With Federal Emissions

***This article was updated by Technical Service Bulletin 51-65-57, dated September, 1995.***

Diagnostic Chart:

Electrical Schematic:

CIRCUIT DESCRIPTION
When the Oxygen Sensor (O2S) reaches operating temperature it produces a varying voltage between 0.10 volts (exhaust is lean) and 1.0 volts (exhaust is rich). The PCM monitors this voltage and determines the concentration of gases in the exhaust.

The PCM then uses this input to make fuel corrections to maintain the optimum air/fuel ratio.

DTC 21 will set if the following condition is met:
^ O2S voltage input at the PCM is constant between 0.35 and 0.70 volts for 60 seconds or more with engine speed above 1500 rpm and engine coolant temperature above 80°C (176°F).

DTC CHART TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.
1. This checks to see if the 02S is faulty.
2. This checks for an open in the BLK wire between the PCM and O2S.
3. This checks for an open in the BRN wire between the PCM and ground G106.
4. This checks for a short to ground in the BLK wire between the PCM and O2S, an open in the BRN wire to the O2S and for a faulty PCM.

DIAGNOSTIC AIDS
Make sure that ground G1O6 is clean and tight.

INTERMITTENT MALFUNCTIONS
An intermittent may be caused by a poor connection, rubbed through wire insulation, or a wire broken inside the insulation. Inspect hamess connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals and poor terminal-to-wire connections before component replacement.