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

Tools and steps for updating a PCM

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


We Recommend