Knowledgebase: General
OBD-2 Sensors
Posted by Alex (Im) E. on 01 February 2013 12:43 AM


Usually located on the cylinder head or intake manifold, this sensor is used to monitor the temperature of the engine's coolant. It's resistance changes in proportion to coolant temperature. Input from the coolant sensor tells the computer when the engine is warm so the PCM can go into closed loop feedback fuel control and handle other emission functions (EGR, canister purge, etc.) that may be temperature dependent.

The coolant sensor is a pretty reliable sensor, but if it fails it can prevent the engine control system from going into closed loop. This will result in a rich fuel mixture, excessive fuel consumption and elevated carbon monoxide (CO) emissions -- which may cause the vehicle to fail an OBD II emissions test.

A bad coolant sensor can be diagnosed by measuring its resistance and watching for a change as the engine warms up. No change, or an open or closed reading would indicate a bad sensor.



The oxygen (O2) sensor is the key sensor in the fuel mixture feedback control loop. Mounted in the exhaust manifold, the O2 sensor monitors the amount of unburned oxygen in the exhaust. On many V6 and V8 engines, there are two such sensors (one for each bank of cylinders).

The O2 sensor generates a voltage signal that is proportional to the amount of unburned oxygen in the exhaust. When the fuel mixture is rich, most of the oxygen is consumed during combustion so there is little unburned oxygen in the exhaust.

The difference in oxygen levels between the exhaust inside the manifold and the air outside creates an electrical potential across the sensor's platinum & zirconium tip. This causes the sensor to generate a voltage signal.

The sensor's output is high (up to .9v) when the fuel mixture is RICH (low oxygen), and low (down to .1v) when the mixture is LEAN (high oxygen).

The sensor's RICH/LEAN signal is monitored by the computer and is used to rebalance the fuel mixture for lowest emissions. When the sensor reads lean the PCM increases the on time of the injectors to make the fuel mixture go rich, and when the sensor reads rich the PCM shortens the on time of the injectors to make the fuel mixture go lean.

This causes a rapid back-and-forth switching from rich to lean and back again as the engine is running. The net result is an "average" mixture that is almost perfectly balanced for clean combustion. The rate at which this happens is slowest in older throttle body injection systems, and fastest in multiport sequential fuel injection.

If the O2 sensor's output is monitored on an oscilloscope, it will produce a zigzagging line that dances back and forth from rich to lean. Think of it as a kind of heart monitor for the engine's air/fuel mixture.

The sensor's responsiveness and voltage output can diminish with age and exposure to certain contaminants in the exhaust such as lead, sulfur, silicone (coolant leaks) and phosphorus (oil burning).

If the sensor becomes contaminated, it may not respond very quickly to changes in the air/fuel mixture causing a lag in the PCM's ability to control the air/fuel mixture. The sensor's voltage output may also decline giving a lower than normal reading.

This may cause the PCM to react as if the fuel mixture were leaner than it really is resulting in an overly rich fuel mixture.

How common is this problem? One EPA study found that 70% of the vehicles that failed an I/M 240 emissions test needed a new O2 sensor.

Checking the health of an O2 sensor is a job that requires a scan tool or oscilloscope.

The other alternative is to simply replace the O2 sensor for preventive maintenance. Unheated 1 or 2 wire wire O2 sensors on pre-OBD II 1976 through early 1990s applications should be replaced every 30,000 to 50,000 miles to assure reliable performance.

Heated 3 and 4-wire O2 sensors on pre-OBD II mid-1980s through mid-1990s applications should be changed every 60,000 miles. And on 1996 and newer OBD II vehicles, the recommended replacement interval is 100,000 miles.



This sensor is mounted on or connected to the intake manifold to monitor intake vacuum. It changes voltage or frequency as manifold pressure changes. The computer uses this information to measure engine load so ignition timing can be advanced and retarded as needed. It performs essentially the same job as the vacuum advance diaphragm on an old fashioned mechanical distributor.

On engines with a "speed density" type of fuel injection, the MAP sensor also helps the PCM estimate airflow.

Problems here may cause an intermittent check engine light (light comes on when accelerating or when the engine is under load), hesitation when accelerating, elevated emissions and poor engine performance.

The engine will run with a bad MAP sensor, but it will run poorly. Some PCMs can substitute "estimated data" for a missing or out of range MAP signal, but engine performance will be drastically reduced.

Some MAP sensor problems are not the fault of the sensor itself. If the vacuum hose that connects the MAP sensor to the intake manifold is loose, leaking or plugged, the sensor can't produce an accurate signal.

Also, if there is a problem within the engine itself that causes intake vacuum to be lower than normal (such as a vacuum leak, EGR valve that's stuck open, or leaky PCV hose), the MAP sensor's readings may be lower than normal.



Mounted on the throttle shaft of the carburetor or throttle body, it changes resistance as the throttle opens and closes.

The computer uses this information to monitor engine load, acceleration, deceleration and when the engine is at idle or wide open throttle. The sensor's signal is used by the PCM to enrichen the fuel mixture during acceleration, and to retard and advance ignition timing.

Many TPS sensors require an initial voltage adjustment when installed. This adjustment is critical for accurate operation.

On some engines, a separate idle switch and/or wide open throttle (WOT) switch may also be used. Drivability symptoms can be similar to those caused by a bad MAP sensor.

The engine will run without this input, but it will run poorly.



Mounted ahead of the throttle body on multiport fuel injected engines, this sensor monitors the volume of air entering the engine.

The sensor uses either a hot wire or heated filament to measure both airflow and air density.

The sensing element in a MAF sensors can be easily contaminated causing hard starting, rough idle, hesitation and stalling problems.



A mechanical flap-style sensor that is used on Bosch and other import multiport fuel injected engines, this sensor also measures airflow.

The function is the same as a mass airflow sensor, but air pushing against a spring-loaded flap moves a rheostat to generate an electronic signal.

The drivability symptoms are the same as those of a mass airflow sensor if the sensor fails.



Mounted on the intake manifold, this sensor changes resistance to monitor incoming air temperature.

The sensor's input is used to adjust the fuel mixture for changes in air density.

Problems here can affect the air/fuel mixture, causing the engine to run rich or lean.



Used on engines with distributorless ignition systems, the crankshaft position sensor serves essentially the same purpose as the ignition pickup and trigger wheel in an electronic distributor. It generates a signal that the PCM needs to determine the position of the crankshaft and the number one cylinder.

This info is necessary to control ignition timing and the operation of the fuel injectors. The signal from the crank sensor also tells the PCM how fast the engine is running (engine rpm) so ignition timing can be advanced or retarded as needed.

On some engines, a separate camshaft position sensor is also used to help the PCM determine the correct firing order. The engine will not run without this sensor's input.

There are two basic types of crankshaft position sensors: magnetic and Hall effect. The magnetic type uses a magnet to sense notches in the crankshaft or harmonic balancer. As the notch passes underneath, it causes a change in the magnetic field that produces an alternating current signal.

The frequency of the signal gives the PCM the information it needs to control timing. The Hall effect type of crank sensor uses notches or shutter blades on the crank, cam gear or balancer to disrupt a magnetic field in the Hall effect sensor window.

This causes the sensor to switch on and off, producing a digital signal that the PCM reads to determine crank position and speed.

If a crank position sensor fails, the engine will die.

It will crank but won't start. Most problems can be traced to faults in the sensor's wiring harness.

A disruption of the sensor supply voltage (Hall effect types), ground or return circuits can cause a loss of the all-important timing signal.



Detects engine vibrations that indicate detonation is occurring so the computer can momentarily retard timing.

Some engines have two knock sensors. A failure here can cause spark knock and engine damaging detonation because the PCM won't know to retard ignition timing if knock is occurring.



On engines with EGR (exhaust gas recirculation) valves, the valve may be operated by vacuum, by a small electric motor or by pulsing solenoids to open and close the valve.

The EGR valve opens a port between the intake and exhaust manifolds so intake vacuum can suck a small amount of exhaust back into the engine. This reduces combustion temperatures to reduce the formation of oxides of nitrogen (NOX).

Various means may be used to monitor the operation of the EGR valve. The PCM may check the MAP sensor when the EGR valve is commanded open to see if there is a drop in intake vacuum (there should be if the EGR valve is opening).

On other applications, the EGR valve may be equipped with a position sensor to signal the PCM when the valve is open or closed. The valve position sensor works like a throttle position sensor and changes resistance. The voltage signal typically varies from 0.3 (closed) to 5 volts (open).



Monitors vehicle speed so the computer can regulate torque converter clutch lockup, shifting, etc.

The sensor may be a magnetic pickup or a Hall effect pickup. The magnetic type generate an alternating current signal that increases in frequency and amplitude with speed. The Hall effect type generate a digital square wave signal that increases in frequency with speed.

The vehicle speed sensor sensor may be located on the transmission, differential or transaxle.

Magnetic sensors sometimes attract metal particles that stick to the tip and reduce the strength of the signal. A problem here can disable the cruise control system as well as affect transmission shifting and converter engagement.



The important thing to remember about all of these different types of sensors is the basic GIGO rule: garbage in, garbage out.

If a sensor signal is missing or is reading out of its normal range, it will usually (but not always) set a fault code.

But a sensor code does not always mean the sensor itself needs to be replaced. Often the problem is elsewhere in the sensor's wiring circuit. For this reason, accurate diagnosis is a must to rule out other possibilities.

Replacing a sensor won't solve a drivability or emissions problem if the problem isn't the sensor.

Common conditions such as fouled spark plugs, bad plug wires, a weak ignition coil, a leaky EGR valve, vacuum leaks, low compression, dirty injectors, low fuel pressure or even low charging voltage can all cause drivability symptoms that may be blamed on a bad sensor.

If there's are no sensor-specific fault codes, these kinds of possibilities should be ruled out before much time is spent on electronic diagnosis.


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