PFI Chart A-9: Left O2 Sensor Lean

The ECM provides a .45-volt reference signal to oxygen sensor on circuit No. 412. When warm, a properly operating oxygen sensor will drive the reference voltage lower (below .45 volts) to indicate a lean mixture and higher (above .45 volts), to indicate a rich mixture.

Fixed low oxygen sensor voltage readings are normally evidence that the air fuel mixture is lean and the closed loop system is unable to compensate sufficiently due to some malfunction in the engine emission or fuel system. Less likely is the possibility that the sensor has failed and is giving an incorrect low reading. If this is the case, the closed loop fuel system will be overcompensating and causing rich operation (black smoke, fouled plugs, poor fuel economy, high HC and CO, etc.) while the O2 sensor is indicating lean.

NOTE: The following step numbers refer to circled numbers on diagnostic chart. See Fig 1 . For circuit identification, see Figure .

If code EO17 is stored do not use this chart, go to PFI CHART A-7: LEFT O2 SENSOR NOT READY .
With the sensor disconnected, the oxygen sensor reference voltage should appear. If parameter ED32 reading is less than .38 volts then check for grounded circuit No. 412 or open ground circuit No. 413.
Checks if sensor circuitry is able to record rich readings. DVOM will drive circuit No. 412 to above .64 volts (rich). Similar results may be obtained by placing one finger on the battery positive post and another finger on oxygen sensor circuit No. 412 harness terminal.
The ECM compares oxygen sensor signal voltage received on circuit No. 412 to ground voltage on circuit No. 413. If the ECM does not have a good ground on circuit No. 413, the O2 sensor signal can appear falsely high or low. With engine running, using a digital voltmeter with a minimum 10-megaohms input impedance, measure voltage from the oxygen sensor at the exhaust manifold to pin "EL5" at ECM. If voltage is less than -0.5 volts or greater than +.05 volts, repair poor connection between ECM pin "E15" and O2 ground eyelet on front of rear (toward firewall) engine cylinder head.
If O2 sensor circuit is okay, then lean exhaust problem is caused by the fuel or emission systems not controlling properly.

Fig 1: Identifying PFI Chart A-9 (Left O2 Sensor Lean) Flow Chart

Courtesy of GENERAL MOTORS CORP.