Lambda (oxygen) sensor

How it works

The lambda (oxygen) sensor is screwed into the exhaust, ahead of the catalytic converter, on most modern fuel-injected bikes. It measures the oxygen left in the exhaust gas, which shows whether the air-fuel mixture is too rich or too lean. The ECU uses this signal to constantly correct fuelling, to limit consumption and emissions. Many sensors are electrically heated so they work right after starting. Over time the sensor can get fouled (oil, poor fuel, a long-running rich mixture) or its heater can fail: the ECU then runs in a fallback mode, the engine warning light comes on, consumption rises and idle may become uneven. An exhaust leak near the sensor also skews its reading. Diagnosis is done with a fault-code reader; replacement is often simple with the right socket, on a cold engine.

Maintenance tips

• ✓If the engine warning light comes on, have the fault codes read before replacing the sensor.
• ✓Check for exhaust leaks near the sensor: an air leak distorts its reading.
• ✓Use good-quality fuel and fix any oil-burning problem, both of which foul the sensor.
• ✓Remove the sensor on a cold engine with an oxygen-sensor socket, and keep the connector dry.
• ✓A persistently rich mixture (black smoke) must be fixed: it destroys the sensor and the catalytic converter.