Test TC-8A - Oxygen Sensor (O2S) Stays At Center (DTC 21)

WARNING: This page is about a different car, the 1995 Jeep Wrangler, 1995 Jeep Grand Cherokee, and 1995 Jeep Cherokee. However, it is still accessible from the selected car via links, so may be relevant.

NOTE: Perform TEST TC-1A before proceeding with this test. For connector terminal identification, see CONNECTOR IDENTIFICATION at beginning of article. For wiring diagrams, see WIRING DIAGRAMS at the end of this article. Perform TEST VER-2A after each repair.

Start engine. Allow engine to run until normal operating temperature is reached. Using scan tool, set engine speed to 1500 RPM. Using scan tool, read O2S state. If O2S is not switching from rich to lean, go to step 4). If O2S is switching from rich to lean, wiggle sensor connector and wiring harness while watching scan tool display.
If O2S was locked at center at any time, inspect all related wiring and connectors and repair as necessary. If O2S was not locked at center at any time, condition required to set trouble code message is not present at this time. O2S STAYS AT CENTER trouble code message sets if O2S output voltage stays at .5 volt for 1.5 minutes with engine temperature more than 170°F (77°C) and engine running for 2 minutes. Possible causes are: open O2S circuit or O2S failure. Go to next step.
Inspect all related wiring and connectors and repair as necessary. If no problems were found with wiring and connectors, see INACTIVE DTC CONDITION in this article. Test is complete.
Turn ignition off. Disconnect O2S connector and ensure connector is clean and not damaged. Using scan tool in ohmmeter mode, check resistance of ground circuit (Black/Light Blue wire) at O2S connector harness side. If resistance is less than 5 ohms, go to next step. If resistance is more than 5 ohms, repair open in Black/Light Blue wire.
Connect a jumper wire between O2S signal circuit (Black/Dark Green wire on XJ and YJ bodies or Black/Orange wire on ZJ body) and battery positive terminal. Using scan tool, read O2S voltage. If O2S voltage is less than one volt, go to next step. If O2S voltage is more than one volt, replace O2S.
Turn ignition off. Disconnect Powertrain Control Module (PCM) connector and ensure connector is clean and not damaged. Disconnect jumper wire. Using an external ohmmeter, check resistance of signal circuit (Black/Dark Green wire on XJ and YJ bodies or Black/Orange wire on ZJ body) between O2S connector harness side and PCM connector terminal No. 41. If resistance is less than 5 ohms, replace PCM. If resistance is more than 5 ohms, repair open in O2S signal circuit.