Diagnostic Aids

Check for poor connection at PCM. Inspect harness connectors for backed-out terminals, improper mating, broken locks, improperly formed or damaged terminals and poor terminal connection. Observe scan tool while moving all related harnesses 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. If DTC cannot be duplicated, check scan tool FAILURE RECORDS to determine mileage since last DTC was set.