Chart A7 - Lean Exhaust Signal
The ECM provides a .45-volt reference signal to oxygen sensor on circuit No. 412. When oxygen sensor is cold (less than 200°C), the output voltage will be around .45 volt. The ECM will keep system in open loop operation. When warm, a properly operating oxygen sensor will drive the .45-volt reference lower (less than .45 volt) to indicate a lean mixture and higher (greater than .45 volt) to indicate a rich mixture.
The oxygen sensor signal voltage will swing from rich to lean rapidly (one swing every 2 seconds), if ECM is in control of air/fuel mixture. The low oxygen sensor voltage readings are normally evidence that the air/fuel mixture is lean and closed loop system is unable to compensate sufficiently due to a failure in some part of engine emission or fuel systems.
Less likely is the possibility that the oxygen sensor has failed and is giving an incorrectly low reading. However, if this is the case, closed loop fuel system will be overcompensating and causing rich operation while oxygen sensor is indicating lean readings.
- If Code EO13 is stored, DO NOT use this chart. See CHART A6 - OXYGEN SENSOR NOT READY.
- With sensor disconnected, oxygen sensor reference voltage (.38-.63 volt) should appear. If parameter ED07 reading is less than .38 volt, check for grounded signal circuit No. 412 or open ground circuit No. 413.
- Check for ECM and oxygen sensor circuitry ability to record rich readings. The DVOM set on "VOLTS" scale will provide a few billionths of an amp to drive circuit No. 412 to greater than .64 volt (rich). Similar results may be obtained by placing one finger on battery positive terminal 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. 413B. If the ECM does not have a good ground to engine on circuit No. 413B, oxygen sensor can appear falsely high or low. With engine running, use a voltmeter to measure voltage from oxygen sensor to terminal A1 at ECM. If voltage is -.05 volt to +.05 volt, ground circuit is okay. If voltage is less than -.05 volt or more than +.05 volt, repair poor ground connection between ECM terminal A1 and oxygen sensor ground eyelet on engine, at rear bank of cylinders, with a ground eyelet on power steering line retainer clip.
- If oxygen sensor circuit is okay, lean exhaust is caused by fuel or emission systems not controlling the following:
- A fuel delivery system which is not functioning properly may cause a lean fuel mixture. This malfunction can be caused by fuel pressure less than 9 psi at injectors or by defective injectors. See CHART A4A - FUEL SYSTEM DIAGNOSIS.
- A loss of EGR will cause a lean fuel mixture. See CHART C6 - EGR DIAGNOSIS.
- If AIR management system were to send air to exhaust ports at all times, this would give a lean indication at oxygen sensor. See CHART C6 - AIR MANAGEMENT DIAGNOSIS.
- If CHART A9 - OXYGEN SENSOR DIAGNOSIS indicates that sensor is okay, check all connectors and terminals for an intermittent condition.
- Check for vacuum leaks at hoses, intake manifold and throttle body gaskets.
- Check for intermittent injector operation. See CHART A5 - INJECTOR SYSTEM DIAGNOSIS.