Error Code P0134 is described as Oxygen Sensor Circuit No Activity Detected (Bank 1, Sensor 1). This means there’s a problem in the front oxygen sensor on Bank 1, as the O2 sensor is inactive.


The PCM (Powertrain Control Module) is an automotive component, and is a control unit that generally combines with ECM (Engine Control Unit) and TCU (Transmission Control Unit). It is essentially the brain of the engine’s control system. And in relation to Error Code P0134, the PCM provides the baseline voltage of around 450 mV on the O2 sensor signal circuit.

The purpose of the oxygen sensor is to measure the content of O2 in the exhaust gas after it leaves the process of combustion from the engine. This data is crucial for the engine to produce the optimal amount of power while simultaneously producing the lowest possible amount of air pollution. If there’s not enough O2 in the exhaust, it will force the engine to run too rich and use excessive amount of fuel. This both wastes fuel and pollutes the air with carbon monoxide. When this happens, the PCM will take over and reduce the amount of fuel it sends to the engine.

If there’s too much O2 however, this means the engine is running to lean, and pollutes the air with nitrogen oxide and raw hydrocarbons (both are poisonous substances that harm the environment). When this happens, the PCM will take over and increase the amount of fuel it sends to the engine.

When the temperature is cold, the PCM will detect the internal resistance from the sensor as high. The sensor will then warm up to lower the resistance and start making voltage based O2 content in the exhaust. When the PCM detects that it took the sensor a significantly longer time to warm up (more than 1 minute), or if the voltage is inactive (not reading (outside 391-491 mV), it will trigger the error code.

Also, when the PCM determines that the O2 voltage is somewhere around 450mV (this value varies in different makes of vehicles, while a conventional oxygen sensor continuously switch above and below 450mV), it will trigger the Error Code P0134.

Common Symptoms

  • Check Engine light is on
  • Rough riding of the vehicle, engine misfire or idling
  • Engine dies or stutters
  • Bad fuel consumption
  • Bad smelling exhaust, exhaust emits black smoke

Possible Causes

This error code may be caused by one or more of the following conditions:

  • Defective Oxygen Sensor
  • Defective Fuel Ratio Sensor
  • Damaged heater circuit in the Oxygen Sensor
  • Broken or frayed wiring and connectors in the sensor
  • Damaged (blown) heater circuit fuse
  • Leaks in Exhaust System
  • Leaks in Air System
  • Faulty Coolant Temperature Sensor
  • Outdated software or failure of Powertrain Control Module

How to Check

When the code is set, record the freeze frame data in detail. Then, duplicate the conditions that set the code through a test drive, and make sure you pay close attention to the load, MPH and RPM. Use a factory quality data streaming scan tool with dedicated live data, and make sure you verify the code conditions before you go to the next step.

If you CAN’T verify the code setting malfunction

Check the connections of the sensor. Verify that there are 12V heater signal(s) and good ground(s) in the sensor, and they follow the needed times, based on manufacturer’s diagnostic documentation. Verify the signal from the O2 sensor to the PCM is “seen” by back checking the O2 sensor connector, and if needed, back check the PCM’s signal wire as well. Check the harness of the sensor and make sure it is not scraped, damaged or grounding anywhere. Also, make sure you wiggle test. You need to use a high impedance DVOM (Digital Volt Ohm Meter) for these electrical tests.

Test drive the vehicle and duplicate the conditions that set the error code. If the code doesn’t come back, you can now replace the O2 sensor, as it is most likely the one causing the problem.

If you CAN verify the code setting malfunction

Check the connections of the sensor, including the exhaust system. Make sure there are no upstream leaks in the Air Fuel Ratio Sensor. Make sure there are no 12V heater signal(s) and ground(s) to the sensor, and it follows the manufacturer’s diagnostic documentation. Check the signal from the O2 sensor that connects to the PCM, make sure it is “seen” by back probing the O2 sensor connector, and if needed, back probe the signal wire at the PCM as well. Check the harness of the sensor as well, and make sure it is not chafed or grounded.  Again, you will need high impedance DVOM for these tests.

Here are different ways to test the Air Fuel Ratio Sensor:

With the key on, engine off, disconnect the sensor from its two key wires (there are lots of wires in this component, find the right ‘two’ key wires) and probe the harness that connects to the PCM. One wire should be 3V, while the other one should be 3.3V. The other wires should be 12V power(s) and ground(s) for the circuit of the heater. In some vehicles, you may need to start the engine and let it idle to get the right voltage and find the wires.

Connect the sensor the harness using jumper wires. Then, connect the DVOM in series with 3.3V wire. Set the tool to milliamp scale, start your engine and let it idle. The 3.3 volt wire should cross-count between +/- 10 milliamps. Change the RPM when adding and subtracting throttle, you will notice the signal respond to the subtle changes of mixture. If you don’t always see +/- 10 milliamp variations in this wire, than means your Air Fuel Ratio Sensor is defective.

If you still could get verifiable results, then remove the Air Fuel Ratio Sensor. The spark should have a light tan color. If the sensor probe comes with white and chalky parts, this means it has been lagging between switching phases, and should be replaced.

How to Fix

Different causes require different repairs. Here are some of the most common fixes to get rid of this error code.

The most common fix for this error code is to replace the O2 sensor. However, that doesn’t always solve the problem or get rid of the Check Engine light.

  • Check the connectors and wiring for problems, and solve the problem accordingly
  • Excessive blowing of amperage of the heater fuse require replacement of both the sensor and the blown fuse
  • PCM replacement. This however, should be the last resort, when all other possibilities are covered and no solution seems to fix the problem.

Parting Tips

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