diff --git a/automotive/vehicle/2.1/types.hal b/automotive/vehicle/2.1/types.hal
index 2e361a3..569e1a5 100644
--- a/automotive/vehicle/2.1/types.hal
+++ b/automotive/vehicle/2.1/types.hal
@@ -249,7 +249,10 @@
       | VehicleArea:GLOBAL),
 };
 
-/** The status of the vehicle's fuel system. */
+/**
+ * The status of the vehicle's fuel system.
+ * These values come from the SAE J1979 standard.
+ */
 enum Obd2FuelSystemStatus : int32_t {
   OPEN_INSUFFICIENT_ENGINE_TEMPERATURE = 1,
   CLOSED_LOOP = 2,
@@ -264,7 +267,10 @@
   COMPRESSION = 1,
 };
 
-/** These ignition monitors are common to both SPARK and COMPRESSION. */
+/**
+ * Ignition monitors common to both SPARK and COMPRESSION.
+ * These values come from the SAE J1979 standard.
+ */
 enum Obd2CommonIgnitionMonitors : int32_t {
   COMPONENTS_AVAILABLE = 0x1 << 0,
   COMPONENTS_INCOMPLETE = 0x1 << 1,
@@ -276,7 +282,10 @@
   MISFIRE_INCOMPLETE = 0x1 << 5,
 };
 
-/** Ignition monitors available for SPARK vehicles. */
+/**
+ * Ignition monitors available for SPARK vehicles.
+ * These values come from the SAE J1979 standard.
+ */
 enum Obd2SparkIgnitionMonitors : Obd2CommonIgnitionMonitors {
   EGR_AVAILABLE = 0x1 << 6,
   EGR_INCOMPLETE = 0x1 << 7,
@@ -303,7 +312,10 @@
   CATALYST_INCOMPLETE = 0x1 << 21,
 };
 
-/** Ignition monitors only available for COMPRESSION vehicles. */
+/**
+ * Ignition monitors only available for COMPRESSION vehicles.
+ * These values come from the SAE J1979 standard.
+ */
 enum Obd2CompressionIgnitionMonitors : Obd2CommonIgnitionMonitors {
   EGR_OR_VVT_AVAILABLE = 0x1 << 6,
   EGR_OR_VVT_INCOMPLETE = 0x1 << 7,
@@ -324,7 +336,10 @@
   NMHC_CATALYST_INCOMPLETE = 0x1 << 17,
 };
 
-/** The status of the vehicle's secondary air system. */
+/**
+ * The status of the vehicle's secondary air system.
+ * These values come from the SAE J1979 standard.
+ */
 enum Obd2SecondaryAirStatus : int32_t {
   UPSTREAM = 1,
   DOWNSTREAM_OF_CATALYCIC_CONVERTER = 2,
@@ -332,7 +347,10 @@
   PUMP_ON_FOR_DIAGNOSTICS = 8,
 };
 
-/** The fuel type(s) supported by a vehicle. */
+/**
+ * The fuel type(s) supported by a vehicle.
+ * These values come from the SAE J1979 standard.
+ */
 enum Obd2FuelType : int32_t {
   NOT_AVAILABLE = 0,
   GASOLINE = 1,
@@ -366,14 +384,16 @@
  * Some of the properties are represented as an integer mapping to another enum. In those cases
  * expect a comment by the property definition describing the enum to look at for the mapping.
  * Any value greater than the last reserved index is available to vendors to map their extensions.
+ * While these values do not directly map to SAE J1979 PIDs, an equivalence is listed next
+ * to each one to aid implementors.
  */
 enum DiagnosticIntegerSensorIndex : int32_t {
   /** refer to FuelSystemStatus for a description of this value. */
-  FUEL_SYSTEM_STATUS = 0,
-  MALFUNCTION_INDICATOR_LIGHT_ON = 1,
+  FUEL_SYSTEM_STATUS = 0, /* PID 0x03 */
+  MALFUNCTION_INDICATOR_LIGHT_ON = 1, /* PID 0x01 */
 
   /** refer to IgnitionMonitorKind for a description of this value. */
-  IGNITION_MONITORS_SUPPORTED = 2,
+  IGNITION_MONITORS_SUPPORTED = 2, /* PID 0x01 */
 
   /**
    * The value of this sensor is a bitmask that specifies whether ignition-specific
@@ -381,39 +401,39 @@
    * bits in this value are given by, respectively, SparkIgnitionMonitors and
    * CompressionIgnitionMonitors depending on the value of IGNITION_MONITORS_SUPPORTED.
    */
-  IGNITION_SPECIFIC_MONITORS = 3,
-  INTAKE_AIR_TEMPERATURE = 4,
+  IGNITION_SPECIFIC_MONITORS = 3, /* PID 0x01 */
+  INTAKE_AIR_TEMPERATURE = 4, /* PID 0x0F */
 
   /** refer to SecondaryAirStatus for a description of this value. */
-  COMMANDED_SECONDARY_AIR_STATUS = 5,
-  NUM_OXYGEN_SENSORS_PRESENT = 6,
-  RUNTIME_SINCE_ENGINE_START = 7,
-  DISTANCE_TRAVELED_WITH_MALFUNCTION_INDICATOR_LIGHT_ON = 8,
-  WARMUPS_SINCE_CODES_CLEARED = 9,
-  DISTANCE_TRAVELED_SINCE_CODES_CLEARED = 10,
-  ABSOLUTE_BAROMETRIC_PRESSURE = 11,
-  CONTROL_MODULE_VOLTAGE = 12,
-  AMBIENT_AIR_TEMPERATURE = 13,
-  TIME_WITH_MALFUNCTION_LIGHT_ON = 14,
-  TIME_SINCE_TROUBLE_CODES_CLEARED = 15,
-  MAX_FUEL_AIR_EQUIVALENCE_RATIO = 16,
-  MAX_OXYGEN_SENSOR_VOLTAGE = 17,
-  MAX_OXYGEN_SENSOR_CURRENT = 18,
-  MAX_INTAKE_MANIFOLD_ABSOLUTE_PRESSURE = 19,
-  MAX_AIR_FLOW_RATE_FROM_MASS_AIR_FLOW_SENSOR = 20,
+  COMMANDED_SECONDARY_AIR_STATUS = 5, /* PID 0x12 */
+  NUM_OXYGEN_SENSORS_PRESENT = 6, /* PID 0x13 */
+  RUNTIME_SINCE_ENGINE_START = 7, /* PID 0x1F */
+  DISTANCE_TRAVELED_WITH_MALFUNCTION_INDICATOR_LIGHT_ON = 8, /* PID 0x21 */
+  WARMUPS_SINCE_CODES_CLEARED = 9, /* PID 0x30 */
+  DISTANCE_TRAVELED_SINCE_CODES_CLEARED = 10, /* PID 0x31 */
+  ABSOLUTE_BAROMETRIC_PRESSURE = 11, /* PID 0x33 */
+  CONTROL_MODULE_VOLTAGE = 12, /* PID 0x42 */
+  AMBIENT_AIR_TEMPERATURE = 13, /* PID 0x46 */
+  TIME_WITH_MALFUNCTION_LIGHT_ON = 14, /* PID 0x4D */
+  TIME_SINCE_TROUBLE_CODES_CLEARED = 15, /* PID 0x4E */
+  MAX_FUEL_AIR_EQUIVALENCE_RATIO = 16, /* PID 0x4F */
+  MAX_OXYGEN_SENSOR_VOLTAGE = 17, /* PID 0x4F */
+  MAX_OXYGEN_SENSOR_CURRENT = 18, /* PID 0x4F */
+  MAX_INTAKE_MANIFOLD_ABSOLUTE_PRESSURE = 19, /* PID 0x4F */
+  MAX_AIR_FLOW_RATE_FROM_MASS_AIR_FLOW_SENSOR = 20, /* PID 0x50 */
 
   /** refer to FuelType for a description of this value. */
-  FUEL_TYPE = 21,
-  FUEL_RAIL_ABSOLUTE_PRESSURE = 22,
-  ENGINE_OIL_TEMPERATURE = 23,
-  DRIVER_DEMAND_PERCENT_TORQUE = 24,
-  ENGINE_ACTUAL_PERCENT_TORQUE = 25,
-  ENGINE_REFERENCE_PERCENT_TORQUE = 26,
-  ENGINE_PERCENT_TORQUE_DATA_IDLE = 27,
-  ENGINE_PERCENT_TORQUE_DATA_POINT1 = 28,
-  ENGINE_PERCENT_TORQUE_DATA_POINT2 = 29,
-  ENGINE_PERCENT_TORQUE_DATA_POINT3 = 30,
-  ENGINE_PERCENT_TORQUE_DATA_POINT4 = 31,
+  FUEL_TYPE = 21, /* PID 0x51 */
+  FUEL_RAIL_ABSOLUTE_PRESSURE = 22, /* PID 0x59 */
+  ENGINE_OIL_TEMPERATURE = 23, /* PID 0x5C */
+  DRIVER_DEMAND_PERCENT_TORQUE = 24, /* PID 0x61 */
+  ENGINE_ACTUAL_PERCENT_TORQUE = 25, /* PID 0x62 */
+  ENGINE_REFERENCE_PERCENT_TORQUE = 26, /* PID 0x63 */
+  ENGINE_PERCENT_TORQUE_DATA_IDLE = 27, /* PID 0x64 */
+  ENGINE_PERCENT_TORQUE_DATA_POINT1 = 28, /* PID 0x64 */
+  ENGINE_PERCENT_TORQUE_DATA_POINT2 = 29, /* PID 0x64 */
+  ENGINE_PERCENT_TORQUE_DATA_POINT3 = 30, /* PID 0x64 */
+  ENGINE_PERCENT_TORQUE_DATA_POINT4 = 31, /* PID 0x64 */
   LAST_SYSTEM_INDEX = ENGINE_PERCENT_TORQUE_DATA_POINT4,
 };
 
@@ -421,79 +441,81 @@
  * This enum provides the canonical mapping for sensor properties that have a floating-point value.
  * The ordering of the values is taken from the OBD2 specification.
  * Any value greater than the last reserved index is available to vendors to map their extensions.
+ * While these values do not directly map to SAE J1979 PIDs, an equivalence is listed next
+ * to each one to aid implementors.
  */
 enum DiagnosticFloatSensorIndex : int32_t {
-  CALCULATED_ENGINE_LOAD = 0,
-  ENGINE_COOLANT_TEMPERATURE = 1,
-  SHORT_TERM_FUEL_TRIM_BANK1 = 2,
-  LONG_TERM_FUEL_TRIM_BANK1 = 3,
-  SHORT_TERM_FUEL_TRIM_BANK2 = 4,
-  LONG_TERM_FUEL_TRIM_BANK2 = 5,
-  FUEL_PRESSURE = 6,
-  INTAKE_MANIFOLD_ABSOLUTE_PRESSURE = 7,
-  ENGINE_RPM = 8,
-  VEHICLE_SPEED = 9,
-  TIMING_ADVANCE = 10,
-  MAF_AIR_FLOW_RATE = 11,
-  THROTTLE_POSITION = 12,
-  OXYGEN_SENSOR1_VOLTAGE = 13,
-  OXYGEN_SENSOR1_SHORT_TERM_FUEL_TRIM = 14,
-  OXYGEN_SENSOR1_FUEL_AIR_EQUIVALENCE_RATIO = 15,
-  OXYGEN_SENSOR2_VOLTAGE = 16,
-  OXYGEN_SENSOR2_SHORT_TERM_FUEL_TRIM = 17,
-  OXYGEN_SENSOR2_FUEL_AIR_EQUIVALENCE_RATIO = 18,
-  OXYGEN_SENSOR3_VOLTAGE = 19,
-  OXYGEN_SENSOR3_SHORT_TERM_FUEL_TRIM = 20,
-  OXYGEN_SENSOR3_FUEL_AIR_EQUIVALENCE_RATIO = 21,
-  OXYGEN_SENSOR4_VOLTAGE = 22,
-  OXYGEN_SENSOR4_SHORT_TERM_FUEL_TRIM = 23,
-  OXYGEN_SENSOR4_FUEL_AIR_EQUIVALENCE_RATIO = 24,
-  OXYGEN_SENSOR5_VOLTAGE = 25,
-  OXYGEN_SENSOR5_SHORT_TERM_FUEL_TRIM = 26,
-  OXYGEN_SENSOR5_FUEL_AIR_EQUIVALENCE_RATIO = 27,
-  OXYGEN_SENSOR6_VOLTAGE = 28,
-  OXYGEN_SENSOR6_SHORT_TERM_FUEL_TRIM = 29,
-  OXYGEN_SENSOR6_FUEL_AIR_EQUIVALENCE_RATIO = 30,
-  OXYGEN_SENSOR7_VOLTAGE = 31,
-  OXYGEN_SENSOR7_SHORT_TERM_FUEL_TRIM = 32,
-  OXYGEN_SENSOR7_FUEL_AIR_EQUIVALENCE_RATIO = 33,
-  OXYGEN_SENSOR8_VOLTAGE = 34,
-  OXYGEN_SENSOR8_SHORT_TERM_FUEL_TRIM = 35,
-  OXYGEN_SENSOR8_FUEL_AIR_EQUIVALENCE_RATIO = 36,
-  FUEL_RAIL_PRESSURE = 37,
-  FUEL_RAIL_GAUGE_PRESSURE = 38,
-  COMMANDED_EXHAUST_GAS_RECIRCULATION = 39,
-  EXHAUST_GAS_RECIRCULATION_ERROR = 40,
-  COMMANDED_EVAPORATIVE_PURGE = 41,
-  FUEL_TANK_LEVEL_INPUT = 42,
-  EVAPORATION_SYSTEM_VAPOR_PRESSURE = 43,
-  CATALYST_TEMPERATURE_BANK1_SENSOR1 = 44,
-  CATALYST_TEMPERATURE_BANK2_SENSOR1 = 45,
-  CATALYST_TEMPERATURE_BANK1_SENSOR2 = 46,
-  CATALYST_TEMPERATURE_BANK2_SENSOR2 = 47,
-  ABSOLUTE_LOAD_VALUE = 48,
-  FUEL_AIR_COMMANDED_EQUIVALENCE_RATIO = 49,
-  RELATIVE_THROTTLE_POSITION = 50,
-  ABSOLUTE_THROTTLE_POSITION_B = 51,
-  ABSOLUTE_THROTTLE_POSITION_C = 52,
-  ACCELERATOR_PEDAL_POSITION_D = 53,
-  ACCELERATOR_PEDAL_POSITION_E = 54,
-  ACCELERATOR_PEDAL_POSITION_F = 55,
-  COMMANDED_THROTTLE_ACTUATOR = 56,
-  ETHANOL_FUEL_PERCENTAGE = 57,
-  ABSOLUTE_EVAPORATION_SYSTEM_VAPOR_PRESSURE = 58,
-  SHORT_TERM_SECONDARY_OXYGEN_SENSOR_TRIM_BANK1 = 59,
-  SHORT_TERM_SECONDARY_OXYGEN_SENSOR_TRIM_BANK2 = 60,
-  SHORT_TERM_SECONDARY_OXYGEN_SENSOR_TRIM_BANK3 = 61,
-  SHORT_TERM_SECONDARY_OXYGEN_SENSOR_TRIM_BANK4 = 62,
-  LONG_TERM_SECONDARY_OXYGEN_SENSOR_TRIM_BANK1 = 63,
-  LONG_TERM_SECONDARY_OXYGEN_SENSOR_TRIM_BANK2 = 64,
-  LONG_TERM_SECONDARY_OXYGEN_SENSOR_TRIM_BANK3 = 65,
-  LONG_TERM_SECONDARY_OXYGEN_SENSOR_TRIM_BANK4 = 66,
-  RELATIVE_ACCELERATOR_PEDAL_POSITION = 67,
-  HYBRID_BATTERY_PACK_REMAINING_LIFE = 68,
-  FUEL_INJECTION_TIMING = 69,
-  ENGINE_FUEL_RATE = 70,
+  CALCULATED_ENGINE_LOAD = 0, /* PID 0x04 */
+  ENGINE_COOLANT_TEMPERATURE = 1, /* PID 0x05 */
+  SHORT_TERM_FUEL_TRIM_BANK1 = 2, /* PID 0x06 */
+  LONG_TERM_FUEL_TRIM_BANK1 = 3, /* PID 0x07 */
+  SHORT_TERM_FUEL_TRIM_BANK2 = 4, /* PID 0x08 */
+  LONG_TERM_FUEL_TRIM_BANK2 = 5, /* PID 0x09 */
+  FUEL_PRESSURE = 6, /* PID 0x0A */
+  INTAKE_MANIFOLD_ABSOLUTE_PRESSURE = 7, /* PID 0x0B */
+  ENGINE_RPM = 8, /* PID 0x0C */
+  VEHICLE_SPEED = 9, /* PID 0x0D */
+  TIMING_ADVANCE = 10, /* PID 0x0E */
+  MAF_AIR_FLOW_RATE = 11, /* PID 0x10 */
+  THROTTLE_POSITION = 12, /* PID 0x11 */
+  OXYGEN_SENSOR1_VOLTAGE = 13, /* PID 0x14 */
+  OXYGEN_SENSOR1_SHORT_TERM_FUEL_TRIM = 14, /* PID 0x14 */
+  OXYGEN_SENSOR1_FUEL_AIR_EQUIVALENCE_RATIO = 15, /* PID 0x24 */
+  OXYGEN_SENSOR2_VOLTAGE = 16, /* PID 0x15 */
+  OXYGEN_SENSOR2_SHORT_TERM_FUEL_TRIM = 17, /* PID 0x15 */
+  OXYGEN_SENSOR2_FUEL_AIR_EQUIVALENCE_RATIO = 18, /* PID 0x25 */
+  OXYGEN_SENSOR3_VOLTAGE = 19, /* PID 0x16 */
+  OXYGEN_SENSOR3_SHORT_TERM_FUEL_TRIM = 20, /* PID 0x16 */
+  OXYGEN_SENSOR3_FUEL_AIR_EQUIVALENCE_RATIO = 21, /* PID 0x26 */
+  OXYGEN_SENSOR4_VOLTAGE = 22, /* PID 0x17 */
+  OXYGEN_SENSOR4_SHORT_TERM_FUEL_TRIM = 23, /* PID 0x17 */
+  OXYGEN_SENSOR4_FUEL_AIR_EQUIVALENCE_RATIO = 24, /* PID 0x27 */
+  OXYGEN_SENSOR5_VOLTAGE = 25, /* PID 0x18 */
+  OXYGEN_SENSOR5_SHORT_TERM_FUEL_TRIM = 26, /* PID 0x18 */
+  OXYGEN_SENSOR5_FUEL_AIR_EQUIVALENCE_RATIO = 27, /* PID 0x28 */
+  OXYGEN_SENSOR6_VOLTAGE = 28, /* PID 0x19 */
+  OXYGEN_SENSOR6_SHORT_TERM_FUEL_TRIM = 29, /* PID 0x19 */
+  OXYGEN_SENSOR6_FUEL_AIR_EQUIVALENCE_RATIO = 30, /* PID 0x29 */
+  OXYGEN_SENSOR7_VOLTAGE = 31, /* PID 0x1A */
+  OXYGEN_SENSOR7_SHORT_TERM_FUEL_TRIM = 32, /* PID 0x1A */
+  OXYGEN_SENSOR7_FUEL_AIR_EQUIVALENCE_RATIO = 33, /* PID 0x2A */
+  OXYGEN_SENSOR8_VOLTAGE = 34, /* PID 0x1B */
+  OXYGEN_SENSOR8_SHORT_TERM_FUEL_TRIM = 35, /* PID 0x1B */
+  OXYGEN_SENSOR8_FUEL_AIR_EQUIVALENCE_RATIO = 36, /* PID 0x2B */
+  FUEL_RAIL_PRESSURE = 37, /* PID 0x22 */
+  FUEL_RAIL_GAUGE_PRESSURE = 38, /* PID 0x23 */
+  COMMANDED_EXHAUST_GAS_RECIRCULATION = 39, /* PID 0x2C */
+  EXHAUST_GAS_RECIRCULATION_ERROR = 40, /* PID 0x2D */
+  COMMANDED_EVAPORATIVE_PURGE = 41, /* PID 0x2E */
+  FUEL_TANK_LEVEL_INPUT = 42, /* PID 0x2F */
+  EVAPORATION_SYSTEM_VAPOR_PRESSURE = 43, /* PID 0x32 */
+  CATALYST_TEMPERATURE_BANK1_SENSOR1 = 44, /* PID 0x3C */
+  CATALYST_TEMPERATURE_BANK2_SENSOR1 = 45, /* PID 0x3D */
+  CATALYST_TEMPERATURE_BANK1_SENSOR2 = 46, /* PID 0x3E */
+  CATALYST_TEMPERATURE_BANK2_SENSOR2 = 47, /* PID 0x3F */
+  ABSOLUTE_LOAD_VALUE = 48, /* PID 0x43 */
+  FUEL_AIR_COMMANDED_EQUIVALENCE_RATIO = 49, /* PID 0x44 */
+  RELATIVE_THROTTLE_POSITION = 50, /* PID 0x45 */
+  ABSOLUTE_THROTTLE_POSITION_B = 51, /* PID 0x47 */
+  ABSOLUTE_THROTTLE_POSITION_C = 52, /* PID 0x48 */
+  ACCELERATOR_PEDAL_POSITION_D = 53, /* PID 0x49 */
+  ACCELERATOR_PEDAL_POSITION_E = 54, /* PID 0x4A */
+  ACCELERATOR_PEDAL_POSITION_F = 55, /* PID 0x4B */
+  COMMANDED_THROTTLE_ACTUATOR = 56, /* PID 0x4C */
+  ETHANOL_FUEL_PERCENTAGE = 57,/* PID 0x52 */
+  ABSOLUTE_EVAPORATION_SYSTEM_VAPOR_PRESSURE = 58, /* PID 0x53 */
+  SHORT_TERM_SECONDARY_OXYGEN_SENSOR_TRIM_BANK1 = 59, /* PID 0x55 */
+  SHORT_TERM_SECONDARY_OXYGEN_SENSOR_TRIM_BANK2 = 60, /* PID 0x57 */
+  SHORT_TERM_SECONDARY_OXYGEN_SENSOR_TRIM_BANK3 = 61,/* PID 0x55 */
+  SHORT_TERM_SECONDARY_OXYGEN_SENSOR_TRIM_BANK4 = 62, /* PID 0x57 */
+  LONG_TERM_SECONDARY_OXYGEN_SENSOR_TRIM_BANK1 = 63, /* PID 0x56 */
+  LONG_TERM_SECONDARY_OXYGEN_SENSOR_TRIM_BANK2 = 64, /* PID 0x58 */
+  LONG_TERM_SECONDARY_OXYGEN_SENSOR_TRIM_BANK3 = 65, /* PID 0x56 */
+  LONG_TERM_SECONDARY_OXYGEN_SENSOR_TRIM_BANK4 = 66, /* PID 0x58 */
+  RELATIVE_ACCELERATOR_PEDAL_POSITION = 67, /* PID 0x5A */
+  HYBRID_BATTERY_PACK_REMAINING_LIFE = 68, /* PID 0x5B */
+  FUEL_INJECTION_TIMING = 69, /* PID 0x5D */
+  ENGINE_FUEL_RATE = 70, /* PID 0x5E */
   LAST_SYSTEM_INDEX = ENGINE_FUEL_RATE,
 };