The Electronic Control Modules (ECM) installed on heavy truck engines record data during normal operation. While their primary purpose is to control the mechanical and electrical systems on the engine, they monitor other vehicle systems as well. Stored data from the ECM can be downloaded into reports that are useful for tracking fleet performance, scheduling maintenance or troubleshooting engine problems. Many of the current production heavy trucks also record event information that may be useful when analyzing or reconstructing accidents. It is not uncommon for a vehicle involved in a frontal accident to experience sudden decelerations and/or sustain damage to the coolant system or rupture the oil pan. Sustained data outside normal operating parameters from sensors on the engine will typically trigger the ECM to generate a fault code. On a Caterpillar ECM, a short event record called a Diagnostic Snapshot may also be generated with a fault code. Numerous operator controls, engine sensor parameters and vehicle speeds from the truck/tractor are documented for a specified period of time both prior to and subsequent to the trigger condition. The Caterpillar ECM monitors the vehicle speed and is capable of generating what is known as a Quick Stop Snapshot record (from here on referred to as “Quick Stop”) if the vehicle decelerates at greater than or equal to a predetermined rate. A Quick Stop also logs engine parameters and operator control information. This paper discusses the timing and accuracy of data recorded by Diagnostic Snapshot and Quick Stop records and the reliability of the data therein for use in collision analysis.