Diagnostic Aids

An intermittent may be caused by a poor connection, rubbed-through wire insulation or a broken wire inside insulation. Inspect harness connectors for backed-out terminals, improper mating, broken locks, improperly formed or damaged terminals, poor terminal-to-wire connection, and damaged harness.

To diagnose a faulty HO2S heater or heater circuit. After a cool down period, use scan tool to observe HO2S voltage. Turn ignition on, engine not running. When conditions are as specified, normal HO2S voltage will be 455-460 millivolts. A voltage reading of 1000 millivolts or more, indicates a short to voltage in signal line of suspect HO2S. A voltage reading of less than 5 millivolts indicates a short to ground in signal line, or high and low signal lines are shorted together. To test, disconnect suspect HO2S 4-pin harness connector. Connect test light between ignition feed and heater ground circuits. See WIRING DIAGRAMS article. Turn ignition on. Test light should illuminate. If test light does not illuminate, repair open in ignition feed or ground. If test light illuminates, replace suspect HO2S.

Using scan tool, monitor signal voltage while wiggling HO2S related connectors and wiring harness with a warm engine running at part throttle in closed loop. If a failure is induced, signal voltage will change from normal fluctuating voltage to a fixed voltage. A change in voltage will assist in isolating location of fault. DO NOT solder HO2S wires. If DTC cannot be duplicated, reviewing scan tool FAILURE RECORDS data can be useful in determining mileage and frequency of DTC occurrence.