Code 27, Sub-O2 Sens CKT, Open Or Shorted CKT (Ca Emissions): Notes

Sub-oxygen (O2) sensor produces a varying voltage as engine reaches normal operating temperature. Voltage will vary from .1 (lean) to 1.0 (rich) volt. PCM monitors voltage and determines concentration of gases in exhaust after converter. PCM uses this input and Oxygen (O2) sensor input, and adjusts fuel to maintain air/fuel ratio.

Code 27 will set when engine is at operating temperature and engine is off idle for 2 seconds or more with O2 sensor input at PCM .45 volt or more, and sub-O2 sensor input at PCM .45 volt or less.

1. Checks for faulty sub-O2 sensor.
2. Checks for open in Black wire between PCM and sub-O2 sensor.
3. Checks for open in Brown wire between PCM and G106.
4. Checks for short to ground in Black wire between PCM and sub-O2 sensor, open in Brown wire to sub-O2 sensor, and a faulty PCM.