The heated oxygen sensors are mounted on the front side and the rear sied of Catalytic Converter (warm-up catalytic converter), which detects the oxygen concentration in the exhaust gas. The front HO2S signal is used to control air/fuel ratio (closed loop fuel control) and the rear HO2S signal is used to monitor front HO2S and catalyst for proper operation. The heated oxygen snesor (HO2S) produces a voltage that varies between 0V and 1V. When the air/fuel ratio is lean, the oxygen concentration in the exhaust gas increases and the front HO2S outputs a low voltage (approximately 0 ~ 0.1 V).
When the air/fuel ratio is rich, the oxygen concentration in the exhaust gas decreases and the front HO2S output a high voltage (approximately 0.8 ~ 1 V). The ECM constantly monitors the HO2S and increases or decreases the fuel injection duration by using the HO2S signal, which is called closed-loop fuel control operation.
ECM assumes that O2 sensor heater coil is abnormal and shows P0135 when the corrected resistance value of upstream O2 sensor heater is higher than the specification.
Item
Detecting Condition
Possible Cause
DTC Strategy
Signal check
Poor connection
Open or Short to ground in power circuit
Open in control circuit
S1
Enable
Conditions
Exhaust Gas temperature > 450°C
Heating period > 50sec
Battery voltage : 10.7V ~ 16.1V
Threshold Value
Internal resistance > Threshold value (Calculated by exhaust gas temp. or heating power)
Diagnostic Time
60sec
MIL ON Condition
3 driving cycle
? S1 : upstream oxygen sensor / S2 : downstream oxygen sensor
The HO2S requires a minimum temperature to provide a closed loop fuel control system. So the HO2S contains a heater element to reduce its warm-up time and ensure its performance during all driving conditions. The HO2S heater is controlled ON after engine start except for Cold condition and high speed accelleration. The ECM controls this heater element by duty cycle. The main relay supplies voltage to the heater and the ECM provides a ground circuit for activating the heater.