally by voltage. Charging voltage, battery voltage, 5 volt reference voltage, modulated voltage, pulse width, duty cycle, on and on the list goes. Start messing up some of these tight tolerances on what the computer expects to see, and it may very well puke out a code.

Take for example, one of the most popular OBD-I codes listed throughout most forums for the 6.5 is the code 78. "Wastegate code" is what it gets termed as. Most folks just jump in and replace the wastegate solenoid. This may or may not solve the problem. You have to understand that the computer is looking at the boost level that is being given to it from the boost sensor. It has no clue what's really going on in the engine. Your whole turbo could be trashed (not very common, but just using this as an example) The computer looks for an expected level of boost. It doesn't see it. It doesn't know why or how it's not arriving at the desired/expected boost level. All's it knows is, it's not there, and lets throw a code.

After understanding each of the systems that is represented by a "code" you will better understand what the code actually means.

Some (including myself) have had codes that have come up on their vehicle and thought that a specific part is at fault. When really, the issue is an external factor.

The Injection pump is one of those parts that has many codes associated with it. Not understanding the potiential reasons that you could have a code come up, may result in an unnecessary pump replacement.

There are many different circuits that make our injection pumps work. However, they are are the mercy of some external factors. Too many to list here, but some would be: system voltage, condition of grounds, condition of harness connections, and the fuel supply to the pump itself. You may see a code that represents a fuel solenoid closure time problem. This could be one of two different things. This could be that you may have a) a geniune problem with the fuel solenoid, or b) a fuel delivery problem. (which could be a bad lift pump) Now, given the choice, would you rather replace a $100 lift pump, or a $1200 injection pump? Base it solely on the code, such a mistake can happen.

All this to say, don't take the code as the ultimate authority on the problem, but use it as a tool to indicate where the problem may lie.

Hopefully following in this thread in the near future we can have a few listings for what you can expect if you see a specific "code".
 

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How true, nice post.

Pull the next layer off and look at that same PCM through a scanner and the basic diagnostic codes seem more like dummy lights.
 
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