Diagnostic Aids

Check for faulty connections or damaged harness. Inspect PCM harness connector EGR control circuit for backed-out terminal. Observe scan tool while moving all related harness and connectors. A change in scan tool display indicates fault location.

Ensure engine has reached operating temperature. Place gear selector in Park. Using scan tool, monitor HO2S 1 and HO2S 2 values while using scan tool IAC RPM function to maintain MAF at 10 grams per second. Compare activity between HO2S 1 and HO2S 2 for 30 seconds. If activity between both HO2S is about equal, a problem exists. Repeat DTC P0420.