How Trouble Codes Relate to Readiness Monitors

A scan tool may show diagnostic trouble codes and readiness monitor statuses on separate screens. They answer different questions. A trouble code reports a diagnostic condition the vehicle has detected; readiness indicates whether particular onboard checks have completed in the relevant period. You need both pieces of information to avoid treating an unfinished test as a proven broken part. 1 2

Stored or Confirmed Codes

Stored/confirmed codes are records the ECU reports under a particular diagnostic mode. Depending on the vehicle and fault conditions, some confirmed emission-related faults may command the malfunction indicator light (MIL). A reading with a confirmed problem deserves diagnostic attention before attempting to make monitors complete simply by driving.

Pending Codes

Pending codes can indicate that the ECU has detected a condition not yet confirmed by the relevant logic. Not entering any pending codes is not the same as checking that none exist. If your scanner cannot display pending codes or you did not check, the assistant will explicitly say that information is unknown. 3

Permanent Codes

Permanent DTCs may be retained differently from ordinary fault memory. In jurisdictions that evaluate permanent codes, a blank normal DTC screen or MIL-off status does not automatically establish compliance. The rules differ by program and vehicle; consult the current authority for inspections. 2 2

The Check Engine Light Is Another Data Point

Distinguish the ECU-commanded MIL reported by a tool from the light that you personally saw on the dashboard. They usually relate but are not interchangeable observations. A flashing Check Engine Light or severe drivability symptoms call for special caution and professional assessment; a web-based diagnostic wizard should not provide an adventurous drive-cycle procedure to 'force readiness'. 4

Why Erasing Codes Is Not the Solution

Clearing DTCs can erase useful evidence and reset readiness monitors. That can make an inspection less likely to be testable immediately after a reset and make it harder to diagnose the original problem. If an actual fault exists, removing the code display is not the same as repairing it. 1

Bring Better Information to a Technician

When you capture the exact code strings, whether they were confirmed or pending, commanded MIL state and monitor statuses, you can provide a far more useful report than 'my car won't pass'. The tool records what you observed and identifies what's missing; it never uses a generic code alone to guarantee which part should be replaced.

Record your scanner observations.