What happens if the lambda sensor gets dirty?

Answer 1

The most noticeable symptom will be decreased O2 crosscounts, and the system will eventually begin to enrichen the air / fuel mixture, resulting in a rich condition, with all of its associated symptoms, such as (not an all inclusive list):

• Poor fuel economy;
• Stalling or sagging during acceleration, especially under load or uphill;
• Rough idle;
• Emissions test failure; and
• Fouled plugs.

When replacing an O2 sensor, it is important to identify the cause of failure, unless the sensor is just at the end of its service life. Sensors can be contaminated by silicon sealers used elsewhere in the engine, as well as by antifreeze leaking into the combustion chambers or even anti-seize compounds applied carelessly.

It is important to remember that a partially restricted catalytic converter will mimic some of the symptoms of a degraded O2 sensor. To tell the difference, observe the long and short term fuel trims with a scan tool. With both a bad O2 sensor and restricted catalytic converter, the fuels trims will be consistently negative, indicating that the system is trying to lean out the mixture. Observe the O2 crosscounts. If they are high, suspect a restricted converter. If they are low, suspect a bad O2 sensor first.