WASHER 8F6820 - Caterpillar

Use this procedure to troubleshoot the electrical system if a problem is suspected with the oxygen sensors. Also, use this procedure if one of the diagnostic codes in Table 1 is active or easily repeated.
Table 1
Diagnostic Codes Table    
Description    Conditions which Generate this Code    System Response    
3217-3 Aftertreatment Intake %O2 : Voltage Above Normal     The Electronic Control Module (ECM) may have detected a short circuit to the +Battery. The ECM may have detected an open circuit in the signal circuit for the oxygen sensor at the turbocharger exhaust outlet.     The code is logged.
The amber warning lamp is illuminated.
The ECM uses the last good value from the oxygen sensor for the control of exhaust emissions.    
3227-3 Aftertreatment Outlet %O2 : Voltage Above Normal     The ECM may have detected a short circuit to the +Battery. The ECM may have detected an open circuit in the signal circuit for the oxygen sensor at the outlet of the catalytic converter.    
Illustration 1 g02037921
Schematic of the circuit for the oxygen sensor
Illustration 2 g02036913
Oxygen sensor and buffer (1) Oxygen sensor (2) Sensor bufferThe oxygen sensor is used to monitor the emission level of the engine exhaust. The ECM utilizes the signal from the oxygen sensor to adjust the air/fuel ratio. This strategy ensures that the engine burns the fuel efficiently. Two oxygen sensors are used for this application. One of the oxygen sensors is mounted in the turbocharger exhaust outlet. The other oxygen sensor is mounted in the output flange of the catalytic converter. Each sensor circuit consists of an oxygen sensor and a sensor buffer. A voltage converter is used to power both sensor circuits.The oxygen sensor is used to detect the level of oxygen in the engine exhaust. The sensor generates a voltage signal that is directly proportional to the concentration of oxygen. The usable range of this signal is 150 mV to 750 mV. A lean concentration level will produce a 150 mV signal and a rich concentration level will produce a 750 mV signal. This analog signal is sent to the sensor buffer. The oxygen sensor utilizes a heating element in order to heat the sensing element of the sensor. The use of the heating element improves the warm-up time and the accuracy of the sensor.Note: The sensor has an operating range of 600 to 800° C (1100 to 1500° F). The tip of the sensor may be hot. Use caution when you handle the oxygen sensor.The sensor buffer is used to filter the electronic signal that is sent from the oxygen sensor. The buffer acts as a filter in order to condition the analog signal before the signal is sent to the ECM. The voltage converter converts 24 VDC from the ECM to 12 VDC that is used by the sensor circuit. The voltage converter provides 12 VDC to each of the buffers. Each buffer supplies 12 VDC to the oxygen sensor. Each sensor supply is used to power the heating element