Heated Oxygen Sensor (HO2S) Monitor
HEATED OXYGEN SENSOR (HO2S) MONITORHeated Oxygen Sensor Monitor:
Heated Oxygen Sensor Monitor - PZEV (Partial Zero Emission Vehicle) Focus Only:
The HO2S Monitor is an on-board strategy designed to monitor the HO2S sensors for a malfunction or deterioration which can affect emissions. The fuel control or Stream 1 HO2S sensors are checked for proper output voltage and response rate (the time it takes to switch from lean to rich or rich to lean). Stream 2 HO2S sensors used for Catalyst Monitoring, and Stream 3 HO2S sensors used for FAOS (Fore-Aft Oxygen Sensor) control are also monitored for proper output voltage. Input is required from the ECT or CHT, IAT, MAF and CKP sensors to activate the HO2S Monitor. The Fuel System Monitor and Misfire Detection Monitor must also have completed successfully before the HO2S Monitor is enabled.
1. The HO2S sensor senses the oxygen content in the exhaust flow and outputs a voltage between zero and 1.0 volt. Lean of stoichiometric (air/fuel ratio of approximately 14.7:1), the HO2S will generate a voltage between zero and 0.45 volt. Rich of stoichiometric, the HO2S will generate a voltage between 0.45 and 1.0 volt. The HO2S Monitor evaluates the Stream 1 (Fuel Control) and Stream 2 (Catalyst Monitor) and the Stream 3 (FAOS Control) HO2Ss for proper function.
2. The time between HO2S switches is monitored after vehicle startup and during closed loop fuel conditions. Excessive time between switches or no switches since startup indicates a malfunction. Since lack of switching malfunctions can be caused by HO2S sensor malfunctions or by shifts in the fuel system, DTCs are stored that provide additional information for the lack of switching malfunction. Different DTCs indicate whether the sensor was always indicates lean/disconnected (P1131 or P2195, P1151 or P2197), or always indicates rich (P1132 or P2196, P1152 or P2198). 2004 MY vehicles will monitor the HO2S signal for high voltage, in excess of 1.1 volts and store a unique DTC. (P0132, P0152). An over voltage condition is caused by a HO2S heater or battery power short to the HO2S signal line.
A functional test of the rear HO2S sensors is done during normal vehicle operation. The peak rich and lean voltages are continuously monitored. Voltages that exceed the calibratable rich and lean thresholds indicate a functional sensor. If the voltages have not exceeded the thresholds after a long period of vehicle operation, the air/fuel ratio may be forced rich or lean in an attempt to get the rear sensor to switch. This situation normally occurs only with a green catalyst (less than 500 miles). If the sensor does not exceed the rich and lean peak thresholds, a malfunction is indicated. 2004 MY vehicles will monitor the rear HO2S signal for high voltage, in excess of 1.1 volts and store a unique DTC. (P0138, P0158). An over voltage condition is caused by a HO2S heater or battery power short to the HO2S signal line.
3. The MIL is activated after a fault is detected on two consecutive drive cycles.
4. Some 2004 Partial Zero Emission Vehicles (PZEV Focus) will utilize three sets of HO2S sensors. The front sensors (HO2S11/HO2S21) are the primary fuel control sensors. The next sensors downstream in the exhaust are utilized to monitor the light-off catalyst (HO2S12/HO2S22). The last sensors downstream in the exhaust (HO2S13/HO2S23) are utilized for very long term fuel trim in order to optimize catalyst efficiency (Fore Aft Oxygen Sensor Control). Ford's first PZEV vehicle uses a 4-cylinder engine so only the Bank 1 DTCs are utilized.
The HO2S Monitor DTCs can be categorized as follows:
- HO2S signal circuit malfunction - P0131, P0136, P0151, P0156.
- HO2S slow response rate - P0133, P0153.
- HO2S circuit high voltage - P0132, P0138, P0144, P0152, P0158, P0164.
- HO2S heater circuit malfunction - P0135, P0141, P0155, P0161, P0147, P0167.
- HO2S heater current malfunction - P0053, P0054, P0055, P0059, P0060, P0061.
- Downstream HO2S not running in on-demand self test - P1127.
- Swapped HO2S connectors - P0040, P0041, P1128, P1129, P2278.
- HO2S lack of switching - P1131, P1132, P1151, P1152, P2195, P2196, P2197, P2198.
- HO2S lack of switching (Sensor indicates lean) - P1137, P1157, P2270, P2272, P2274, P2276.
- HO2S lack of switching (Sensor indicates rich) - P1138, P1158, P2271, P2273, P2275, P2277.