How do lambda sensors work?

Question

Virtually every modern passenger vehicle today utilizes lambda sensors from the factory.

How critical are they to a vehicle's operation? How do they work?

Answer 1

Lambda sensors, often referred to as oxygen sensors or O2 sensors, are the bedrock of any modern-day EFI system. Without them, fuel injection management is essentially flying blind and has to resort to other less-desirable means to determine how much air and fuel need to be mixed.

Their primary role is to provide feedback to the fuel management to determine if the air-fuel mixture being fed into the engine is being mixed in the right ratio. They do so by measuring "lambda", the ratio of actual AFR to stoich.

Two lambda sensors sandwiching a catalytic converter can also be used to assess if the cat is running properly.

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Lambda sensor types & Operation

Lambda sensors work at elevated temperatures, around 300 °C (600 °F); many lambda sensors contain a resistive heater element to help get them up to temperature quickly.

1. Narrowband

   These qualitatively detect whether the exhaust gases are rich or lean.

   The most prevalent type of sensor is the zirconia-based narrowband, which generates its own voltage as signal output:

   • useful voltage range 0.1 - 0.9 V
   • stoichiometric AFR at 0.45 V
   • low voltage = lean condition, high voltage = rich condition

   

   The rarer titania-based variant does not generate its own voltage, but changes electrical resistance based on the oxygen concentration detected.

   • low resistance = rich condition, high resistance = lean condition

2. Wideband

   These quantitatively measure lambda.

   Changes in AFR translate to changes in current output, which is then converted to a corresponding voltage. The voltage range may vary between car manufacturers.

   • low current = rich condition, high current = lean condition

   

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Impact on fuel management

The diagram below nicely summarizes the impact of lambda sensors on fuel injection management:

• lambda sensor senses leaner-than-desired exhaust → increase injector pulsewidth
• lambda sensor senses richer-than-desired exhaust → decrease injector pulsewidth