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OBD Readiness Monitors

Readiness monitors, in plain English

Monitors are self-tests your truck's OBD system runs in the background. CARB Clean Truck Check uses them to confirm your emissions controls are actually working.

The monitors that matter most

  • Fuel system monitor — verifies fuel delivery is in range.
  • Comprehensive components (CCM) — continuous self-test of inputs/outputs.
  • EGR / VVT monitor — verifies the EGR system flows correctly.
  • NOx / SCR catalyst monitors — verifies aftertreatment efficiency.
  • DPF / PM filter monitor — verifies particulate filter performance.
  • Boost pressure monitor — verifies turbo and charge-air system.

“Not ready” vs. “failed”

Not ready means the monitor hasn’t had the chance to run yet. It’s not a failure — it just means there isn’t enough data. Complete a drive cycle and re-scan.

Failed means the monitor ran and the result was outside spec. That typically sets a DTC and triggers a check engine light. A test will not pass until the underlying fault is repaired.

How many monitors can be incomplete?

For CARB Clean Truck Check, most heavy-duty diesels are allowed a very limited number of incomplete monitors at submission. The exact threshold depends on engine family and the OBD specification the vehicle is certified to. Always check the latest CARB guidance, or ask your Certified Tester before your appointment.

Quick troubleshooting checklist

  • Read DTCs first. Any active DTC will block monitor completion.
  • Confirm no recent battery disconnects or DTC clears.
  • Ensure DEF tank is full and SCR has no warnings.
  • Allow active regen events to complete uninterrupted.
  • Re-run the drive cycle and re-scan with a quality OBD tool.
Not a substitute for CARB guidance. CTC Connect publishes plain-English explanations of publicly available compliance information. Always verify rules with the official CARB Clean Truck Check program. Need a Certified Tester? Search the directory.