An OBDII system doesn’t run its tests on demand. It monitors emissions and other systems continuously, performing specific tests under certain driving conditions known as “drive cycles.” If your OBDII system shows “Not Ready,” it means one or more of these tests haven’t completed successfully. This article explains why this happens and how to address it.
Understanding OBDII Readiness Monitors
“Not Ready” status indicates incomplete diagnostic tests. Common causes include:
- Recent Battery Disconnection/Replacement: Disconnecting or replacing the battery resets the OBDII system, requiring it to rerun its tests.
- Low Battery Charge: A low battery can disrupt the OBDII system and prevent tests from completing.
- Cleared Diagnostic Trouble Codes (DTCs): Using a scan tool to clear DTCs also resets the system and its readiness monitors.
- Required Software Update: Outdated OBDII software may prevent proper testing.
- Pending Emission System Issues: A developing problem might not yet trigger the “Check Engine” light but can prevent readiness.
- Improperly Installed Aftermarket Systems: Aftermarket installations (e.g., car stereos, alarms) can interfere with the OBDII system.
How to Set OBDII Readiness Monitors to “Ready”
To enable the OBDII system to complete its tests, follow these steps:
- Fuel Level: Ensure your fuel tank is between 25% and 75% full.
- Cold Start: Let the vehicle sit for at least eight hours without running, allowing the engine to cool down to ambient temperature. This ensures a “cold start” condition.
- Drive Cycle: After the cold start, drive the vehicle under normal conditions for one to two weeks, including some highway driving. This allows the system to run through its various tests.
Troubleshooting Persistent “Not Ready” Status
If the monitors remain “Not Ready” after following the above steps, check these components:
- Aftermarket System Wiring: Inspect wiring for interference with the OBDII system or battery.
- Fuses: Verify all fuses related to the OBDII system are functioning.
- Thermostat/Coolant Temperature: Ensure the engine is reaching and maintaining proper operating temperature.
- Recalls/TSBs/Extended Warranties: Check for outstanding issues related to the emissions system or OBDII computer.
- Professional Diagnostic Scan: Have a mechanic perform a diagnostic analysis using a generic OBDII scan tool connected to the data link connector (DLC) under the dash.
Important Considerations
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Pending Codes: Check for pending DTCs using a scan tool.
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Drive Cycle Information: Consult your vehicle’s owner’s manual or manufacturer for specific drive cycle instructions.
A properly functioning OBDII system is crucial for passing emissions tests. While you can’t force the system to run its tests immediately, understanding how it works and following the recommended procedures will help ensure it completes its diagnostics successfully.
