The computer on this Blazer shows 2 codes, p301.

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The computer on this Blazer shows 2 codes, p301 and p0131, what are they?
Saturday, January 26th, 2013 AT 9:49 PM

1 Reply



IMPORTANT: If the voltage is measured with a 10 megohm Digital Voltmeter, the voltage may read as low as 0.32 volt (320 mV).

The Vehicle Control Module (VCM) supplies a voltage of about 0.45 volt (450 mV) between the HO2S Signal and HO2S Low circuits. The Heated Oxygen Sensor varies the voltage within the range of about 1.0 volt (1000 mV) if the exhaust is rich, down to about 0.10 volt (100 mV) if the exhaust is lean.

The sensor is like an open circuit and produces no voltage when it is below 360 C (600 F). The DTC P0131, P0132, an open signal circuit, or a cold sensor causes Open Loop operation. The DTC P0131 determines if the HO2S sensor or the signal circuit is shorted to low by checking for a lean condition during a steady throttle and power enrichment (PE).


The following conditions will set the DTC:

No Throttle Position (TP) sensor DTCs.
No Intake Air Temperature (IAT) sensor DTCs.
No Manifold Absolute Pressure (MAP) sensor DTCs.
No Engine Coolant Temperature (ECT) sensor DTCs.
No Mass Air Flow (MAF) sensor DTCs.
No intrusive test in progress.
No device controls active.
The system voltage measures at least 9.0 volts.

Test Enable Lean Test

Closed Loop.
Closed Loop low MAP not active.
Air to fuel ratio is between 14.5-14.8.
The TP is between 3.5-99%.
The above met for 5 seconds.
The HO2S voltage is less than 86 mV (0.086 V).

Test Enable PE Lean Test

Closed Loop.
Power Enrichment mode active.
High speed fuel cutoff not active.
Time elapsed since test enable is at least 1 second.
The HO2S voltage is less than 598 mV (0.598 V).


A current DTC P0131 set causes the system to operate in an Open Loop. With a DTC P0131 set, the Malfunction Indicator Lamp (MIL) turns ON at the first test failure.


The VCM turns the MIL OFF after 3 consecutive drive trips when the test has Run and Passed and not Failed.
A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 22 C (40 F) from the start-up coolant temperature and the engine coolant temperature exceeds 70 C (180 F) during the same ignition cycle).
Use the scan tool Clear Information function.
Disconnect the VCM battery feed for 30 seconds.


IMPORTANT: Never solder or attempt to repair HO2S wires.

Using the scan tool, observe the Long Term (LT) fuel trim values at the different RPM and the air flow conditions. The scan tool also displays the fuel trim cells, so the LT fuel trim values can be checked in each of the cells in order to determine when the DTC may have set. If the conditions for this DTC exist, the LT fuel trim values measure around 158 or greater.

Check for the following conditions:

Display the HO2S voltage with a scan tool. If the voltage is less than 0.086 V (86 mV) and increases to about 0.450 V (450 mV) after the HO2S is disconnected, then the HO2S is internally shorted and should be replaced.
A sensor pigtail may be mispositioned and contacting the exhaust system.
An intermittent short to ground in the signal circuit between the VCM connector and HO2S.
A poor VCM to the engine block ground.
Lean injectors: Perform the Injector Balance Test. Refer to Fuel Injector Balance Test for the 4.3L.

See: Powertrain Management Computers and Control Systems Testing and Inspection Component Tests and General Diagnostics Fuel Injector Balance Test

Fuel Contamination: Water, even in small amounts, near the in-tank fuel pump inlet can be delivered to the injectors. The water causes a lean exhaust and can also set this DTC.
Fuel pressure: If the pressure is too low, the system will be lean. In order to confirm, monitor a fuel pressure while driving the vehicle at various speeds and loads. Refer to Fuel System Diagnosis.

See: Powertrain Management Computers and Control Systems Testing and Inspection Component Tests and General Diagnostics System Diagnosis - Fuel System

Exhaust leaks: If there is an exhaust leak, the engine may pull the outside air into the exhaust. And past the sensor.
Vacuum or the Crankcase leaks can cause a lean condition or a possibly a high idle.


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

Running the engine at 1200 RPM keeps the oxygen sensor hot in order to ensure the oxygen sensor remains active and can indicate the exhaust oxygen content accurately.
Opening the HO2S Signal circuit should result in a displayed voltage between the 350 and 550 mV. If the display is fixed below 350 mV, the fault is a short to ground in the HO2S Signal circuit or a faulty VCM.

301 is a missfire code for number 1 cylinder, compression, plug, wire, injector and wiring need to be checked

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Saturday, January 26th, 2013 AT 11:25 PM

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