The rear HO2S(DOWN) is used to measure the oxygen content of the gases and installed downstream of the TWC. To measure the oxygen content, the HO2S requires a supply of ambient air as a reference.Since this is supplied through the wiring, the lead must not be clamped or damaged in any other way.The purposes are to 1) make the fuel trim function more accurate 2) monitor the front HO2S for ageing 3) monitor the operation of the TWC.The rear HO2S differs from the front unit in that it reacts somewhat more slowly and that the sensor element is slightly different in appearance.
ECM sets DTC P0137 if the ECM detects signal voltage lower than the possible range of a properly operating rear heated oxygen sensor (HO2S).
Item
Detecting Condition
Possible Cause
DTC Strategy
Signalcheck,Low
Contact resistance in connections.
Short to ground in signal circuit
Faulty HO2S(DOWN)
Faulty ECM
General Enable
Conditions
After enough heated
Battery voltage > 10.7V
Catalyst temperature (model) = 250? ~800?
Case1
Enable
Engine speed : 1800~3400 rpm
Engine load : 22.5~65.3 %
No misfire and No purge valve error
Threshold Value
B1S2 sensor voltage < 0.04 V
Diagnostic Time
25 sec
Case2
Sensor in cold condition
0.1 sec
MIL ON Condition
3 driving cycle
Output voltage
Heater
Approx. 0.5V & Idle
9.0 ? (20?)
The amplitude of the signal output of the rear HO2S is small compared to the front HO2S because the rear HO2S detects emission gas purified by the catalytic converter. This is the normal signal waveform of the rear HO2S at idle.