Verifying A Repair (Drive Cycle)

The purpose of the OBD II Drive Cycle is to run all of the on-board diagnostic monitors. Some Diagnostic Trouble Codes (DTCs) will be set immediately when the Powertrain Control Module (PCM) recognizes a fault, but others are only detected by the diagnostic monitors. The diagnostic monitor must observe the system it is watching under certain road and operating conditions before it can decide if that system is operating properly.

When a DTC is cleared, all of the diagnostic monitors will be reset. When verifying a repair to a vehicle after a monitored DTC is cleared, it will be necessary to complete part or all of the vehicle drive cycle, and it may be necessary to complete two trips. Until the appropriate part of the drive cycle has been completed, the PCM will not reset a DTC that is detectable by a monitor if a fault still exists.

When verifying a repair requiring part or all of the drive cycle, it will always be necessary that Mode 1 is complete. If the battery or PCM was never disconnected, it is probable that Mode 1 will be completed, because the learned air/fuel ratios will still be in memory. Modes 2 and 3 will be performed depending on-what fault was repaired. For Exhaust gas Recirculation (EGR) or fuel system repair verification, Mode 2 will be performed. For catalyst or Oxygen Sensor (O2S) system repair verification, Mode 3 will be performed.

FAILURE TO COMPLETE MODE 1

NOTE: Whenever Mode 1 is attempted, check for the availability of a Mode 1 Parameter Identification (PID). This way you can assure that Mode 1 was successfully completed before attempting Modes 2 or 3. This will also potentially save much time should Mode 1 fail to complete.

Faults that affect air/fuel ratio such as vacuum leaks or faulty injectors will stop Mode 1 from passing. When Mode 1 of the Drive Cycle is not complete, Modes 2 and 3 will not run. Therefore, if Modes 2 and/or 3 are performed and no monitors are completed, it is probable that Mode 1 was not completed. If only Mode 2 (or 3) was performed and failed to complete, then Mode 3 (or 2) should also be performed to see if a Mode 1 fault is probable. Inspect the vehicle PID list and check the status of the Mode 1 PID (if available) to see if Mode 1 has been completed. If the Mode 1 PID is not available and it is suspected that Mode 1 cannot be completed, the following items could be at fault:

- A diagnostic trouble code is set

- Air/vacuum leaks

- Faulty fuel injector(s)

- Pressure regulator malfunction

- Clogged air filter/air intake system leak

- EVAP canister purge system malfunction

Short Term Fuel PID:

Any fault that affects air/fuel ratio can prevent Mode 1 from completion. Using the New Generation STAR (NGS), it is possible to determine if there is an imbalance by observing the short term fuel trim PID. The short term fuel trim PID should read O% +/- 15% if the system is operating as designed. Always check for DTCs first, and observe the short term fuel trim PID with the engine at operating temperature and the engine speed at idle. For vehicles with bank 1 and bank 2 short term fuel trim PIDs, the readings should both be close to 0% +/- 15%.

For more information on diagnosing a vehicle for a Mode 1 concern, refer to Pinpoint Test QC. QC - OBD II Monitor Testing Not Complete (DTC P1000)