Dr4 - Code 21, Oxygen Sensor Circuit To Logic Module
- With diagnostic readout box connected to engine harness connector, turn ignition switch to the "OFF" position. Disconnect oxygen sensor connector. Connect one end of jumper wire to harness end of oxygen sensor connector. Start engine and touch other end of jumper wire, in turn, to battery positive and negative posts.
- When wire touches positive post, display on readout box should increase towards 5 volts. When negative post is touched, display on readout box should decrease towards zero volts. If voltage readings change, replace oxygen sensor. If voltage readings do not change, leave oxygen sensor disconnected.
- Turn ignition switch to the "OFF" position. Connect a digital voltmeter to cavity No. 18 of logic module Black connector (Blue on turbo) and ground. Turn the ignition switch to the "RUN" position. Voltmeter should read at least .3 volts.
- If voltage is okay, repair the wire of cavity No. 18 for open circuit to oxygen sensor. If voltage is zero, replace the logic module, making sure that the terminal in cavity No. 18 is not crushed.