Changing the Program: Updating a PCM is easy with the right tools

Tools and steps for updating a PCM

Tool Briefing offers real-world tips and advice for using specific tools to accomplish specific tasks. The step-by-step tasks are shown below in the Identifix Direct-Hit box, along with related vehicles, concerns and factory bulletin numbers. But remember, the tool information might apply to many other vehicles too.

Tools needed for this job

  • smoke machine
  • scan tool
  • pass thru device

When the “check engine” light turns on, reach for a scan tool first. The light turns on when the Powertrain Control Module (PCM) thinks tailpipe emissions are above the legal limit. The trouble codes in our example indicate both cylinder banks are running lean, which would increase NOx emissions. Many things could cause these codes, and without a driveability complaint, finding the real problem will require a little digging.


Codes that start with “P0” are generic and can be retrieved with a basic code reader, but you need a scan tool to monitor short-term (STFT) and long-term fuel trim (LTFT). Most scan tools report fuel trim as numbers and also as a graph. Some can graph multiple data channels, and scan tool software for laptop computers can graph up to eight channels at once.


The scan tool will also show Mass Air Flow (MAF) sensor data. Depending on the type of sensor, it will appear as grams per second (g/s), frequency (Htz) or voltage. On the Ford described here, MAF readings are reported as voltage, and if the voltage isn’t correct at idle, that might indicate a vacuum leak.



A good way to locate a vacuum leak is with a smoke machine. There are several on the market, ranging from small canisters to wheeled console units. Many techs flow smoke with nitrogen instead of shop air to minimize the risk of fire. Newer smoke machines are compatible with nitrogen, and some even generate their own.


Smoke machines are simple to use, and people are finding imaginative ways to utilize them. The smoke is made from common baby oil, and when combined with ultraviolet dye, it’s one of the fastest ways to find a leak.


Reprogramming a PCM requires a J2534 Pass Thru device. Only 2004 and newer vehicles can be reprogrammed.


A pass thru device is quite literally a box that acts as a translator, allowing the PCM to communicate with a Microsoft Windows operating system. Pass thru tools are completely self-contained; some don’t even have an on/off button. There are a dozen or more on the market, and some scan tools have pass thru capabilities too.


To reprogram a PCM, the tool must comply with SAE Standard J2534-1. You can also use a J2534-2 pass thru tool that can reprogram other control modules as well as the PCM.


Not all pass thru tools work on every vehicle, although some are more versatile than others. Most automakers test them all, and those that have been “validated” to work properly on their vehicles are listed on their service information website (more on that later). This is valuable information when deciding which scan tool or pass thru tool to buy.


Pass thru tools are now in the second generation, and newer models are generally more reliable. That said, reprogramming doesn’t always go smoothly. Often the PCM will reprogram correctly but another control unit will not, causing the whole operation to stall or abort. Sometimes it takes multiple attempts to complete the operation, and sometimes it’s necessary to skip over some modules (on Fords it’s often the PATS module). If the programming won’t run to completion, the tool company’s technical support line is an important resource.


Reprogramming a PCM requires an external power supply. Reprogramming is a delicate operation requiring careful attention to detail. At best, getting it wrong will cost time. On some vehicles, a botched reprogramming operation can ruin the PCM. Throughout the reprogramming operation, battery voltage absolutely must remain constant. That’s why you need external power.


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