Diagnosing Intermittents Using PCM "SNAPSHOTS"

PCM snapshots may be very useful in diagnosing powertrain problems. Data stored through Customer Snapshot, while in on-board diagnostics, or when a PCM code was set may be retrieved and used to determine if a particular input was incorrect when the problem occurred. Many of the PCM DTC flow charts will use Snapshot on code set information to pinpoint a problem if it is not current.

For example, the flow chart for DTC P028 uses the snapshot data parameter PD76 (gear ratio) to indicate what gear the transaxle was in when the code set. This information may be used along with the transaxle pressure switch indication to zero in on the circuit(s) that are intermittently shorted or open. Snapshot on code set data may even be used, as in DTC P023 flow chart, to determine whether the engine was running or not. This is an important piece of data in diagnosing DTC P023.

In order to use Customer Snapshot data or PCM snapshot data that was recorded in on-board diagnostics, you must first understand what problem the driver was trying to capture in the snapshot. Once this is understood, any appropriate parameters may be retrieved and any abnormal parameter readings used to pinpoint what sensor or system is causing the problem.

For example, say a vehicle comes in with an intermittent rough engine condition and a Customer Snapshot was taken while the engine was running rough. Several data parameters may be retrieved and an abnormal parameter noted. MAP sensor, ECT sensor, IAT sensor, and knock sensor parameters may be compared to normal values of those parameters and any values that are out of the normal range investigated. Also, the second digit of PCM output fault status B (PD17) may be retrieved. If it is a "1", the PCM noted a bad fuel injector circuit which may be causing the rough engine problem and should be investigated. While this method may result in increased diagnosis time, it also results in less actual repair time and fewer service comebacks.