Code P0138, O2 Sens No. 2 (Catalyst Monitor) High Sig VLTGE: Notes

A three-way catalytic converter is used to maintain reasonably low emissions of hydrocarbons (HC), carbon monoxide (CO) and oxides of nitrogen (NOx). The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, thus converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen and free oxygen.

The PCM is capable of monitoring this process using HO2S No. 2. HO2S No. 2, which is located in the exhaust stream past the three-way catalytic converter, produces an output signal which indicates the oxygen storage capacity of the catalyst. This in turn indicates the catalyst's ability to convert exhaust emissions effectively.

HO2S No. 2 signal will be far less active than that produced by HO2S No. 1 if the catalyst is properly functioning. PCM will set Code P0138 if a problem exists which causes the PCM to detect an excessively high HO2S No. 2 signal voltage for an extended period of time. Code P0138 will set when the following conditions are present.

• Throttle angle is between 2-40 percent.
• HO2S No. 2 signal voltage is less than .9 volt.
• Above conditions for more than 4 1/2 minutes.

Code P0138 will not illuminate Malfunction Indicator Light (MIL).