Snip:

The so called lambda sensor is an exhaust gas oxygen sensor which is
utilized in motor vehicles with three-way catalysts for the reduction of
pollutants in the exhaust gas. It monitors the concentration of oxygen in
the exhaust gas in order to keep the air-fuel ratio within tight limits,
close to the optimum catalyst efficiency which is at the stochiometric ratio
for complete combustion of about 14.7 kg air per 1 kg gasoline.

The lambda sensor utilizes quite the same solid state electrolyte material
that was used in the Nernst lamp (yttrium stabilized zirconia). It is
characterized by its high conductivity for O2- ions at elevated temperatures
(about 700 K).

Usually, at the heart of the sensor is a hollow cylinder of solid
electrolyte. The inner and outer walls are covered with microporous platinum
electrodes. The outer wall is immersed in the stream of exhaust gas whereas
the inner wall is exposed to ambient air whose oxygen content of about 21 %
by volume serves as a reference.

For the reduction reaction of oxygen

O2 + 4 e- = 2 O2-

according to the Nernst equation the sensor voltage is approximately

Us = (R T / 4 F) ln [p(O2, reference) / p(O2, exhaust)]

(temperature T, partial pressure p, gas constant R, Faraday constant F)

The largest manufacturers of lambda sensors are Bosch, Germany and NTK,
Japan

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Its probabaly either the sensor or the CAT thats at fault, the CAT should be
able to be tested on the car to determine if its faulty, its always woth
driving the car at high speed for 20 to 30 miles just before you take it in
for its test, helps warm up the CAT & cleares some of the deposits that
build up in it from short runs,

Steve68s.