Automotive Diagnostic OBD2 Scanner

The most important OBD II sensors found in all modern engines.

Oxygen Sensors (O₂)

The oxygen sensors determine the unburnt oxygen found in the exhaust, providing the data to ECU to regulate the fuel mixture (lean or rich).


O₂ malfunction symptoms:

• Poor Fuel Economy
• Engine running rich on fuel (black soothed plugs)
• Fails the smog test
• ‘Check Engine’ (MIL) ON

Diagnosis & Repair:

1.	Run the engine to approximately normal working temperature.
2.	Rev up and keep the engine about 2000 RPM.
3.	Test the front catalytic converter sensors by graphing the O2 sensor output. There should be a constant oscillation on the graph of about -0.2 volts to +0.8 volts (sine wave format graph). The graph wave should also respond to sharp throttle revving. If the graph doesn’t show the variation readings -0.2 to +0.8 volts, and nothing seems to happen with some quick engine rev blasts, during the test — the sensor is bad.
4.	Test the rear catalytic converter sensors with the same method above. These sensors should keep a steady constant reading of approximately +0.5 volts. Even with sudden throttle revs the reading should vary very fractionally with no sudden spikes. If the sensor reading is near the 0 volts reading — the sensor is bad. If the sensor seems to follow the front sensor oscillations the sensor is good but the catalytic converter is bad.

Important: Never assume a sensor is bad just by running a scantest only — or worst by the common fault-elimination presumption . Test each sensor individually.

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Mass Air Flow Sensor (MAF)

The MAF sensor measures the amount of air flowing in the engine.


MAF malfunction symptoms:

• Engine starts to misfire
• Rough idle
• Power loss while pressing the gas pedal
• Occasionally stalling

Diagnosis & Repair:

1.	Check all air ducts and hoses for any air leaks. This is the most common MAF related trouble codes as ducts come loose through engine vibration and pipes perish due to continuous engine heat and oil contamination.
2.	Run the MAF sensor through the diagnostic OBD II scanner and check the voltage reading. A steady voltage supply should be visible with the engine running at idle speed.
3.	Check if the sensor is defective or on its way out by tapping the sensor housing gently. If sudden voltage spikes changes when doing so the sensor is likely to be bad. A good sensor should not react to vibration — voltage should remain unaltered.
4.	Rev the engine and check that the graph (voltage) rises smoothly and in proportion to the rising revs. Any notable sudden discrepancy, or slow reaction in proportion to the revs means the sensor is either bad, or contaminated with debris. Before replacing the sensor try any clean them with care with electronics component cleaner or special automotive sensor spray. [The electronics spray is more gentle on the sensor — even though it may take more than just one cleaning attempt. Do not be tempted by using any harsh solvents.

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Intake Air Temperature Sensor (IAT)

The IAT sensor measures the incoming air temperature and sends this data to the ECU management system. This data is used by the Obd Software to calculate the exact air density and fine-tunes the air to fuel mixture. The reading when driving is normally somewhere near the approximate outdoor temperature.


IAF malfunction symptoms:

• Rough idling when cold
• Stalling
• Poor fuel economy

Diagnosis & Repair:

1.	Check for loose plug terminal connection of the IAF sensor. Also check that the terminal connection points are clean by unplugging it for inspection. Once connected start the engine and leave on idle. Wiggle the plug gently and repeat with the wire. See if any noticeable variations occur. [A common occurrence seems to be that the plug will be forgotten during service and left unplugged]
2.	Certain modified air-filters (pancake, open style without proper air-box) can easily unbalance the IAT sensor limitations and give partially false lean readings. The software in turn gives an over-rich fuel mixture to compensate for the lean situation.

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Throttle Position Sensor (TPS)

The TPS gives the exact opening position of the throttle body.


TPS malfunction symptoms:

• Hesitation when trying to accelerate
• Stumbling

Diagnosis & Repair:

1.	Turn the ignition key ON with the engine OFF and measure the TPS voltage reading. Gradually press the accelerator pedal until full throttle is achieved. The voltage should rise smoothly without and sudden abrupt readings — positive or negative.
2.	Check for any visual wear on the throttle body moving parts and the TPS sensor. The sensors usually wear just outside the idling position. Replace if found defective.

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Engine Coolant Temperature Sensor (ECT)

ECT measures the engine coolant’s temperature.


ECT malfunction symptoms:

• Bad overall running performance
• Poor fuel economy
• Overheating (noticeable lack of performance once over normal operating temperature)

Diagnosis & Repair:

1.	Leave the car cool down completely, preferably over-night.
2.	Check initial temperature with diagnostic scanner (if function is available on particular scanner).
3.	Start engine — after a few moments the temperature should start increasing. If scanner or reader function is unavailable, it is possible to check resistance smooth variation with a multimeter. Reading variation should be similar to a warming engine reading in Degrees Celsius / Fahrenheit.

* * If in any doubt just go and change the sensor. These are normally inexpensive and is a blind safety precaution. While I do not usually suggest changing anything without proper testing and diagnosis — this is an exemption of the rule. It is much safer than overheating and damaging your engine. * *

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Exhaust Gas Recirculation (EGR)

* * Valve not Sensor * *
The EGR is a vacuum or electrically controlled valve that mixes a controlled amount of exhaust gas with the intake air to control the combustion process better.


EGR malfunction symptoms:

• Detonation under acceleration
• Fails the Smog Test

Diagnosis & Repair:
Only on vacuum controled EGR valves:

1.	Start engine and leave it idle for a few seconds.
2.	Manually open the EGR valve by hand and check MAF and, or O2 sensor readings. If the do not react or react very sluggishly the EGR valve passageways are blocked with carbon.
3.	Clean the EGR passageways.
4.	Retest after cleaning up and installing the EGR valve back in position.