Contour Enthusiasts Group Archives Forums General Troubleshooting P0136, and P0156

You'll typically see normal O2 sensor voltage fluctuate through most of that 0.1-1 volt range, mabye more like .2-.8. It 'switches' back and forth across 0.5v, which is the optimum fuel-air mixture. For both good fuel control, and to keep the catalyst supplied with oxygen, the system is continually dithering between slightly rich and slightly lean. The response curve resembles AC voltage. The thing to remember is it should NOT be consistently below or above .5v. When it stops 'switching', something's not right.