crash course in OBDII sensors

[TonyMazz]

I receive a periodic mailing from the folks at autotap. This one explains at a fairly high level the basics in our OBDII sensors....thought it would be of interest to some.
FYI -


* Oxygen sensors provides information about the fuel mixture. The PCM
(Powertrain Control Module) uses this feedback to constantly re-adjust
and fine tune the air/fuel ratio. This keeps emissions and fuel
consumption to a minimum. A bad O2 sensor will typically make an engine run rich, use more fuel and pollute. O2 sensors deteriorate with age and may be contaminated if the engine burns oil or develops a coolant leak.

* OBDII vehicles have several oxygen sensors. You’ll typically find one in
each exhaust manifold (so on a “V” engine you’ll often find two). You’ll
also find another “downstream” O2 sensor behind the catalytic converter to monitor converter efficiency.

* Though most O2 sensors have no recommended replacement interval (replace "as needed" only), sluggish O2 sensors can be replaced to restore like-new performance. On OBD II vehicles, you should be able to expect 100,000 miles (sometimes more) of life from an O2 sensor.

* The coolant sensor monitors engine temperature. The PCM uses this
information to regulate a wide variety of ignition, fuel and emission
control functions. When the engine is cold, for example, the fuel
mixture needs to be richer to improve drivability. Once the engine
reaches a certain temperature, the PCM starts using the signal from the
O2 sensor to vary the fuel mixture. This is called "closed loop"
operation, and it is necessary to keep emissions to a minimum.

* The throttle position sensor (TPS) keeps the PCM informed about throttle position. The PCM uses this input to change spark timing and the fuel mixture as engine load changes. A problem here can cause a flat spot during acceleration (like a bad accelerator pump in a carburetor) as well as other drivability complaints.

* The Airflow Sensor, of which there are several types, tells the PCM how much air the engine is drawing in as it runs. The PCM uses this to
further vary the fuel mixture as needed. There are several types of
airflow sensors including hot wire mass airflow sensors and the older
flap-style vane airflow sensors. All are very expensive to replace.

* The crankshaft position sensor does two things: It monitors engine rpm and helps the computer determine relative position of the crankshaft so the PCM can control spark timing and fuel delivery in the proper sequence. The PCM also uses the crank sensor's input to regulate idle speed, which it does by sending a signal to an idle speed control motor or idle air bypass motor. On some engines, an additional camshaft position sensor is used to provide additional input to the PCM about valve timing.

The manifold absolute pressure (MAP) sensor measures intake vacuum,
which the PCM also uses to determine engine load. The MAP sensor's input affects ignition timing primarily, but also fuel delivery.

Knock sensors are used to detect vibrations produced by detonation. When the PCM receives a signal from the knock sensor, it momentarily retards timing while the engine is under load to protect the engine against spark knock.

The EGR position sensor tells the PCM when the exhaust gas recirculation (EGR) valve opens (and how much). This allows the PCM to detect problems with the EGR system that would increase pollution.

The vehicle speed sensor (VSS) keeps the PCM informed about how fast the vehicle is traveling. This is needed to control other functions such as torque converter lockup. The VSS signal is also used by other control
modules, including the antilock brake system (ABS).