Error Code P0133 is described as Oxygen Sensor Circuit Slow Response (Bank 1, Sensor 1). This code means the air/fuel ratio in the engine is not being automatically adjusted by the O2 sensor signal or the ECM (Engine Control Module) the way they are supposed to do, or not being adjusted as expected when the engine warms up or under normal engine use.


The job of the O2 sensor is to measure the amount of oxygen in the exhaust gas when it leaves the engine after the combustion process. This information is important for the engine to give off the most power, while at the same time minimize its contribution to air pollution. If there’s not enough oxygen in the exhaust, the engine is running too rich, and using a lot of fuel.

This wastes fuel and pollutes the air with CO2. When this occurs, the PCM (Powertrain Control Module) reduces the fuel it sends to the engine. If there’s too much O2 on the other hand, the engine is running too lean, which means it’s polluting the air with NOx (Nitrogen Oxides) and raw hydrocarbons. When this occurs, the PCM will add fuel it sends to the engine.

The advanced broadband version of the O2 sensor is called the Air-Fuel Ratio Sensor.

The Error Code P0133 is set when the PCM identifies that the O2 sensor voltage switches from 400mV to 450mV takes too long. This value usually varies from one vehicle to another, but usually, the O2 sensor will only need around 100ms to switch its voltage. The PCM oversees the lean-rich switching time of Air Fuel Ratio sensor, but uses different values. This is because the air fuel ratio sensor is a broadband device, which means it is capable of controlling the air-fuel ratio of the engine throughout the all phases of operation, including wide open throttle, and when cold starting the vehicle from freezing weather.

Common Symptoms

  • Check Engine light is on
  • Engine dies, rough idle and running
  • Excessive fuel consumption
  • Black smoke emission, bad smell from the exhaust

Possible Causes

  • Defective Oxygen sensor or Air Fuel Ratio Sensor
  • Defective Oxygen sensor or Air Fuel Ratio Sensor Heater circuit
  • Low Fuel Pressure
  • Broken or frayed wiring in the sensor
  • Intake Air System leak
  • Exhaust leak
  • Defective ECT (Engine Coolant Temperature) sensor
  • Defective PCM or outdated software

How to Check

Record the freeze frame data in full detail when you see the code is set. Then, duplicate the code-setting conditions through a test drive, pay attention to the engine load, MPH and RPM. Use factory quality data streaming scan tool through a dedicated live data tool. Verify the code conditions before proceeding to next step.

If you can’t verify the code setting malfunction

Check the sensor connections. Make sure there is 12V heater signal(s) and good grounds(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 as “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 scrapped, 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 have 3V, while the other one should have 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 signal respond to the subtle changes of mixture. If you don’t always see +/- 10 milliamp variation 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

The easiest way to get rid of the Error Code P0133 is to reset the code and see if it comes back after a few drives. If the code indeed comes back, the problem is most likely in the front Bank 1 O2 sensor. This means you need to replace the sensor, however, you should also consider some of these solutions:

  • Check and repair exhaust leaks
  • Check wiring problems such as shorted or frayed wires
  • Check both the frequency and amplitude of the O2 sensor
  • Check for worsening or contaminated O2 sensor, replace if required
  • Check for inlet air leaks
  • Check the MAF (Mass Flow) sensor for the right operation

Parting Tips

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