Circuit Description

The DTC P0137 Heated Oxygen Sensor (HO2S) Circuit Low Voltage Sensor 2 diagnostic monitors the voltage on the signal circuit of the post catalyst oxygen sensor. In order to control emissions of hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), a 3-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting the HC and the CO into harmless water vapor and carbon dioxide. The catalyst also reduces NOx by converting the NOx to nitrogen. The powertrain control module (PCM) has the ability to monitor this process using the heated oxygen sensors HO2S 1 and HO2S 2. The HO2S 1 produces an output signal which indicates the amount of oxygen present in the exhaust gas entering the 3-way catalytic converter. The HO2S 2 produces an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalysts ability to convert exhaust gases efficiently. If the catalyst is operating efficiently, the HO2S 1 signal will be far more active than that produced by the HO2S 2.