PZEV Model
Advanced DiagnosticsDTC P2251: Rear Air/Fuel Ratio (A/F) Sensor (Bank 1, Sensor 1) VS Circuit High Voltage (PZEV Model)
General Description
The rear air/fuel ratio (A/F) sensor (bank 1, sensor 1) is installed in the exhaust manifold and detects oxygen content in the exhaust gas. The rear A/F sensor transmits a signal to the powertrain control module (PCM). The PCM controls fuel injection duration by comparing the target air/fuel ratio with the rear A/F sensor signal. The sensor includes the VS cell, the pump cell, the atmospheric reference cavity, the diffusion layer, and the heater, and it enables overall feedback control. When the VSB1 terminal voltage and the PCM internal signal voltage are more than a specified value for more than a predetermined time, the PCM detects a malfunction and a DTC is stored.
Monitor Execution, Sequence, Duration, DTC Type, OBD Status
Enable Conditions
Malfunction Threshold
After the sensor becomes active, the VSB1 terminal voltage is 6.0 V or more for at least 5 seconds.
Before the sensor becomes active, the VSB1 terminal voltage is 5.3 V or more for at least 38 seconds.
Driving Pattern
Start the engine. Hold the engine speed at 3,000 rpm without load (in P or N) until the radiator fan comes on, then let it idle for 2 minutes.
Diagnosis Details
Conditions for illuminating the MIL
When a malfunction is detected during the first drive cycle, a Temporary DTC is stored in the PCM memory. If the malfunction recurs during the next (second) drive cycle, the MIL comes on and the DTC and the freeze frame data are stored.
Conditions for clearing the MIL
The MIL will be cleared if the malfunction does not recur during three consecutive drive cycles in which the engine conditions are similar to the first time the malfunction was detected. The MIL, the DTC, the Temporary DTC, and the freeze frame data can be cleared by using the scan tool Clear command or by disconnecting the battery.