21 Upstream O2S Response - (P 0139)

21 DOWNSTREAM O2S RESPONSE - (P 0139)

Since the switch rate of the downstream sensor is extremely slow, the Powertrain Control Module (PCM) waits until it recognizes commands for wide open throttle (WOT) and/or deceleration from the Throttle Position Sensor (TPS). When the TPS sends a command to the PCM for WOT, the PCM goes into open loop. While in open loop, the injection pulse width increases to an air/fuel ratio less than stoichiometric. The resulting increase in fuel causes the downstream sensor to switch to a rich signal.

When the TPS sends a command to the PCM for deceleration, the PCM goes into a deceleration open loop and decreases the injection pulse width to an air/fuel ratio that is greater than stoichiometric. This results in a switch to lean signal from the downstream O2 sensor.

The Malfunction Indicator Lamp (MIL) illuminates and a DTC is stored when the PCM recognizes a slow response from the downstream 02 sensor switching rich when the PCM enriches the air/fuel ratio during WOT or lean when the PCM leans out the air/fuel ratio during deceleration.

The Downstream Slow Response malfunction could possibly be caused by problems with any of the following components:

- Downstream O2 sensor
- Downstream O2 sensor wiring or related connectors (open or shorted)
- Exhaust system (leak)
- PCM