466 Auto Electricity and Electronics
A vehicle speed sensor is tested in much the same way
as an engine speed sensor. Sensor resistance or voltage out-
put is usually measured and compared to specs.
Oxygen (O
2
) Sensor Service
A bad oxygen or O2 sensor will primarily upset the fuel
injection system or the computerized carburetor system.
The voltage signal from the sensor represents air-fuel ratio.
If the oxygen sensor produces a false output (incorrect volt-
age), the computer cannot precisely control how much fuel
is metered into the engine. A rich mixture and high fuel
consumption normally results from a lazy oxygen sensor
signal, but a lean misfi re condition can also result.
O
2
Sensor Contamination
Normally, oxygen sensors are designed to last about
50,000 miles (81,000 km). However, its life can be short-
ened by contamination, blocked outside air, shorts, road
contaminants, and poor electrical connections. Oxygen
sensor contamination can be caused by:
❑
Leaded fuel.
❑
Silicone.
❑
Carbon.
Leaded fuel is the most common cause of oxygen sensor
contamination. Lead coats the ceramic element, and the sen-
sor cannot produce enough voltage output for the computer.
Silicone forms a glassy coating on a sensor. Sources of
silicone are antifreeze, volatile RTV silicone sealers, water-
proofi ng sprays, and gasoline additives.
Carbon contamination results from rich fuel mixtures.
Carbon in fuel coats the sensor. Carbon and moderate lead
contamination can sometimes be reversed. Run the engine
at high speeds with a large vacuum hose (PCV hose for
example) removed and with only unleaded fuel in the tank.
This will sometimes burn off light lead and most carbon
deposits. The sensor may start working normally again.
O
2
Sensor Inspection
Also, check that the outside of the sensor and its elec-
trical connection are free of oil, dirt, undercoating, and
other deposits. If outside air cannot circulate through the
oxygen sensor, the sensor will not function.
An oxygen sensor only generates a tiny voltage (an
average of about 0.5 volts). A poor electrical connection
can prevent this small voltage from reaching the computer.
Always check the sensor’s electrical connections.
O
2
Sensor Testing
Discussed earlier, most computer systems will produce
a trouble code indicating when the output from the oxygen
sensor is not within normal parameters. This would tell you
to do further tests on the oxygen sensor and its circuit.
Many computer systems have a limp-in mode. If the oxy-
gen sensor or some other sensor fails and produces an incorrect
output, the system will go into this emergency limp-in mode.
A predetermined oxygen sensor voltage (0.5 volts for example)
or other output will be simulated by the computer and used to
keep the engine running well enough to drive in for repairs.
Remember from the previous chapter that some ana-
lyzers will read oxygen sensor voltage when connected to
the computer system. This allows you to check O2 sensor
output without removing the wires to the sensor. This is
also true for some other sensors.
A digital voltmeter can also be used to test the output
of an oxygen sensor. Warm the engine to full operating
temperature to shift the computer system into closed loop
mode. The sensor must be hot (about 600°F or 315°C) to
operate properly. You may have to warm the engine at fast
idle for up to 15 minutes with some cars. Note that a few
systems can drop out of closed loop mode at idle.
Caution: Only use a high-impedance, digital
meter to measure oxygen sensor voltage. A
conventional analog or low-resistance meter
can draw too much current and damage the O2 sensor.
Unplug the sensor leads. Then, connect a digital volt-
meter to the oxygen sensor, as shown in Figure 24-7.
O
2
Sensor Output Voltage
Oxygen sensor output voltage should cycle up and
down from about 0.2 volts (200 mV) to 0.8 volts (800 mV),
Figure 24-8. A 0.2 volt or low reading would show a lean air-
fuel ratio condition, and a 0.8 volt or high reading would show
a rich condition. A high or low reading does not always mean
the O2 sensor is bad. Another problem, such as leaking or
clogged fuel injector, could make the sensor read high or low.
A quick test to see if the oxygen sensor reacts to a
change in air-fuel mixture is to pull off a large vacuum hose,
like the PCV valve. This extra air should make the oxygen
sensor try to enrich the fuel mixture and compensate for the
air leak (lean condition). The output voltage should then go
down (to about 0.3 or 0.4 volts) to signal a need for more
fuel to adjust for the vacuum leak or extra air.
When the engine throttle is snapped open and closed,
O2 sensor output should also cycle up and down to show
the change in air-fuel mixture.
If you block the air inlet at the air cleaner or inject pro-
pane gas into the air inlet (creating a rich mixture), the oxy-
gen sensor voltage should increase (go up to about 0.8 to 0.9
volts). It should try to signal the computer that too much fuel,
or not enough air, is entering the combustion chambers.
If the oxygen sensor voltage does not change properly
as you simulate rich and lean air-fuel ratios, the oxygen
sensor is faulty. You might try running the engine at high
speeds with a large vacuum hose removed to clean off light
lead or carbon contamination.
A faulty oxygen sensor will usually be locked at one
voltage output level and will not cycle voltage up and down
normally. It also may not produce enough voltage.