3.1.6 Example for Use of Standardized Test IDs for Misfire Monitor
On Board Diagnostic (OBD) regulations require reporting the number of misfire events detected during the current driving cycle (Test ID $OC) and the average number of misfire events detected during the last 10 driving cycles (Test ID $0B) for each cylinder. Therefore, for a 4-cylinder engine, eight pieces of data must be reported for both Test IDs.
The purpose of the misfire data is to help identify which cylinders are currently misfiring ($0C) and identify which cylinders have been consistently misfiring in the past 10 driving cycles ($0B). The actual misfire event counts will depend on how the vehicle was driven, how long it was driven, etc. Misfire counts for cylinders are only to be compared relative to each other. If some cylinders have many more misfire events than other cylinders, troubleshooting should begin with the cylinders that have the highest numbers of misfire events.
The Test ID $0B registers contain the EWMA (Exponential Weighted Moving Average) value for misfire events counted during the last 10 driving cycles. The EWMA value is only re-calculated once per driving cycle. This calculation is carried out during every power-down sequence due to the control module having a short stay alive period after the ignition key is turned off. The EWMA value uses the misfire event counts collected during the last/current driving cycle. The value of the $0C counters, after the driving cycle ends, is the number of misfire events counted during the current/last driving cycle. The software takes the contents of the $0B register (this is the previous average) multiply by 0.9 and adds the contents of the $0C register (this is the current misfire event counts) multiplied by 0.1. This becomes the new EWMA value.
The Test ID $0C counters counts misfire events for each cylinder and saves them in Keep Alive or Non-Volatile Memory. They update continuously, in 200 or 1000 revolution increments, as a minimum. When the engine starts, the $0C misfire counters are reset to zero. Prior to engine start-up, the last value from the previous driving cycle is retained, so that the number of misfire events that occurred during the last drive cycle can be displayed. If a vehicle has constant misfire in one or more cylinders, Test ID $0C can be used to monitor the misfire event counters while the vehicle is being driven, up to a maximum of 65,535 events.
There are no minimum or maximum misfire monitor threshold limits for misfire counts. Test IDs $0B and $0C just accumulate the number of misfires that occur. These counts should accumulate with or without a misfire Diagnostic Trouble Code (DTC). If there was a small misfire, but not enough to store a DTC, Test ID $0B and $0C values for each cylinder should still show the number of misfire events that occurred.