DTC P0138: O2 Sensor No. 2 (Catalyst Monitor) Hi SGNL 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 converter promotes a chemical reaction which oxidizes HC and CO present in exhaust gas, thus converting them into harmless water vapor and carbon dioxide.

Catalyst also reduces NOx, converting it to nitrogen and free oxygen. PCM is capable of monitoring this process using sensor No. 2. Sensor No. 2, which is located in exhaust stream past three-way catalytic converter, produces an output signal which indicates oxygen storage capacity of catalyst. This in turn indicates catalyst's ability to convert exhaust emissions effectively.

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

• Throttle angle is 2-40 percent.
• Sensor No. 2 signal voltage is less than .9 volt.
• Conditions exist for more than 4 1/2 minutes.