The rear heated oxygen sensor is mounted on the rear side of the Catalytic Converter (warm-up catalytic converter) or in the rear exhaust pipe, which is able to detect catalyst efficiency. The rear heated oxygen sensor (HO2S) produces a voltage between 0V and 1V. This rear heated oxygen sensor is used to estimate the oxygen storage capability. If a catalyst has good conversion properties, the oxygen fluctuations are smoothed by the oxygen storage capacity of the catalyst. If the conversion provided by the catalyst is low due to aging, poisoning or misfiring, then the oxygen fluctuations are similar to signals from the front oxygen sensor.
Checking output signals from HO2S under detecting condition, if an output signal is above 1.3V for more than predeterminate time, ECM sets P0138. MIL(Malfunction Indication Lamp) turns on when the malfunction lasts till consecutive 2 driving cycle.
Item | Detecting Condition | Possible Cause |
DTC Strategy |
•
Monitor signal voltage | 1. Poor connection 2. Short to battery in harness 3. HO2S(B1/S2) 4. ECM |
Enable Conditions |
•
Battery voltage ≥ 10V
•
Engine running ≥ 60 sec
•
Engine warm-up state | |
Threshold value |
•
The voltage of HO2S(B1/S2) > 1.3V | |
Diagnosis Time |
•
Continous (more than 12.5 sec.failure for every 15 sec.test) | |
MIL On Condition |
•
2 Driving Cycles |
A/F Ratio (λ) | Output Voltage (V) |
RICH | Min. 0.8 |
LEAN | Max. 0.1 |
Fig.1) Normal graph of front HO2S and rear HO2S at idle.
Fig.2) Normal waveforms of front HO2S(the upper) and rear HO2S.(the lower) (chassis ground used)
Fig.3) Normal waveforms of front HO2S(the upper) and rear HO2S.(the lower) (sensor ground used)
After warming-up, if accelerator pedal is released suddenly around 4000rpm, the HO2S signal reading will be lower than 200mV resulting from Fuel cut-off for the moment. Conversely, if suddenly accelerator pedal is depressed, HO2S signal reading will be around 0.6V~1.0V. At idle, Normally HO2S signal will switch from lean to rich with 3 Hz. And as racing, Its frequency rises.
