Oxygen sensors should be replaced every 60 – 90k miles. Worn
oxygen sensors cause engine surging, especially upon start up,
and hesitation when accelerating enthusiastically. Upon replacing
your sensor, usually a 3 to 5 M.P.G. improvement is noticed
along with improved smooth running.

Oxygen Sensor Diagnosis

Generic Method

Here are some fast and
reliable diagnostic procedures which you can use to check out
most oxygen sensors. A great time to do this is when you are
performing a tune-up.

The following symptoms will help tip you off to a failed oxygen
sensor: Surging and/or hesitation Decline in fuel economy
Unacceptable exhaust emissions Premature failure of the
catalytic converter You will need the following equipment:

• A handheld volt meter
  
• A propane enrichment device
  
• An oxygen sensor socket.The manufacturer’s vehicle specific test instructions It
  should take less than 10 minutes to perform a diagnostic
  check on most vehicles.
  
  1. Verify the basic engine parameters per the
  manufacturer’s specifications for the following: timing,
  integrity of the electrical system (supply voltage), fuel delivery/
  mixture performance and internal mechanical considerations.
  
  2. Test the rich mixture performance as follows:

  A. Disconnect the sensor lead to the control unit.

  B. Run the engine at 2500 rpm.

  C. Artificially enrich the fuel mixture by directing
  propane into the intake until the engine speed drops by 200rpm.
  Or, if you’re working on a vehicle with electronic fuel injection,
  you can remove and plug the vacuum line to the fuel pressure
  regulator.
  
  3. Test the lean mixture performance as follows:

  A. Induce a small vacuum leak.

  B. If the volt meter rapidly drops to .2 volts or below
  in less than a second, then the oxygen sensor is correctly
  measuring the lean mixture. But, if the volt meter responds
  sluggishly, or if it stays above .2 volts, then the sensor
  should be replaced. 4. Test dynamic performance as follows:

  A. Reconnect the sensor lead.

  B. Set the mixture to specification.

  C. Run the engine at 1500 rpm.

  D. The sensor output should fluctuate around .5 volts. If it
  doesn’t, replace the sensor. One-wire and two-wire “”unheated”
  type oxygen sensors should be checked or replaced every 30,000-
  50,000 miles. These sensors rely solely on hot exhaust gas to heat
  up to operating temperature, and therefore are designed to allow
  a large volume of exhaust gas to make contact with the active
  ceramic element. Therefore, these sensors are exposed to
  contamination. One wire oxygen sensors are typically found on
  pre ’84 Audi’s. “Heated” type oxygen sensors have a built-in heater
  which heats the sensors up to operating temperature. Therefore,
  much less exhaust gas needs to contact the ceramic element,
  making these sensors less prone to contamination. “Heated” type
  sensors can also be located further downstream (closer to the
  catalytic converter), which increases their life expectancy by
  reducing thermal shock. In addition, the latest versions feature
  improved ceramic elements which are more resistant to silicone,
  oil, and lead contamination. “Heated” type oxygen sensors should
  be checked or replaced every 60,000-100,000 miles. Three wire
  sensors are typically found on ’84-onward Audi’s.
  
  Bosch – 3 Wire (fits most Audi’s, O.E. direct fit)
  
  AC-DELCO – (HJS) 3 Wire
  (German mfg., direct fit, O.E. quality)

  Universal – 3-wire oxygen sensor kit
  (splicing required, fits all 3 wire Audi’s)

  Bosch Single Wire