Chart A-6 Oxygen Sensor Test

Chart A-6:

Oxygen Sensor Circuit:

DESCRIPTION:

The ECM provides a 0.45 volt reference signal to the oxygen sensor on CKT 412. When the oxygen sensor is cold (below 200°C), the output voltage will be around 0.45 volts and the ECM will keep the system in open loop operation. When warm, a properly operating oxygen sensor will drive the 0.45 volt reference lower (below 0.45 volts) to indicate a lean mixture and higher (above 0.45 volts) to indicate a rich mixture. The oxygen sensor signal voltage will swing from rich to lean rapidly, at least one swing every two seconds, if the ECM is in good control of the air/fuel mixture. Low oxygen sensor voltage readings are evidence that the air/fuel mixture is lean and the closed loop system is unable to compensate sufficiently due to a failure in some part of engine emission or fuel system. Fixed high oxygen sensor voltage readings are evidence that the air-fuel mixture is rich and the closed loop system is unable to compensate sufficiently due to a failure in some part of the engine emission or fuel system. Less likely is the possibility that the oxygen sensor has failed and is giving an incorrect reading. If the oxygen sensor is giving false rich readings, the closed loop fuel system will be overcompensate and cause a lean operation while the 02 sensor is indicating rich. Likewise, a lean indication will cause a rich operation (i.e. black smoke, fouled spark plugs, poor fuel economy, high HC and CO, etc.) while the 02 sensor is indicating lean.

NOTES ON FAULT TREE:

1. Steps contained in this section of the diagnostic tree could cause both an indicated rich or lean operation as follows:

a. A fuel delivery system which is not functioning properly may cause an incorrect fuel mixture. This malfunction can be caused by fuel pressure which is greater than 12 PSI or less than 9 PSI at the injectors, by defective injectors, etc. Refer to CHART A-4A FUEL SYSTEM DIAGNOSIS for additional information.
b. Injector dripping or not injecting can cause rich or lean exhaust (depending on failure). To check injectors, see Chart A-5.
c. Excessive EGR flow displaces oxygen and causes a rich exhaust indication. A loss of EGR will cause a lean exhaust indication. Refer to EGR DIAGNOSIS CHART C-7, for additional information.

2. If a code was not set, the DFI SYSTEM CHECK did not refer to the OXYGEN SENSOR CIRCUIT CHECK which will be required before further diagnosis.

3. Steps contained in this section could cause only an indicated rich operation as follows:

a. Carbon canister loaded with fuel can cause rich operation, see CHART C-3.
b. Vacuum leak to the MAP hose can cause a false high MAP reading. High MAP readings cause the ECM to deliver too much fuel for current driving conditions.
c. A restricted air cleaner could cause a rich fuel mixture. Inspect the air cleaner and replace if necessary.
d. If none of the above have indicated a problem, the code should be cleared and a road test should be performed. This code can be the result of extreme purge rates under high temperature conditions, in such a case no further diagnosis is required and no repairs should be attempted.

4. Steps contained in this section could cause only an indicated lean operation as follows:

a. If the AIR management system were to send air to the exhaust ports at all times, this would give a lean indication at the oxygen sensor. Refer to AIR MANAGEMENT DIAGNOSIS CHART C-6, for additional information.
b. Check for vacuum leaks at hoses, intake manifold and throttle body gaskets.
c. Check for fuel contamination.
d. If no problems found, replace the oxygen sensor.