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Using the Scan Tool For Maintenance, Part 2

Preventive maintenance is all about two things: eliminating downtime and saving money. Anytime you can have the vehicle in the shop for a routine service and find a problem before it manifests itself as a symptom or a Malfunction Indicator Light (MIL), you have saved yourself a trip to the shop and kept the cruiser on the street. Do that a few times and you will see a dramatic gain in efficiency of your fleet operations.

In the Nov-Dec 2009 issue of Police Fleet Manager

, we looked at how monitoring the data stream can make a technician aware of possible problems before the MIL is illuminated.

In this article we’ll be looking at Diagnostic Trouble Codes (DTC).

Trouble codes are exactly what they sound like. When the Powertrain Control Module (PCM) or any of a dozen interconnected modules detects a problem, it often sets a code. In the pre-OBD II days, each manufacturer had his own list of codes, his own set of definitions, and even his own nomenclature for components. (Ever replace a BOO switch?) Since the mid 1990s, however, the federal government stepped in and mandated consistent code definitions, component terminology and even abbreviations. A P0420 in a Lexus describes the same problem as a P0420 in a Crown Vic.

The “P” means it is a Powertrain code, usually engine or transmission-related. The first “0” means it is a generic code. This means that it applies consistently to all makes and models. The “4” tells us it is related to emission control, and the “20” signifies catalytic converter efficiency.

Diagnostic challenges exist for many reasons. First, not all codes are generic. The feds understood that different manufacturers needed a little leeway to build their own things, so if the “P” is followed by a “1” instead of a “0,” you have a manufacturer-specific code.

Second, not all codes will turn on the MIL. It will be stored, but not necessarily displayed to the driver. The MIL is entirely related to emissions. Actually, if the vehicle software is written in such a way as to have the tailpipe emissions under control even with a running problem, the MIL may never come on.

In the case of Ford, they apparently determined that a customer would more likely bring a vehicle into the shop with a powertrain operation problem than with a MIL displayed. (“Well, Officer Jones, how long has the Check Engine light been on?” “Oh, about three years.”)

If a Ford CVPI starts to misfire, the computer will pick that up by monitoring the crank sensor. If it appears to the PCM that the misfire is cylinder specific, the PCM can shut down the injector on that cylinder. This does two things—one good and one questionable. First, it eliminates the need for the MIL because a cylinder that gets no fuel produces no harmful emissions. Second, the tech could spend hours diagnosing an inoperative injector, assuming that this is what is causing the misfire, unless he knows that this may be not a cause, but an effect.

Importantly, not all codes are powertrain related. The “P” means Powertrain. If the code begins with a “C,” you have a Chassis code. If it’s a “B,” you have a Body code, and a “U” represents a communication code.

Powertrain Codes

There are two ways to access and understand Powertrain codes. If you have a scan tool, you can enter the VIN information and read all the codes and data from what is referred to as the “manufacturer” side. However, you can also access code and data information from the “generic” side. The generic side simply has federally mandated codes and information.

The advantage of the generic side is that the PCM can’t lie to you about data. The advantage of the manufacturer side is that you get more information specific to the vehicle being looked at. The more the tech knows about how the system works, the more accurately he will be able to perform his diagnosis because he will know when to use the generic side and when to use the manufacturer side.

When checking a vehicle that is simply in for maintenance, this is one option for checking possible problems. Verify the MIL operation. Key-on, engine-off; does the MIL come on? If not, you need to find out why. Run the engine; does the MIL go out? If not, you need to find out why. If it does go out, there may still be a problem unrelated to emissions. Check the data stream for the specific parameters you have determined to verify at each PM.

Check for codes using the manufacturer side of the scan tool. This offers more information than the generic side. If no codes are found, you are done. If codes are found, then research needs to be done. If data needs to be examined, it is often best to access the data on the generic side of the scan tool. You will not get as much information, but you will not be misled either.

The reason for checking certain Parameter Identification (PID) powertrain data streams before checking trouble codes is that the adaptive capabilities of the PCM will shift to the fringe of its operation parameter before, perhaps long before, a trouble code will be set. By knowing what to look for, the tech can find problems and address them before a code is set. If the tech then finds a code, he can evaluate the data PIDs related to that code. In this event, it may be best to examine the data on the generic side.

The tech will, through practice, develop his own preventive maintenance procedures based on the vehicle in question and his experience. By using the scan tool as a means of gathering information before a symptom occurs, downtime can be minimized.

Kevin Roberts is the president of Roberts Repair in Rhinelander, WI. The company has specialized in emergency vehicle maintenance since 1989. Roberts is an ASE Certified Master Automobile and Master Truck Technician. He can be reached at

Published in Police Fleet Manager, Mar/Apr 2010

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