Circuit Description
The ECU supplies a voltage of about 0.45 volt between the HO2S High and HO2S Low circuits. The Heated Oxygen Sensor (HO2S Bank 2 Sensor 1) varies the voltage within the range of about 1.0 volt (1000 mV) if the exhaust is rich, down to about 0.1 volt (100 mV) if the exhaust is lean.
The sensor is like an open circuit. It produces no voltage when it is below 360°C (600°F). This will result in Open Loop operation.
The HO2S heater provides for a faster sensor warm-up. This allows the sensor to become active in a shorter period of time and remain active during a long extended idle.
DTC 64 determines if the HO2S sensor or the circuit is shorted to low by checking for an extended lean condition.
This is a type B DTC.
Conditions for Setting the DTC
The following conditions will set the DTC:
| • | The vehicle is in the CNG operation mode. |
| • | The CNG fuel voltage is greater than 1.95 volts (fuel level above 1/8 tank). |
| • | The ECT is greater than 70°C (160°F). |
| • | The oxygen sensor has been warm for at least 120 seconds (the oxygen sensor is determined to be warm by the ECU if the ECT is greater than 40°C (104°F) and the mass air flow is above 11 g/sec for at least 120 seconds). |
| • | The ECU is in closed loop operation. |
| • | The HO2S (Bank 2, Sensor 1) voltage is less than 0.02 volt (20 mV). |
Action Taken When the DTC Sets
The MIL will turn ON after 2 consecutive trips with test failures. The system will operate in Open Loop if any DTC of the second HO2S (Bank 1, Sensor 1) is set (DTC 42, 44 or 45).
Conditions for Clearing the MIL/DTC
The ECU turns the MIL OFF after three consecutive driving trips without a fault condition present. A DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 40°C (72°F) from the start-up coolant temperature and the engine coolant temperature exceeds 70°C (160°F) during that same ignition cycle) or the code clearing procedure has been used.
Diagnostic Aids
Always diagnose ANY VCM codes that are present before beginning this diagnostic procedure.
Check for the following conditions:
| • | A heated oxygen sensor wire. A sensor pigtail may be mis-routed and contacting the exhaust manifold. |
| • | An intermittent ground in the wire between the connector and the sensor. |
| • | A poor ECU to the engine block ground. |
| • | A faulty mixture control valve. |
| • | A faulty high pressure regulator. |
| • | A faulty low pressure regulator. |
| • | Exhaust leaks. The engine may pull the outside air into the exhaust and past the sensor if there is an exhaust leak. |
| • | Vacuum or crankcase leaks can cause a lean condition and possibly a high idle. |
If the above conditions do not exist, the problem is a faulty heated sensor.
Never solder the HO2S wires. Refer to Repair Procedures in Electrical Diagnosis in the C/K Truck Service Manual for proper wire and connector repair.
Test Description
The number(s) below refer to the step number(s) on the diagnostic table.
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Removing the CNG system fuse will force the system to start on gasoline (the AF ECU is deactivated, but the Keep Alive Memory (KAM) contents is still retained). The gasoline operation will then allow the VCM to enable HO2S sensor diagnostics that may detect a similar fault. This will isolate the fault to the base engine wiring harness or the HO2S.
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This test determines if the sensor wiring is the cause of DTC 64.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Was the Alternative Fuels (AF) Powertrain On-Board Diagnostic (OBD) System Check performed? | -- | Go to Step 2 | Go to Alternative Fuels (AF) Powertrain On Board (OBD) System Check |
The fuel indicator light will remain OFF due to the removal of the CNG system fuse. Is the action complete? | 1200 RPM | Go to Step 3 | -- | |
3 | Perform the AF Powertrain OBD System Check again. Is the action complete? | -- | Go to Step 4 | -- |
4 | Are there any Powertrain DTCs displayed that have not been diagnosed? | -- | Go to the applicable Powertrain DTC table | Go to Step 5 |
Is there continuity? | -- | Go to Step 6 | Go to Step 8 | |
6 | Check for a poor connection at the ECU. Was a problem found? | -- | Go to Step 11 | Go to Step 7 |
7 | Replace the ECU. Refer to Engine Control Unit Replacement Is the action complete? | -- | Go to Step 13 | -- |
8 |
Is there continuity? | -- | Go to Step 9 | Go to Step 11 |
9 |
Is there continuity? | -- | Go to Step 10 | Go to Step 11 |
10 | Check for a poor connection at connector C017. Was a problem found? | -- | Go to Step 11 | Go to Step 12 |
11 | Repair the circuit as necessary. Refer to Repair Procedures in Electrical Diagnosis in the C/K Truck Service Manual. Is the action complete? | -- | Go to Step 13 | -- |
12 |
Is the action complete? | -- | Go to Step 13 | -- |
13 |
Did the MIL illuminate? | 1500 RPM | Go to Step 2 | Go to Step 14 |
14 | Are any ECU or VCM DTCs displayed that have not been diagnosed? | -- | Go to the applicable DTC table | System OK |
