A technician is performing a pre-purchase inspection on a used vehicle. The Check Engine Light (CEL) is off, but when checking emission monitor status with a scan tool, the technician finds that the 'Catalyst Monitor' and 'EVAP System Monitor' are 'Not Ready'. All other monitors are 'Complete'. The vehicle has recently had its battery replaced. What is the most likely reason for the 'Not Ready' status of these specific monitors?
- AThe vehicle's PCM is faulty.
- BThe vehicle has not completed the required drive cycle for these monitors.
- CThe oxygen (O2) sensors are faulty.
- DA pending DTC exists for these systems.
Show answer & explanationAnswer & explanation
Correct answer: B. The vehicle has not completed the required drive cycle for these monitors.
After a battery disconnect or DTC clearing, all OBD-II monitors reset to 'Not Ready'. The Catalyst and EVAP monitors are notoriously difficult to set and require specific driving conditions (drive cycles) to run and complete. The recent battery replacement explains why they are not ready, as the vehicle hasn't been driven enough or in the correct conditions since the reset.
Why the other options are wrong
- A. A faulty PCM would likely cause more widespread issues or multiple DTCs, not just two specific monitors being 'Not Ready' after a battery replacement.
- C. Faulty O2 sensors would likely cause other monitors to fail (e.g., fuel system monitor) or set specific O2 sensor DTCs, and wouldn't be the primary reason for a 'Not Ready' status on the Catalyst and EVAP monitors following a battery replacement.
- D. Pending DTCs would indicate a fault, but wouldn't prevent a monitor from running unless the fault itself was preventing it. 'Not Ready' simply means the test hasn't run yet.
OBD-II Monitor Readiness
OBD-II monitors self-test various emission-related systems. After a battery disconnect or DTC clear, monitors reset to 'Not Ready' and require specific driving conditions (a 'drive cycle') to run and report 'Complete'. Catalyst and EVAP monitors are often the last to set.
- Monitors reset after battery disconnect/DTC clear.
- Require specific drive cycles to set to 'Complete'.
- Catalyst and EVAP monitors are typically the most challenging to set.
Memory trick: Monitors aren't 'Ready' because the 'Drive Cycle' hasn't 'Finished' yet.