Contour Enthusiasts Group Archives Forums General Troubleshooting po161 code O2 sensor but which of 4 help?

Just to clear up a common misconception, the downstream (or rear) oxygen sensors ARE used for fuel control in a secondary manner in addition to their primary duty as catalyst monitors. As the upstream oxygen sensors age, they shift lean or rich in the way they meter fuel. The downstream sensors can detect this fuel shift and feed back a trim bias to the EEC to keep the fuel control and pre-cats happy.