PFI Chart A-11 Left O2 Sensor Rich
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 high oxygen sensor voltage readings are normally evidence that the air fuel-mixture is rich 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 high reading. If this is the case, the closed loop fuel system will be overcompensating and causing lean operation while the O2 sensor is indicating rich.
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 ED32 reading is greater than .64 volts then check for circuit No. 412 shorted to voltage or poor ground circuit No. 412.
- Checks if sensor circuitry is able to record lean readings.
- 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 "B6" at ECM. If voltage is less than -0.5 volts or greater than +.05 volts, repair poor connection between ECM pin "B6" and O2 ground eyelet on front of rear (toward firewall) engine cylinder head.
- If O2 sensor circuit is okay, then rich exhaust problem is caused by fuel or emission systems not controlling properly. Possible causes for a rich condition are:
- A fuel delivery system which is not functioning properly may cause a rich fuel mixture. This malfunction can be caused by fuel pressure which is greater than 50 psi (3.5.kg/cm2) at injectors or by defective injectors. Refer to PFI CHART A-4: FUEL SYSTEM DIAGNOSIS for additional information.
- Excessive EGR flow displaces oxygen and causes a rich exhaust indication. See ECM DTC EO48: EGR SYSTEM FAULT.
- Injector dripping or injector leaks can cause rich exhaust. To check injectors, see PFI CHART A-5: INJECTOR SYSTEM DIAGNOSIS .
- Carbon canister loaded with fuel can cause rich operation. See PFI CHART C-3: CANISTER PURGE CONTROL DIAGNOSIS .
- Vacuum leak at MAP sensor can cause a false high MAP reading. High MAP readings cause ECM to deliver too much fuel for current driving conditions.
- If PFI CHART A-13: LEFT O2 SENSOR DIAGNOSIS , indicated that sensor is okay, check all connectors and terminals for an intermittent code.