Heated Oxygen Sensor Monitor: Description

WARNING: This page does not describe the selected car, but rather 32 other vehicles, including the 2003 Mercury Sable, 2003 Mercury Mountaineer, 2003 Mercury Marauder, 2003 Mercury Grand Marquis, and 2003 Lincoln Town Car. However, it is still accessible from the selected car via links, so may be relevant.

Heated Oxygen Sensor (HO2S) monitor is an on-board strategy designed to monitor HO2S sensors for a malfunction or deterioration which can affect emissions. The fuel control or upstream HO2S is checked for proper output voltage and response rate (time it takes to switch from lean to rich or rich to lean). Downstream HO2S sensors used for catalyst monitor are also monitored for proper output voltage. Input is required from ECT or CHT, IAT, MAF and CKP sensors to activate HO2S monitor. Fuel system monitor and misfire detection monitor must also have completed successfully before HO2S monitor is enabled.

The HO2S sensor senses the oxygen content in the exhaust flow and outputs a voltage between zero and one 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 one 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.
Once the HO2S Monitor is enabled, the Stream 1 HO2S signal voltage amplitude and response frequency are checked. Excessive voltage is determined by comparing the HO2S signal voltage to a maximum calibratable threshold voltage. A fixed frequency closed loop fuel control routine is executed and the Stream 1 HO2S voltage amplitude and output response frequency are observed. A sample of the Stream 1 HO2S signal is evaluated to determine if the sensor is capable of switching or has a slow response rate. A HO2S heater circuit fault is determined by turning the heater on and off and looking for a corresponding change in the Output State Monitor (OSM) and by measuring the current going through the heater circuit. Since the 2002 Model Year, vehicles will monitor the HO2S signal for a high voltage, in excess of 1.5 volts.
MIL is activated after a fault is detected on 2 consecutive drive cycles.

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.