Heated Oxygen Sensor (HO2S) Monitor Description
HEATED OXYGEN SENSOR (H025) MONITOR DESCRIPTIONThe Heated Oxygen Sensor (H02S) Monitor detects deterioration of oxygen sensor performance. The oxygen sensor creates a specific voltage (between zero and one volt) in accordance with the current Air/Fuel ratio by sensing the oxygen content in the exhaust. Lean of stoichiometric, the HO2S will generate a voltage between 0.00 and 0.45 volt. Rich of stoichiometric, the HO2S will generate a voltage between 0.45 and 1.00 volt. This function deteriorates as time and mileage increase. The HO2S Monitor determines there is a problem if sensor response is slow, absent or fixed. The Fuel System Monitor and the Misfire Monitor must pass before the HO2S Monitor will run.
^ The PCM monitors and compares both the front and rear oxygen sensors.The front oxygen sensor (upstream) is used for fuel injection control. The rear oxygen sensor (downstream) is used to monitor the condition of the Catalytic Converter.
^ The Heated Oxygen Sensor Monitor evaluates the upstream and downstream oxygen sensors for proper function. After both the Misfire and Fuel System Monitors have run and passed, the Heated Oxygen Sensor Monitor will run. This monitor checks the peak voltage and the frequency of voltage change. An oxygen sensor will fail this monitor if the voltage is above a programmed threshold (too high volts) or if the sensor is lazy with too slow a response frequency.
^ Oxygen sensors do not provide a usable signal until they reach 600 degrees F (316 degrees C). It is necessary to heat oxygen sensors so they can provide a reliable feedback signal as quickly as possible after a cold start.
HO2S Locations
HO2S Monitor DTCs
Signal circuit malfunction: P0131, P0136, P0151, P0156
Heater circuit malfunction: P0135, P0141, P0155, P0161
Slow response rate: P0133, P0153
Swapped sensor connectors: P1126, P1129
Sensor not switching: P1130, P1131, P1132, P1150, P1151, P1152
Sensor not switching, biased lean: P1137
Sensor not switching, biased rich: P1138
Downstream sensor not running in on-demand self test: P1127
HO2S Monitor Service Tips
^ P0133 / P0153: these DTCs are set by the OBDII Response Rate Monitor. This is a test that only runs at a steady state cruise between 30-60 m.p.h. (50-95 kph). You cannot diagnose these DTCs at idle or in the service bay.
^ HO2S Switching Rate: Do not attempt to compare HO2S switching rates from bank to bank or vehicle to vehicle. Because of the adaptive strategy of the PCM, the switching rates are constantly adapting to; customer driving habits, engine and component wear, and engine operating conditions.
^ When removing an HO2S: Always use a 22 mm wrench, when installing an HO2S always use a crows foot and torque wrench and tighten it to 30 +/- 4 lb.-ft. (41 +/- 5 Nm).