DR-4A Fault Code 21-Oxygen Sensor Circuit

1. With diagnostic read-out tool connected to engine harness connector, turn ignition switch Off, then disconnect oxygen sensor connector.
2. Connect a jumper wire to black wire of harness end of 3-way connector.
3. Place system in Sensor Test Mode 02. Note that display readings must be divided by 10.
4. Touch other end of jumper wire to positive post of battery, then to negative battery post.
5. If display read-out increases towards 5 volts when touching positive post or decreases toward 0 volts when touching negative post, replace oxygen sensor.
6. If display read-out does not increase or decrease, proceed to Test 4 step B. DR-4B Oxygen Sensor Circuit at Logic Module