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According to the OBD-II standard, requests to a vehicle's ECU via the OBD-II port are made up of two bytes (excluding header and CRC bytes). The first byte determines the desired mode of operation, and the second byte is the requested parameter identification (PID) number. The ECU will respond with a two byte acknowledgement and possibly some number of data bytes.

[edit] Modes

There are nine modes of operation described in the OBD-II standard (SAE J1979). They are:

1. Show current data
2. Show freeze frame data
3. Show stored trouble codes
4. Clear trouble codes and stored values
5. Test results, oxygen sensors
6. Test results, non-continuously monitored
7. Show pending trouble codes
8. Special control mode
9. Request vehicle information

Vehicle manufactures are not required to support all modes, and they are allowed to include custom modes above #9.

[edit] PIDs

The table below shows the standard OBD-II PIDs as defined by SAE J1979. The expected response for each PID is given, along with information on how to translate the response into meaningful data. Again, not all vehicles will support all PIDs and there can be manufacturer-defined custom PIDs that are not defined in the OBD-II standard.

Note that modes 1 and 2 are basically identical, except that Mode 1 provides current information, whereas Mode 2 provides a snapshot of the same data taken at the point when the last diagnostic trouble code was set. The exceptions are PID 01, which is only available in Mode 1, and PID 02, which is only available in Mode 2. If Mode 2 PID 02 returns zero, then there is no snapshot and all other Mode 2 data is meaningless.

Mode (hex)	PID (hex)	Data bytes returned	Description	Min value	Max value	Units	Formula
01	00	4	PIDs supported				Bit encoded [A7..D0] == [PID 0x01..PID 0x20]
01	01	4	Number of trouble codes and I/M info				Bit encoded. See below.
01	03	2	Fuel system status				Bit encoded. See below.
01	04	1	Calculated engine load value	0	100	%	A*100/255
01	05	1	Engine coolant temperature	-40	215	°C	A-40
01	06	1	Short term fuel % trim—Bank 1	-100 (lean)	99.22 (rich)	%	0.7812 * (A-128)
01	07	1	Long term fuel % trim—Bank 1	-100 (lean)	99.22 (rich)	%	0.7812 * (A-128)
01	08	1	Short term fuel % trim—Bank 2	-100 (lean)	99.22 (rich)	%	0.7812 * (A-128)
01	09	1	Long term fuel % trim—Bank 2	-100 (lean)	99.22 (rich)	%	0.7812 * (A-128)
01	0A	1	Fuel pressure	0	765	kPa (gauge)	A*3
01	0B	1	Intake manifold pressure	0	255	kPa (absolute)	A
01	0C	2	Engine RPM	0	16,383.75	rpm	((A*256)+B)/4
01	0D	1	Vehicle speed	0	255	km/h	A
01	0E	1	Timing advance	-64	63.5	° relative to #1 cylinder	A/2 - 64
01	0F	1	Intake air temperature	-40	215	°C	A-40
01	10	2	MAF air flow rate	0	655.35	g/s	((256*A)+B) / 100
01	11	1	Throttle position	0	100	%	A*100/255
01	12	1	Sec.(?) air status				Bit encoded. See below.
01	13	1	Oxygen sensors present				[A0..A3] == Bank 1, Sensors 1-4. [A4..A7] == Bank 2...
01	14	2	Bank 1, Sensor 1: Oxygen sensor voltage, Short term fuel trim	0 0	1.275 99.2	Volts %	A * 0.005 (B-128) * 0.7812 (if B==0xFF, sensor is not used in trim calc)
01	15	2	Bank 1, Sensor 2: Oxygen sensor voltage, Short term fuel trim	0 0	1.275 99.2	Volts %	A * 0.005 (B-128) * 0.7812 (if B==0xFF, sensor is not used in trim calc)
01	16	2	Bank 1, Sensor 3: Oxygen sensor voltage, Short term fuel trim	0 0	1.275 99.2	Volts %	A * 0.005 (B-128) * 0.7812 (if B==0xFF, sensor is not used in trim calc)
01	17	2	Bank 1, Sensor 4: Oxygen sensor voltage, Short term fuel trim	0 0	1.275 99.2	Volts %	A * 0.005 (B-128) * 0.7812 (if B==0xFF, sensor is not used in trim calc)
01	18	2	Bank 2, Sensor 1: Oxygen sensor voltage, Short term fuel trim	0 0	1.275 99.2	Volts %	A * 0.005 (B-128) * 0.7812 (if B==0xFF, sensor is not used in trim calc)
01	19	2	Bank 2, Sensor 2: Oxygen sensor voltage, Short term fuel trim	0 0	1.275 99.2	Volts %	A * 0.005 (B-128) * 0.7812 (if B==0xFF, sensor is not used in trim calc)
01	1A	2	Bank 2, Sensor 3: Oxygen sensor voltage, Short term fuel trim	0 0	1.275 99.2	Volts %	A * 0.005 (B-128) * 0.7812 (if B==0xFF, sensor is not used in trim calc)
01	1B	2	Bank 2, Sensor 4: Oxygen sensor voltage, Short term fuel trim	0 0	1.275 99.2	Volts %	A * 0.005 (B-128) * 0.7812 (if B==0xFF, sensor is not used in trim calc)
01	1C	1	OBD standards this vehicle conforms to				Bit encoded. See below.
01	1D	1	Oxygen sensors present				Similar to PID 13, but [A0..A7] == [B1S1, B1S2, B2S1, B2S2, B3S1, B3S2, B4S1, B4S2]
01	1E	1	Auxiliary input status				A0 == Power Take Off (PTO) status (1 == active) [A1..A7] not used
01	1F	2	Run time since engine start	0	65,535	seconds	(A*256)+B
01	20	4	PIDs supported 21-40				Bit encoded [A7..D0] == [PID 0x21..PID 0x40]
01	21	2	Distance traveled with malfunction indicator lamp (MIL) on	0	65,535	km	(A*256)+B
01	22	2	Fuel Rail Pressure (relative to manifold vacuum)	0	5177.265	kPa	((A*256)+B) * 0.079
01	23	2	Fuel Rail Pressure (diesel)	0	655350	kPa (gauge)	((A*256)+B) * 10
01	24	4	O2S1_WR_lambda(1): Equivalence Ratio Voltage	0 0	2 8	N/A V	((A*256)+B)*0.0000305 ((C*256)+D)*0.000122
01	25	4	O2S2_WR_lambda(1): Equivalence Ratio Voltage	0 0	2 8	N/A V	((A*256)+B)*0.0000305 ((C*256)+D)*0.000122
01	26	4	O2S3_WR_lambda(1): Equivalence Ratio Voltage	0 0	2 8	N/A V	((A*256)+B)*0.0000305 ((C*256)+D)*0.000122
01	27	4	O2S4_WR_lambda(1): Equivalence Ratio Voltage	0 0	2 8	N/A V	((A*256)+B)*0.0000305 ((C*256)+D)*0.000122
01	28	4	O2S5_WR_lambda(1): Equivalence Ratio Voltage	0 0	2 8	N/A V	((A*256)+B)*0.0000305 ((C*256)+D)*0.000122
01	29	4	O2S6_WR_lambda(1): Equivalence Ratio Voltage	0 0	2 8	N/A V	((A*256)+B)*0.0000305 ((C*256)+D)*0.000122
01	2A	4	O2S7_WR_lambda(1): Equivalence Ratio Voltage	0 0	2 8	N/A V	((A*256)+B)*0.0000305 ((C*256)+D)*0.000122
01	2B	4	O2S8_WR_lambda(1): Equivalence Ratio Voltage	0 0	2 8	N/A V	((A*256)+B)*0.0000305 ((C*256)+D)*0.000122
01	2C	1	Commanded EGR	0	100	%	100*A/255
01	2D	1	EGR Error	-100	99.22	%	A*0.78125 - 100
01	2E	1	Commanded evaporative purge	0	100	%	100*A/255
01	2F	1	Fuel Level Input	0	100	%	100*A/255
01	30	1	# of warm-ups since codes cleared	0	255	N/A	A
01	31	2	Distance traveled since codes cleared	0	65,535	km	(A*256)+B
01	32	2	Evap. System Vapor Pressure	-8,192	8,192	Pa	((A*256)+B)/4 - 8,192
01	33	1	Barometric pressure	0	255	kPa (Absolute)	A
01	34	4	O2S1_WR_lambda(1): Equivalence Ratio Current	0 -128	2 128	N/A mA	((A*256)+B)*0.0000305 ((C*256)+D)*0.00391 - 128
01	35	4	O2S2_WR_lambda(1): Equivalence Ratio Current	0 -128	2 128	N/A mA	((A*256)+B)*0.0000305 ((C*256)+D)*0.00391 - 128
01	36	4	O2S3_WR_lambda(1): Equivalence Ratio Current	0 -128	2 128	N/A mA	((A*256)+B)*0.0000305 ((C*256)+D)*0.00391 - 128
01	37	4	O2S4_WR_lambda(1): Equivalence Ratio Current	0 -128	2 128	N/A mA	((A*256)+B)*0.0000305 ((C*256)+D)*0.00391 - 128
01	38	4	O2S5_WR_lambda(1): Equivalence Ratio Current	0 -128	2 128	N/A mA	((A*256)+B)*0.0000305 ((C*256)+D)*0.00391 - 128
01	39	4	O2S6_WR_lambda(1): Equivalence Ratio Current	0 -128	2 128	N/A mA	((A*256)+B)*0.0000305 ((C*256)+D)*0.00391 - 128
01	3A	4	O2S7_WR_lambda(1): Equivalence Ratio Current	0 -128	2 128	N/A mA	((A*256)+B)*0.0000305 ((C*256)+D)*0.00391 - 128
01	3B	4	O2S8_WR_lambda(1): Equivalence Ratio Current	0 -128	2 128	N/A mA	((A*256)+B)*0.0000305 ((C*256)+D)*0.00391 - 128
01	3C	2	Catalyst Temperature Bank 1, Sensor 1	-40	6,513.5	°C	((A*256)+B)/10 -40
01	3D	2	Catalyst Temperature Bank 1, Sensor 2	-40	6,513.5	°C	((A*256)+B)/10 -40
01	3E	2	Catalyst Temperature Bank 2, Sensor 1	-40	6,513.5	°C	((A*256)+B)/10 -40
01	3F	2	Catalyst Temperature Bank 2, Sensor 2	-40	6,513.5	°C	((A*256)+B)/10 -40
01	40	4	PIDs supported 41-60 (?)				Bit encoded [A7..D0] == [PID 0x41..PID 0x60] (?)
01	41	?	Monitor status this drive cycle	?	?	?	?
01	42	2	Control module voltage	0	65.535	V	((A*256)+B)/1000
01	43	2	Absolute load value	0	25696	%	((A*256)+B)*100/255
01	44	2	Command equivalence ratio	0	2	N/A	((A*256)+B)*0.0000305
01	45	1	Relative throttle position	0	100	%	A*100/255
01	46	1	Ambient air temperature	-40	215	°C	A-40
01	47	1	Absolute throttle position B	0	100	%	A*100/255
01	48	1	Absolute throttle position C	0	100	%	A*100/255
01	49	1	Accelerator pedal position D	0	100	%	A*100/255
01	4A	1	Accelerator pedal position E	0	100	%	A*100/255
01	4B	1	Accelerator pedal position F	0	100	%	A*100/255
01	4C	1	Commanded throttle actuator	0	100	%	A*100/255
01	4D	2	Time run with MIL on	0	65,535	minutes	(A*256)+B
01	4E	2	Time since trouble codes cleared	0	65,535	minutes	(A*256)+B
01	C3	?	?	?	?	?	Returns numerous data, including Drive Condition ID and Engine Speed*
01	C4	?	?	?	?	?	B5 is Engine Idle Request B6 is Engine Stop Request*
02	02	2	Freeze frame trouble code				BCD encoded, see below.
03	N/A	n*6	Request trouble codes				3 codes per message frame, BCD encoded. See below.
04	N/A	0	Clear trouble codes / Malfunction indicator lamp (MIL) / Check engine light				Clears all stored trouble codes and turns the MIL off.
09	02	5x5	Vehicle identification number (VIN)				Returns 5 lines, A is line ordering flag, B-E ASCII coded VIN digits.

In the formula column, letters A, B, C, etc. represent the decimal equivalent of the first, second, third, etc. bytes of data. Where a (?) appears, contradictory or incomplete information was available. Someone with a copy of the 2006 SAE HS-3000 should fact-check these.

[edit] Bitwise encoded PIDs

Some of the PIDs in the above table cannot be explained with a simple formula. A more elaborate explanation of these data is provided here:

Mode 1 PID 01: A request for this PID returns 4 bytes of data. The first byte contains two pieces of information. Bit A7 (the seventh bit of byte A, the first byte) indicates whether or not the MIL (check engine light) is illuminated. Bits A0 through A6 represent the number of diagnostic trouble codes currently flagged in the ECU. The second, third, and fourth bytes give information about the availability and completeness of certain on-board tests:

                 Test available       Test complete
Misfire                B0                   B4           
Fuel System            B1                   B5
Components             B2                   B6
Reserved               B3                   B7
Catalyst               C0                   D0
Heated Catalyst        C1                   D1
Evaporative System     C2                   D2
Secondary Air System   C3                   D3
A/C Refrigerant        C4                   D4
Oxygen Sensor          C5                   D5
Oxygen Sensor Heater   C6                   D6
EGR System             C7                   D7

Mode 1 PID 03: A request for this PID returns 2 bytes of data. The first byte describes fuel system #1. Only one bit should ever be set.

A0     Open loop due to insufficient engine temperature
A1     Closed loop, using oxygen sensor feedback to determine fuel mix
A2     Open loop due to engine load
A3     Open loop due to system failure
A4     Closed loop, using at least one oxygen sensor but there is a fault in the feedback system
A5-A7  Always zero

The second byte describes fuel system #2 (if it exists) and is encoded identically to the first byte.

Mode 1 PID 12: A request for this PID returns a single byte of data which describes the secondary air status. Only one bit should ever be set.

A0     Upstream of catalytic converter
A1     Downstream of catalytic converter
A2     From the outside atmosphere or off
A3-A7  Always zero

Mode 1 PID 1C: A request for this PID returns a single byte of data which describes which OBD standards this ECU was designed to comply with. The hexadecimal and binary representations of the data byte are shown below next to what it implies:

0x01  00000001b    OBD-II as defined by the CARB
0x02  00000010b    OBD as defined by the EPA
0x03  00000011b    OBD ''and'' OBD-II
0x04  00000100b    OBD-I
0x05  00000101b    Not meant to comply with any OBD standard
0x06  00000110b    EOBD

Mode 3: (no PID required) A request for this mode returns information about the DTCs that have been set. The response will be an integer number of packets each containing 6 data bytes. Each trouble code requires 2 bytes to describe, so the number of packets returned will be the number of codes divided by three, rounded up. A trouble code can be decoded from each pair of data bytes. The first character in the trouble code is determined by the first two bits in the first byte:

A7 A6    First DTC character
-- --    -------------------
 0  0    P
 0  1    C
 1  0    B
 1  1    U

The second character in the DTC is a number defined by

A5 A4    Second DTC character
-- --    --------------------
 0  0    0
 0  1    1
 1  0    2
 1  1    3

The third character in the DTC is a number defined by

A3 A2 A1 A0    Third DTC character
-- -- -- --    -------------------
 0  0  0  0    0
 0  0  0  1    1     
 0  0  1  0    2
 0  0  1  1    3
 0  1  0  0    4
 0  1  0  1    5
 0  1  1  0    6
 0  1  1  1    7
 1  0  0  0    8
 1  0  0  1    9

The fourth and fifth characters are defined in the same way as the third, but using bits B7..B4 and B3..B0. The resulting five-character code should look something like "U0158" and can be looked up in a table of OBD-II DTCs.