The PCM supplies and monitors voltage of approximately .45 volt between PCM HO2S low reference circuit (Tan wire) and HO2S signal circuit (Purple wire). If read with a 10 megohm DVOM, voltage may read as low as .32 volt. HO2S varies voltage within a range of approximately 1 volt (rich) to .10 volt (lean). Sensor is like an open circuit and produces no voltage when less than approximately 600°F (360°C). Open loop operation is caused by open oxygen sensor circuit or a cold sensor. Code P0134 will set when the following conditions are present.

Engine coolant temperature is greater than 110°F (43°C).
Engine run time is more than 40 seconds.
HO2S signal voltage is steady between .35-.56 volt.
Throttle angle is greater than 3 percent.
All conditions are met for approximately 30 or more seconds.

Code P0134 will cause system to operate in open loop. PCM will illuminate Malfunction Indicator Light (MIL).

NOTE: Test numbers refer to numbers on diagnostic chart.

If conditions necessary to set Code P0132 exist, system will not go into closed loop.
This will determine if sensor or wiring is cause of Code P0134.
This test checks continuity in reference circuit.

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.

• Brakes feel soft, pull hard to one side, or make grinding noises.

• The engine overheats, stalls repeatedly, or misfires under load.

• You are missing required tools, torque specs, or safe lifting equipment.

• You are not confident in the next step or safety outcome.

Safety Warning

Code P0134, Heated Oxygen Sensor Open Circuit: Notes

Use the Manual With the Right Hardware

When to See a Mechanic