Tip & How-To about Mercedes-Benz E-Class
1.12) Oxygen
(Lambda) sensor
What is it?
This is an electrical device that measures electronically the amount of
oxygen in the exhaust.
Where is it located? The device looks, externally,
a bit like a spark plug set into the exhaust manifold. It is screwed into the manifold and is
identified by a thick wrap cable connected to its top.
How does it work? There are generally
two types of oxygen sensor. The first
has only one wire to it. This is the voltage output wire, the metal body of the
sensor provides the 'ground' in the circuit.
The second type of sensor has four wires within the thick cable. Two of these wires generally of the same
colour (most often white) are part of electrical heater circuit built into the
body of the oxygen sensor (it only works when very hot). The other two wires (often grey and black) are
connected to the oxygen sensing element and these send a cycling voltage output
(0.1 to 0.9 volts) inversely proportionate to the oxygen levels detected, back to
the ECU. If the engine is running very
lean the oxygen levels in the exhaust are high and the sensor has low voltage; conversely
if the engine runs rich the oxygen levels are low and the voltage output of the
sensor is high.
On some systems there
are two oxygen sensors, one on the exhaust manifold before the catalytic
convertor, monitoring engine operation and one after the catalyst that measures
the catalyst performance. With twin separate
exhausts as with V6 and V8 engines it is possible to have four oxygen sensors.
Symptoms of faulty oxygen sensor
Associated OBD2 error codes DTCs: P0130 - P0167
Although perhaps seen as the last element in the
chain of engine feedback sensors the significance of a faulty oxygen sensor
should not be overlooked as it exerts a very strong influence on the ECU:-
·
Fast/erratic idle, poor fuel economy - if the oxygen sensor, in error, reports to the ECU that the engine is
running lean (oxygen levels are high) the ECU may respond by enriching the fuel
mix. This causes the engine to have a
lumpy idle at a higher than normal rate and also increases fuel consumption. (see additional note below)
·
Hesitation and surging - signals from
the oxygen sensor continue throughout the engine performance range so fault
issues that manifest themselves at idle will occur at all engine speeds with
performance consequences.
·
Misfire and stalling - if the oxygen
sensor reports that the exhaust oxygen levels are too low (i.e. engine running
rich) the fuel mix might be reduced, through reduced injection times, to the
point at which the engine misfires or stalls from being made too lean.
NEXT 1.12b) How to check and fix oxygen sensors
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