Oxygen Sensor (Lambda Sensor)

The sensor before the catalytic converter (upstream, sensor 1) is the key feedback for fuel control. The sensor after it (downstream, sensor 2) mainly monitors catalyst efficiency. Older narrowband sensors switch between rich and lean signals; wideband (air-fuel ratio) sensors measure the exact mixture. Both need heat to work, so they have built-in heaters.

Upstream sensors before the catalyst control the mixture; downstream sensors after it monitor catalyst efficiency. Wideband sensors measure the exact air-fuel ratio. Oxygen sensors age with time and can become slow to respond, even before they set a fault code. Contamination from oil, coolant or silicone can shorten their life. Using the correct sensor type and an anti-seize compound only where the manufacturer allows it helps avoid problems after replacement.

A zirconia element compares the oxygen in the exhaust with outside air and produces a signal. The heater brings it to working temperature quickly after a cold start. The sensor compares oxygen in the exhaust with oxygen in outside air. It has a heater so that it reaches operating temperature quickly after a cold start.

A slow or failed sensor causes poor fuel economy, rich or lean running and codes such as P0133 or heater codes like P0135. Exhaust leaks before the sensor give false readings. Faulty sensors can raise fuel consumption and emissions and set codes such as P0130 to P0167. Exhaust leaks near the sensor can make a good sensor read wrongly.