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Crash Course on OBDII sensors
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rpatzer Offline

Posts: 348
Joined: Apr 2008
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Post: #1
Crash Course on OBDII sensors
I thought this article was worth posting:

After last week's little diversion into emissions testing, its time to get back into some meaty info on how things work under the hood. I'd thought we'd use today's email to describe many of the sensors you'll find under the hood of an OBDII vehicle, and what they do. All the built-in diagnostics of OBDII really won't help you fix anything unless you have some basic knowledge of these sensors to do some common-sense type troubleshooting. Let's dive right in.

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).

How's that for a crash course in OBDII sensors? While there can be a few more sensors under the hood, these are the ones that really effect how well your car runs. Keep this info in mind the next time your car stumbles, hiccups or sets a trouble code. A little bit of information goes a long way towards knowing what to fix. For $199, a DIYer armed with AutoTap and some basic info on these sensors can make quick work of the most common problems on today's OBDII vehicles!

Happy Repairs,

Mike Fahrion

Copyright 2006 B&B Electronics
07-03-2008 06:19 AM
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skiers4ever Offline

Posts: 160
Joined: May 2008
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Post: #2
RE: Crash Course on OBDII sensors
Loved it Mike! Thanks!!!! John

2007 Honda Ridgeline 3.5L 4WD
Hydro Super 2 installed on 5/21/08
Colorado - 10,000 Feet!
07-03-2008 10:40 AM
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Walt Offline

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Joined: Jun 2008
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Post: #3
RE: Crash Course on OBDII sensors
Nice article.

On an OBD2 both the O2 and MAP sensors both have a fueling say. Do you know at what RPM range the MAP has a say. My guess is the higher RPMs and highway speeds. When I mess with the O2 sensor I loose low end power and am unchanged on the higher end. I don't see anyone making a EFIE/MAP enhancer yet.
07-03-2008 12:04 PM
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