Code 4-1 Fuel Always Lean

Run engine at 2000 RPM for one minute, and re-test. If code still appears, disconnect jumper wire from Self-Test connector and proceed.

1. Turn ignition off. Disconnect oxygen sensor connector. Set DVOM on the 200 ohm scale. On the oxygen sensor side of the connector, measure resistance between power circuit connector (Grey/Yellow Wire) and ground circuit connector (Black/Lt. Green Wire). See Fig 1 . Resistance should be between 5-20 ohms. If not, replace oxygen sensor.
2. If resistance is within specification, check power circuit. Set DVOM on 20 volt scale. With ignition on and engine off, measure voltage between power circuit connector and ground circuit connector on harness side connector plug.
3. Voltage should be at least between 12-13 volts. Minimum voltage required is 10.5 volts. If 10.5 volts or less is present, test ground circuit or power circuit for continuity. If proper voltage is present, test oxygen sensor signal circuit.
4. Turn ignition off. Disconnect oxygen sensor. Connect jumper wire from oxygen sensor (Dark Green/Pink Wire) to harness. Connect DVOM between engine and jumper. Connect vacuum pump to zone vacuum switches.
5. Run engine at 2000-2300 RPM while applying 10-12 in. Hg to vacuum switch assembly. Wait 2 minutes, and check voltmeter. If voltage remains below 0.5 volts, go to next step. If voltage remains above 0.5 volts, go to step 11). If voltage alternates above and below 0.5 volts, go to step 20).
   Fig 1: Oxygen Sensor & Feedback Motor Circuits

   
6. Turn engine off. Reconnect oxygen sensor. Disconnect thermactor air hose from air pump. Disconnect harness connector at MCU. Connect DVOM between MCU connector Pin No. 3 and engine ground (Pin No. 14). Run engine at idle.
7. Place carburetor on high idle cam. Depress CVR rod on top of carburetor to force system rich and observe voltmeter. If voltage is less than 0.5 volts, go to next step. If greater than 0.5 volts, go to step 11).
8. Turn engine off. Disconnect oxygen sensor (harness at MCU is still disconnected). Connect ohmmeter between MCU connector Pin No. 14 and engine ground. If resistance is 5 ohms or less, go to next step. If resistance is more than 5 ohms, repair circuit, and re-test.
9. Check continuity of Dk. Green/Pink wire. If resistance is 5 ohms or less, go to next step. If resistance is over 5 ohms, repair circuit.
10. Connect ohmmeter between engine ground and Dk. Green/Pink wire. If resistance is less than 1K ohms, repair circuit, and re-test. If resistance is more than 1K ohms, replace oxygen sensor, and re-test.
11. Thermactor hose should still be disconnected from air pump. With ignition off, reconnect harness to MCU and oxygen sensor. Disconnect harness from feedback motor on carburetor. Using a known good feedback motor, connect harness to motor and fully depress shaft. DO NOT  remove motor from carburetor. Apply 10-12 in. Hg vacuum to zone vacuum switches.
12. Start engine and set fast idle cam on low speed step. Observe feedback motor shaft and remove PCV hose from carburetor. Vehicle may run rough due to vacuum leak. If feedback motor shaft fully extends, reconnect PCV hose and thermactor hose, go to step 16). If shaft does not extend, reconnect PCV hose and go to next step. Disconnect substitute motor.
13. Turn ignition off. Disconnect harness connector at MCU and feedback motor. Check continuity of circuits from feedback connector to MCU connector. If resistance is more than 5 ohms in any circuit, repair circuit(s), and re-test. If less than 5 ohms in each circuit, go to next step.
14. With ignition off, connect ohmmeter lead to FBCA terminal 687 (Gry/Yel wire) and to each of 4 outside FBCA terminals in turn. Measure resistance of each FBCA winding. If all resistances are 50-175 ohms at room temperature or above, go to next step. If any reading is not within 50-175 ohms, replace FBCA motor, and re-test.
15. Connect ohmmeter lead to engine ground. Check all circuits in feedback harness connector (except Gry/Yel wire) for continuity with ground. If resistance is more than 1K ohms in all circuits, replace MCU, and re-test. If less than 1K ohms, repair circuit, and re-test.
16. With ignition off, connect ohmmeter lead to feedback motor case and other lead to terminal 687 (Gry/Yel wire) of motor. If resistance is less than 190K ohms, replace motor, and re-test. If greater than 190K ohms, go to next step.
17. Remove feedback motor from carburetor. Check pintle shaft for binding or damage. Connect harness to motor and turn ignition on. Shaft should extend, then retract when ignition is turned off. If pintle binds, replace pintle and re-test. If motor is damaged or if pintle does not move, replace motor, and re-test. If motor is okay, reinstall motor in carburetor and reconnect harness.
18. Turn ignition off and disconnect vehicle harness from TAD solenoid. Connect an ohmmeter across terminals at TAD solenoid. If resistance is 50-100 ohms, go to next step. If resistance is less than 50 ohms or greater than 100 ohms, replace TAD solenoid, and re-test.
19. Disconnect harness connector at MCU. Measure resistance between ground and Pin No. 10 (Wht/Blk wire) in MCU connector. If resistance is less than 1K ohms, repair circuit, and re-test. If more than 1K ohms, reconnect harness at MCU and go to next step.
20. Check connections and operation of TAD solenoid and valve. If wiring and hoses are properly connected, check for carburetor leaks, hose routing errors, and operation of bowl vent solenoid.