Heated Oxygen Sensor (HO2S) Monitor

Heated Oxygen Sensor (HO2S) Monitor

The HO2S monitor is an on-board strategy designed to monitor the HO2Ss for a malfunction or deterioration which can affect emissions. The fuel control or stream 1 HO2Ss 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 HO2Ss are used for catalyst monitoring and stream 3 HO2Ss used for fore-aft oxygen sensor (FAOS) control are also monitored for proper output voltage. A stream 3 H025 is only available on the Focus PZEV. Input is required from the engine coolant temperature (ECT) or cylinder head temperature (CHT) intake air temperature (IAT) mass air flow (MAF) sensors and crankshaft position (CKP) sensors to activate the H025 monitor. The fuel system monitor and misfire detection monitor must also have completed successfully before the H025 monitor is enabled.

1. The H025 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 H025 generates a voltage between zero and 0.45 volt. Rich of stoichiometric the H025 generates a voltage between 0.45 and 1.0 volt. If the reference chamber in the H025 is contaminated or the oxygen in the chamber is depleted~ the sensor undergoes a characteristic shift downward and begins to switch between -1 and 0 volt. When the sensor is in characteristic shift downward software strategy compensates for the negative signal and the vehicle remains in closed loop fuel control. The H025 monitor evaluates the stream 1 (fuel control) stream 2 (catalyst monitor) and stream 3 (FAOS control) H025 for proper function.

2. The time between H025 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 H025 malfunctions or by shifts in the fuel system DTCs are stored that provide additional information for the lack of switching malfunction. Prior to setting lack of switching DTCs stream 1 HO2Ss are checked for a characteristic shift downward concern. A unique fuel control routine is entered into in an attempt to detect characteristic shift downward. When a sensor is indicating lean the fuel system normally compensates by adding fuel. However to detect the characteristic shifi downward the fuel system is instead commanded lean. If characteristic shift downward is present the sofiware strategy compensates for the negative signal and the vehicle remains in closed loop fuel control. When a sensor is indicating rich the fuel system normally compensates by decreasing fuel. However if the sensor was previously identified as having characteristic shift downward the fuel system is instead commanded rich.

Different DTCs indicate whether the sensor always indicates lean/disconnected (P1131 or P2195 P1151 or P2197) or always indicates rich (P1132 or P2196 P1152 or P2198). 2005 MY vehicles will monitor the HO2S signal for high voltage in excess of 1.1 volts and store a unique DTC. (P0132 P0152). An overvoltage condition is caused by a HO2S heater or battery power short to the HO2S signal line.

A functional test of the rear HO2Ss (Stream 2 or Stream 3 [Focus PZEV]) is done during normal vehicle operation. The peak rich and lean voltages are continuously monitored. Voltages that exceed the calibrated rich and lean thresholds indicate a functional sensor. If the voltages have not exceeded the thresholds afier 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. 2005 MY vehicles will monitor the rear HO2S signal for high voltage in excess of 1.1 volts and store a unique DTC. (P0138 P0144 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 afier a fault is detected on 2 consecutive drive cycles.

4. Some 2005 partial zero emission vehicles (PZEV) will use 3 HO2Ss. The front sensor (H02511) is the primary fuel control sensor. The next sensor downstream in the exhaust is used to monitor the light-off catalyst (H02512). The last sensor downstream in the exhaust (H02513) is used for very long term fuel trim in order to optimize catalyst efficiency (FAOS Control). The current PZEV vehicle uses a 4-cylinder engine so only the Bank 1 DTCs are used.

The H025 Monitor DTCs can be categorized as follows:

^ H025 signal circuit malfunction - P0131 P0136 P0151 P0156.
^ H025 slow response rate - P0133 P0153.
^ H025 circuit high voltage - P0132 P0138 P0144 P0152 P0158.
^ H025 heater circuit malfunction - P0135 P0141 P0155 P0161 P0147.
^ H025 heater current malfunction - P0053 P0054 P0055 P0059 P0060.
^ Downstream H025 not running in on-demand self-test - P1127.
^ Swapped H025 connectors - P0040 P0041 P1128 P1129.
^ HO2S lack of switching - P1131, P1132, P1151, P1152, P2195, P2196, P2197, P2198.
^ H025 lack of switching (sensor indicates lean) - P1137 P1157 P2270 P2272 P2274.
^ H025 lack of switching (sensor indicates rich) - P1138 P1158 P2271 P2273 P2275.