NB Miata, OBD Mode 2

Today's post is pretty brief, as I'm planning to do one post per OBD Mode, and there's not a lot to say about Mode 2.

OBD Mode 2 is similar to Mode 1.  However, whereas Mode 1 is used to obtain current diagnostic data, Mode 2 PIDs return freeze frame data.  That is, they report the state of the engine when the last DTC occurred.

For the NB, the following Mode 2 PIDs are supported (a few less than Mode 1):

• 00 - PIDs supported (01-20)
• 02 - Freeze DTC
• 03 - Fuel system status
• 04 - Calculated engine load value
• 05 - Engine coolant temperature
• 06 - Short term fuel % trim - Bank 1
• 07 - Long term fuel % trim - Bank 1
• 0C - Engine RPM
• 0D - Vehicle speed

PID 00 - PIDs supported (01-20)
The subroutine that creates the reply message is at address 2885C.
Return value indicates that 02-07, 0C-0D are supported.

PID 02 - Freeze DTC
The subroutine that creates the reply message is at address 28870.
Returns the DTC which the freeze data is associated with.  If there is no DTC, it returns 0.
If the byte at RAM location 10EF4 is non-zero, it means there is a DTC.
The Freeze DTC value is stored as a word at RAM location 10F28.

PID 03 - Fuel system status
The subroutine that creates the reply message is at address 28886.
The freeze frame data for fuel system status is stored in a byte at RAM location 10F21.

PID 04 - Calculated engine load value
The subroutine that creates the reply message is at address 28894.
The byte at RAM location 10F18 is proportional to calculated freeze frame engine load value.

PID 05 - Engine coolant temperature
The subroutine that creates the reply message is at address 288A4.
The byte at RAM location 10F20 is proportional to (freeze frame engine coolant temperature + 40 degrees C).

PID 06 - Short term fuel % trim - Bank 1
The subroutine that creates the reply message is at address 288B4.
The word at RAM location 10F1C is related to short term fuel % trim.

PID 07 - Long term fuel % trim - Bank 1
The subroutine that creates the reply message is at address 288C4.
The word at RAM location 10F1E is related to long term fuel % trim.

PID 0C - Engine RPM
The subroutine that creates the reply message is at address 288D4.
The word at RAM location 10F1A is proportional to RPM.

PID 0D - Vehicle speed
The subroutine that creates the reply message is at address 288E4.
The byte at RAM location 10F1F is proportional to vehicle speed.