5.5 OBD-II Readiness Monitors & Mode $06 Data

5.5 OBD-II Readiness Monitors & Mode $06 Data

OBD-II systems are not passive code recorders; they are active automated testing platforms. The ECM continuously runs internal self-diagnostic routines designated as Readiness Monitors to verify that all emission-related components and subsystems are fully functional. When advanced driveability issues occur without setting a hard Diagnostic Trouble Code, technicians utilize Mode $06 (On-Board Diagnostic Test Results for Specific Monitored Systems) to inspect raw component test values and pinpoint marginal failures before the MIL illuminates.

---

Continuous vs. Non-Continuous Readiness Monitors

OBD-II monitors are divided into two operational categories:

                       +---------------------------------------+
                       |       OBD-II READINESS MONITORS       |
                       +---------------------------------------+
                                           |
               +---------------------------+---------------------------+
               |                                                       |
               v                                                       v
  +--------------------------+                            +--------------------------+
  |   CONTINUOUS MONITORS    |                            | NON-CONTINUOUS MONITORS  |
  | (Run constantly whenever |                            | (Require strict enable   |
  |  engine is operating)    |                            |  criteria & drive cycle) |
  +--------------------------+                            +--------------------------+
  | 1. Misfire Monitor       |                            | 1. Catalyst Monitor      |
  | 2. Fuel System Monitor   |                            | 2. Heated O2/AFR Monitor |
  | 3. Comprehensive Comp.   |                            | 3. EVAP System Monitor   |
  +--------------------------+                            | 4. EGR / VVT Monitor     |
                                                          | 5. Secondary Air Monitor |
                                                          +--------------------------+

1. Continuous Monitors

These monitors run continuously during all engine operating conditions once the engine is running:

• Misfire Monitor: Uses the CKP sensor to track microsecond crankshaft acceleration pulses, detecting single or multi-cylinder misfires.
• Fuel System Monitor: Continuously monitors fuel trim values in Closed-Loop to detect extreme rich or lean calibration failure.
• Comprehensive Component Monitor (CCM): Continuously checks all major electronic inputs and outputs (sensors, solenoids, relays) for opens, shorts-to-ground, shorts-to-voltage, and out-of-range signal rationality.

2. Non-Continuous Monitors

These monitors run only once per qualifying drive cycle when specific engine operating parameters (Enabling Criteria) are satisfied:

• Catalyst Monitor (TWC)
• Heated Oxygen / AFR Sensor Monitor
• Evaporative Emissions (EVAP) Monitor
• Exhaust Gas Recirculation (EGR) / Variable Valve Timing (VVT) Monitor
• Secondary Air Injection Monitor

---

Enabling Criteria & Drive Cycle Execution

For a non-continuous monitor to run and flip its status flag from 'Not Ready' / 'Incomplete' to 'Ready' / 'Complete', the vehicle must be driven through a specific sequence known as an OBD-II Drive Cycle.

Universal Drive Cycle Enable Criteria Rules

If enabling criteria are violated, the ECM suspends the self-test until the next trip:

1. Cold Start Differential Requirement: Engine Coolant Temperature (ECT) and Intake Air Temperature (IAT) must be within 11°F (6°C) of each other at startup (verifying an overnight cold soak).
2. Fuel Level Window: Fuel tank level must be between 15% and 85%. (EVAP monitors abort if fuel level is <15% to prevent sloshing errors, or >85% due to lack of vapor expansion space).
3. Ambient Temperature Range: Ambient air temperature must be between 40°F and 95°F (4°C - 35°C).
4. Steady Cruise Phase: Sustained steady-throttle driving at 50-60 MPH for 5 to 10 minutes to run Catalyst and O2 sensor monitors.
5. Deceleration Phase: Coasting down from 55 MPH to 20 MPH without depressing the brake or clutch pedal to test EGR and EVAP vacuum draw.

---

Deep-Dive into Mode $06 Diagnostic Data

Standard scan tool OBD-II communications are divided into 10 diagnostic modes (SAE J1979 / ISO 15031-5):

Mode $06 Data Architecture

Mode $06 exposes internal ECM testing registers for non-continuous monitors. Each test displays:

• Monitor ID (MID): Identifies the system being tested (e.g., MID $21 = Catalyst Monitor Bank 1; MID $3C = EVAP 0.020 in. leak monitor; MID $A1 = misfire data aggregated across all cylinders).
• Test ID (TID): Identifies the specific test run inside that monitor (e.g., under the misfire MIDs, TID $0B = misfire average over the last completed drive cycles and TID $0C = misfire counts for the current drive cycle). Catalyst, EGR, and EVAP TIDs are largely manufacturer-defined, so look them up in service information rather than assuming a standard meaning.
• Component ID (CID): Identifies the individual component location.
• Measured Test Value: The actual numeric performance value recorded during the drive cycle test.
• Minimum / Maximum Limits: The calibrated pass/fail thresholds defined by the factory engineers.

Sample Mode $06 Scan Tool Display (CAN Format):
--------------------------------------------------------------
Monitor: MID $21 (Catalyst Monitor Bank 1)
Test:    Oxygen Storage Capacity (manufacturer-defined TID)
Value:   0.042  (Measured Ratio)
Min:     0.000
Max:     0.085  (Fail Threshold)
Status:  PASS   (Margin: 50% under max limit)
--------------------------------------------------------------
Monitor: MID $A4 (Misfire Cylinder 3 Data)
Test:    TID $0C (Misfire counts, current drive cycle)
Value:   42     (Measured Misfires)
Min:     0
Max:     50     (Fail Threshold - DTC P0303 sets at 51)
Status:  PASS   (WARNING: Borderline failure! Misfires occurring without setting DTC)
--------------------------------------------------------------

Advanced Mode $06 Diagnostic Applications

1. Pinpointing Intermittent Misfires: When an engine exhibits a light surge under load but HAS NOT set a P0300 code, check the Mode $06 misfire MIDs (MID $A1 aggregates every cylinder; MIDs $A2 through $AB report cylinders 1 through 10 individually). A cylinder logging 42 misfires against a fail limit of 50 pinpoints an isolated ignition coil or plug failure long before a DTC triggers.
2. Evaluating EVAP Small Leaks (P0442 / P0455): Mode $06 shows raw vacuum decay rates in Pascals or inches of water column. Comparing measured decay against maximum threshold confirms whether a minor seal leak has been resolved after installing a new gas cap.