DTC P0134 or P0154. Chevrolet Express (CNG) 2001

For 1990-2009 cars only

Makes 🢒 Chevrolet 🢒 2001 🢒 Express (CNG) 🢒 DTC 🢒 DTC P0134 or P0154

Circuit Description

The heated oxygen sensor (HO2S) is used in order to determine the oxygen content of the engine's exhaust. The oxygen content of the exhaust indicates when the engine is operating lean or rich. When the engine is operating lean, the exhaust will have more oxygen content. Once at operating temperature, the HO2S will produce a voltage in relation to the oxygen content of the exhaust. The HO2S requires an operating temperature of 360°C (600°F) to produce voltage. The HO2S will produce more voltage when the exhaust is rich and less voltage when the exhaust is lean. The HO2S operating voltage range is between 0.10 volts (100 mV) to 1 volt (1,000 mV).

The gasoline Control Module (VCM/PCM or ECM) supplies a bias (or reference) voltage to the HO2S. This bias voltage is around 450 mV. The Alternative Fuels ECM monitors the bias voltage as well as the voltage produced by the HO2S.

During normal Closed Loop fuel control operation, the Alternative Fuels ECM will add fuel when the HO2S indicates a lean exhaust content. When the HO2S indicates a rich exhaust content, the Alternative Fuels ECM will subtract fuel. This oscillation above and below the bias voltage, sometimes referred to as activity or switching can be monitored with the HO2S signal voltage.

The oxygen sensor contains a heater. The heater is necessary in order to quickly warm the sensor to operating temperature. The heater will also maintain operating sensor temperature during extended idle conditions.

This DTC is used to indicate an HO2S with in-active signal voltage.

Conditions for Running the DTC

•	The engine is operating on alternative fuel.

•	The engine speed is more than 400 RPM.

•	The engine has been operating for at least 4 minutes.

•	The Alternative Fuels ECM determines the HO2S has been at operating temperature for at least 30 seconds. Operating temperature is indicated when engine coolant temperature is more than 40°C (104°F) AND desired gas flow is greater than 0.3 grams per second for 30 seconds.

Conditions for Setting the DTC

The Alternative Fuels ECM monitors an O2S signal voltage stuck between 0.347 V and 0.550 V for 9 seconds continuously.

The Alternative Fuels ECM monitors an O2S signal voltage between 0.00 V and 1.0 V that has not performed a Rich/Lean switch for 15 seconds continuously.

Action Taken When the DTC Sets

The Alternative Fuels ECM will control fuel trim based upon the Bank 2 Sensor 1 signal. If a Bank 2 Sensor 1 DTC sets, the Alternative Fuels ECM will operate in Open Loop fuel control.

•	A first failure of this DTC will NOT illuminate the MIL and will store as Last Test Failed but not as History.

•	The Alternative Fuels ECM will illuminate the MIL and store the DTC as History after 2 consecutive drive trips that the diagnostic runs and fails.

•	The Alternative Fuels ECM will record operating conditions at the time the DTC sets. This information will be stored as Freeze Frame data.

Conditions for Clearing the MIL/DTC

•	The Last Test Failed status will clear when the DTC runs and passes.

•	The Alternative Fuels ECM will turn OFF the MIL after 3 consecutive drive trips that the DTC runs and passes.

•	The History status will clear after 40 consecutive WARM-UP cycles with NO failures of ANY DTC.

•	All DTC Information can be cleared with a scan tool.

Diagnostic Aids

Intermittent Condition

A fault not present indicates the condition that caused the DTC to set is intermittent and not currently present. Refer to the Intermittent Conditions diagnostic information located in Symptoms - Engine Controls. If any additional DTCs were stored proceed to the applicable DTC table in the order stated in Diagnostic System Check - Alternative Fuels (Engine Controls) .

Test Description

The numbers below refer to the step numbers on the diagnostic table.

This step determines if proper sensor activity is present. When in Closed Loop fuel control, the HO2S voltage should rapidly vary between more than and less than the bias voltage.

This step determines if the gasoline Control Module and the High and Low circuits between the Control Module and the HO2S connector are operating properly.

This step determines if the HO2S heater circuit operates correctly up to the HO2S connector.

This step determines if the proper circuit resistance exists between the HO2S Low circuit and Control Module ground.

This step determines if the proper circuit resistance exists between the HO2S Low circuit and Control Module ground with a COLD sensor. A loose HO2S or faulty thread to exhaust electrical contact causes higher resistance when the HO2S is cold. Although 500 ohms is allowed, the typical resistance should be less than 50 ohms.

Step

Action

Value(s)

Yes

Schematic Reference: Engine Controls Schematics

Did you perform the Alternative Fuels Engine Controls Diagnostic System Check?

Go to Step 2

Go to Diagnostic System Check - Alternative Fuels (Engine Controls)

Is DTC P1215 also set?

Go to DTC P1215

Go to Step 3

Use a scan tool in order to establish communication with the Alternative Fuels ECM.
Capture the DTC information from the Alternative Fuels ECM into the scan tool.
Clear the DTC information using a scan tool.
Reset the Fuel Trim values.
Start and idle the engine until a normal operating temperature is reached.
Use the scan tool in order to monitor the HO2S voltage for the sensor that applies to this DTC.

Is the voltage fixed within the specified range?

0.10-1.0 V

Go to Step 6

Go to Step 4

The condition that set this DTC is not present. This DTC may have been set by one of the following conditions:

•	Water intrusion of the HO2S connector

•	An intermittently open HO2S High signal circuit

•	An intermittently open HO2S Low circuit

•	An HO2S Low circuit with high resistance to ground

•	Any Alternative Fuels ECM ground circuits with high resistance to ground

•	An inoperative HO2S heater

•	A faulty HO2S or Control Module connector terminal contact

•	An exhaust system leak, typically within 12 inches up-stream of the HO2S

•	A defective HO2S

Repair any of the above conditions as necessary.

Was a condition found and repaired?

Go to Step 23

Go to Step 5

The fault is not present. Refer to Diagnostic Aids.

Are any DTCs stored that have not been diagnosed?

Go to Diagnostic Trouble Code (DTC) List

Go to Intermittent Conditions

Use the scan tool in order to establish communication with the gasoline Control Module (VCM/PCM or ECM).
Start and idle the engine until a normal operating temperature is reached.
Use the scan tool in order to monitor the HO2S voltage for the sensor that applies to this DTC.

Is the voltage fixed within the specified range?

0.10-1.0 V

Go to Step 7

Go to Step 18

Turn the ignition OFF.
Disconnect the HO2S connector for the sensor that applies to this DTC.
Jumper the HO2S Low circuit to a known good ground.
Jumper the HO2S High signal circuit to a known good ground.
Turn the ignition ON, with the engine OFF.
Use the scan tool in order to monitor the HO2S voltage for the sensor that applies to this DTC.

Is the voltage less than the specified value?

20 mV

Go to Step 10

Go to Step 8

Turn the ignition OFF.
Remove the jumpers from the HO2S terminals.
Disconnect the gasoline Control Module (VCM/PCM or ECM) connector containing the HO2S High and Low circuits.
Use a digital multimeter (DMM) in order to measure the continuity of the HO2S High signal circuit and the HO2S Low circuit between the Control Module connector terminal and the HO2S connector terminal - engine harness side.

Are both resistances less than the specified value?

5 ohms

Go to Step 9

Go to Step 12

Inspect for proper gasoline Control Module (VCM/PCM or ECM) connector terminal contact.
Repair the terminals as necessary. Refer to Connector Repairs in Wiring Systems.

Was a terminal repair made?

Go to Step 23

Go to Step 22

Remove the jumpers from the HO2S terminals.
Connect the J 35616-200 Unpowered Test Lamp between the HO2S heater ignition feed and heater ground circuit terminals - engine harness side.

Turn the ignition ON, with the engine OFF.

Does the test lamp illuminate?

Go to Step 11

Go to Step 13

Turn the ignition OFF.
Reconnect the HO2S connector.
Disconnect the gasoline Control Module connector containing the HO2S Low circuit and the gasoline Control Module connector containing the Control Module ground circuits.
Use the DMM in order to measure the resistance between the HO2S Low circuit terminal and one of the Control Module ground circuit terminals.

Is the resistance less than the specified value?

500 ohms

Go to Step 15

Go to Step 14

Repair the circuit that measured high resistance.

Did you complete the repair?

Go to Step 23

Repair the HO2S heater ignition or ground circuit. Probable causes include the following conditions:

•	An open circuit

•	A high circuit resistance

•	A faulty ground connection

•	A blown heater fuse -- other DTCs may be set

Did you complete the repair?

Go to Step 23

Repair the high resistance between the HO2S Low circuit and the Control Module ground circuit. Probable causes include the following conditions:

•	A faulty HO2S connector terminal contact

•	An open sensor harness Low circuit -- requires HO2S replacement

•	A sensor harness Low circuit with high resistance -- requires HO2S replacement

•	A faulty gasoline Control Module ground connection

•	A high gasoline control Module ground circuit resistance

•	An ungrounded exhaust system

Did you complete the repair?

Go to Step 23

Inspect for exhaust system leaks upstream of the suspect HO2S.

The leak may be very small and is typically within 30 cm (12 inches) of the suspect HO2S.

Repair the exhaust as necessary.

Was an exhaust system repair made?

Go to Step 23

Go to Step 16

Allow engine to cool to ambient temperature.
Use the DMM in order to re-measure the resistance between the HO2S Low circuit terminal and a gasoline Control Module ground circuit terminal.

Is the resistance less than the specified value?

500 ohms

Go to Step 20

Go to Step 17

Remove the HO2S from the exhaust.
Clean the HO2S mounting threads.
Apply anti-seize compound GM P/N 5613695 or equivalent to the sensor threads.
Tighten the HO2S to the proper torque. Refer to Heated Oxygen Sensor Replacement - Bank 1 Sensor 1 in the Engine Controls section of the G Van Service Manual.
Use the DMM in order to measure the resistance between the HO2S Low circuit terminal and a PCM ground circuit terminal.

Is the resistance less than the specified value?

500 ohms

Go to Step 23

Go to Step 21

Inspect the oxygen sensor signal and low circuit for continuity between the Alternative Fuels ECM connector terminals and the oxygen sensor connector terminals.
Inspect these circuits for shorts to other circuits or to ground.

Ensure that the oxygen sensor, gasoline Control Module and the Alternative Fuels ECM connectors are disconnected before testing the circuits.

Repair the wiring as necessary.

Was a wire repair made?

Go to Step 23

Go to Step 19

Replace the Alternative Fuels ECM. Refer to Alternative Fuels Engine Control Module Replacement .
Program the replacement Control Module with the correct vehicle software and calibration.

Did you complete the repair?

Go to Step 23

Perform the AF Fuel System diagnosis and return to this step. Refer to Fuel System Diagnosis .

Was a CNG fuel system problem found and repaired?

Go to Step 23

Go to Step 21

Replace the HO2S sensor. Refer to Heated Oxygen Sensor Replacement - Bank 1 Sensor 1 or Heated Oxygen Sensor Replacement - Bank 2 Sensor 1 in the Engine Controls section of the G Van Service Manual.

Did you complete the repair?

Go to Step 23

Important: The following Control Module replacement procedures must all be completed before attempting to operate the vehicle.

The following procedures are located in the gasoline service manual.

Replace the gasoline Control Module (VCM/PCM or ECM). Refer to Powertrain Control Module Replacement .
Program the replacement Control Module with the correct vehicle software and calibration.

Important: In order to perform the variation learn procedure, a switch-over to gasoline operation is necessary. Remove the CNG 20-amp fuse in order to perform the switch-over.

Perform the Crankshaft Position System Variation Learn .

Did you complete the repair?

Go to Step 23

Reconnect all disconnected or removed components.
Use the scan tool in order to clear all DTC information from BOTH the gasoline Control Module (VCM/PCM or ECM) and the Alternative Fuels ECM.
Use the scan tool in order to communicate with BOTH the gasoline Control Module and the Alternative Fuels ECM in order to reset the Fuel Trim values to zero.
Start the engine.
Operate the vehicle within the Conditions for Running the DTC until the scan tool indicates the diagnostic ran.

Does the scan tool indicate the diagnostic passed?

System OK

Go to Step 2