OBD-II Mode 6 & Mode 8 Difference:

• Mode 6 → Best for diagnosing intermittent issues by reviewing stored test data.
• Mode 8 → Used for active testing and component control, mostly by professionals.

For accurate diagnostics, always refer to manufacturer-specific guidelines and use compatible scan tools. We have some scanner tools which built-in mode 6 & 8 on sale.

Mode 6 (On-Board Diagnostic Test Results – Non-Continuous Monitoring)

Definition

Mode 6 provides access to test results for non-continuous monitors—diagnostic checks that run only under specific conditions (e.g., catalytic converter efficiency, oxygen sensor response).

Purpose

• Stores test results (pass/fail) for components that are not monitored continuously.
• Provides detailed numerical data (e.g., sensor response times, efficiency thresholds).
• Helps diagnose intermittent faults or performance degradation.

Key Data Structure

• TID (Test Identifier) – Specifies the type of test (e.g., oxygen sensor response test).
• CID (Component Identifier) – Identifies the tested component (e.g., Bank 1 Sensor 1).
• Test Value – Raw data or pass/fail status (e.g., “0″ = pass, “1″ = fail).

Common Applications

✔ Checking catalytic converter efficiency (e.g., TID 0×03).
✔ Verifying oxygen sensor response time (e.g., TID 0×05).
✔ Detecting evaporative emissions system leaks (small leaks).

Example

A response like:

• TID = 0×03, CID = 0×12, Value = 120
  → Means an oxygen sensor response test result is 120 ms (must be compared to OEM specs).

Mode 8 (Control of On-Board System, Test or Component)

Definition

Mode 8 allows active control of vehicle subsystems—used to trigger tests, activate components, or adjust parameters.

Purpose

• Forces actuators (relays, valves, etc.) to operate for testing.
• Simulates conditions (e.g., EGR valve opening, fuel pump activation).
• Used in emissions-related maintenance (e.g., DPF regeneration in diesel vehicles).

Common Commands

• Turn ON/OFF cooling fans (to check electrical circuits).
• Activate fuel pump (for pressure testing).
• Cycle the EGR valve (to verify movement).
• Initiate DPF regeneration (in diesel engines).

Important Notes

⚠ Vehicle-specific – Not all cars support Mode 8, and commands vary by manufacturer.
⚠ Requires professional tools – Often needs OEM-level scanners (e.g., GM Tech2, Ford IDS).
⚠ Safety risk – Incorrect commands may damage components or affect drivability.

Mode 6 vs. Mode 8: Key Differences