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

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.


Sometimes the job goes quickly and a jumper pack will provide enough extra power. Often the job takes an hour or more, and even the slight AC ripple produced by a standard battery charger is enough to ruin the job. Purpose-built power supplies and some battery chargers are made to provide clean voltage for reprogramming. When shopping, look for electronically controlled battery chargers and power supplies that mention reprogramming or re-flashing.


Even with a source of clean external power, the vehicle’s battery and charging system must be in good condition. If battery voltage falls too low during that first start-up, the PCM may not initialize properly. This can cause problems that are nearly impossible to diagnose or fix.


Choosing your connection is easy. The pass thru device connects to the PCM through the OBD II connector. The computer connection can be USB, serial, Ethernet, Bluetooth or WiFi. Some people don’t like to risk reprogramming through a wireless connection, and most people use USB 2.0 connection.


On some cars, it’s extremely risky to do this job without a high-speed Internet connection, not just for speed but also to minimize the chance of a corrupted file transfer. Several manufacturers require the vehicle to be connected to their server (via the pass thru tool) during reprogramming.


Once connected to the computer, the pass thru device-driver must be installed and tested. The Windows Hardware Wizard will guide you through this and place a device icon on the computer desktop. Testing communications between the PCM, pass thru device and computer is either menu driven or completely automatic, but you must get on-screen confirmation that the link is established.


Depending on the vehicle, the next step is to load the PCM update into the computer or connect the vehicle to the manufacturer’s service information website. Either way, you’ll need a website subscription. While each manufacturer offers different subscription packages, they all offer a short term subscription at a price that’s reasonable enough to be added to the customer repair order. Some manufacturers offer subscriptions at no charge.



When everything is ready, it’s time to complete the job. Simply follow the instructions on the computer screen. Carefully. Once begun, the job must not be interrupted. It’s particularly important to avoid unplanned changes in battery current draw. Lights, solenoids and fans may activate automatically during reprogramming, but an unplanned current draw, such as opening the car door, could cause the operation to abort.


No matter which pass thru device you use, with practice and patience, reprogramming is no more difficult than any other job. The main thing is to make sure the connections are secure, that communication is firmly established, and that battery power remains stable. The tools and the time spend learning how to use them are an investment that will provide a very handsome return.


2006 Ford F-250 Super Duty King Ranch 5.4L,

Customer Concern:

MIL illumination with trouble codes P0171 and P0174. The engine runs fine. The fuel filter was changed. 


1. Monitor the Long-Term Fuel Trim (LFT) readings at idle and at cruise speed to determine when it is lean.
2. If the readings are highest at idle, look for a vacuum leak.
3. If the LFT numbers are higher at cruise speed, monitor the fuel pressure and check for a dirty Mass Air Flow (MAF) sensor. Try cleaning the MAF sensor and recheck operation.
4. If the MAF sensor is clean, monitor the fuel pressure. It should be roughly 30-45 psi and increase under load. Check voltage between the white and white/red wires at the fuel pump. At idle it should read 6-8 volts.
5. If the fuel trim readings look good under all driving conditions, reprogram the PCM per TSB 07-21-7 if it has not previously been done. 

Potential Causes:

Engine Vacuum Leak
Fuel Pump
Mass Air Flow (MAF) Sensor
Powertrain Control Module (PCM) Programming