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All Info About Auto Repair: Using Powertrain Computer Scan Tools
All Info About Auto Repairs
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Using Powertrain Computer Scan Tools

You have a new scan tool, so here is how to get the most from it.
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  • Drivability Problems. A scan tool will often tell you if the computer has the latest software and hardware and if a software upgrade will cure a problem. Also, if you have access to Technical Service Bulletins (TSB) you can see if there is an upgrade specific to the problem you're having.
  • Hesitation While Accelerating. With the scan tool in Freeze Frame mode you can see if a component, a Throttle Position Sensor (TPS) for example, is showing a drop to zero at the point the hesitation is felt. If this is the case, the computer will think the vehicle is not running and shut off the fuel momentarily.
  • Decreased Fuel Mileage. You can see if the Coolant Temperature Sensor (CTS) is reading accurately. If it is reading cold and the engine is warm the computer will richen the air/fuel mixture.

This can carbon foul the spark plugs and decrease fuel economy. It can also cause your vehicle to fail an emissions test.

  • Faulty Oxygen Sensor. An Oxygen Sensor (HO2S) operates in the .1 to .9 volt range, unlike the rest of the system sensors that operate in the 0 to 5 volt range. The HO2S is designed to measure the amount of oxygen in the exhaust. The too much oxygen in the exhaust indicates a lean fuel mixture while too much indicates a rich mixture. If the PCM sees too much oxygen in the exhaust, it will richen the mixture to compensate. Too much oxygen and the PCM will lean out the mixture.


    A properly operating HO2S will show a fluctuating voltage reading from .1 volts to .9 volts. It is looking for the perfect balance of .5 volts. So a normal HO2S will range up and down from .1 to .9. It is constantly changing trying to get to the .5 volt mark. If the HO2S stays too low for an extended period of time, the PCM will keep richening the mixture causing a decrease in fuel economy and possibly damaging the catalytic converter. It will not set a code, but it could cause your vehicle to fail an emission test. If it stays too high for a time, it could cause poor acceleration and a hesitation.

  • Vehicle Speed Sensor. The Vehicle Speed Sensor (VSS) tells the PCM, and in many cases the speedometer and cruise control, how fast the vehicle is actually going. If it is telling the PCM your vehicle is going faster than it actually is, the PCM could turn off the cooling fans and allow the vehicle to running too hot and possibly overheat.


    It can also cause the torque converter clutch to engage prematurely, which will result in hesitation and/or poor acceleration. A good clue is having the cop tell you that you were going 15 over the limit when you were only 10 over.

  • Knock Sensor. The Knock Sensor (KS) detects detonation, or more commonly called pinging. When the computer sense this it will retard the ignition timing so performance and fuel mileage will drop. You will need to determine if you are using too low an octane fuel or if the Knock Sensor is bad.
  • Mass Airflow Sensor. The Mass Airflow Sensor (MAF) tells the computer how much air is entering the engine. It is measured as pounds per minute, or on some scan tools grams per second. If it is sending a low air flow to the computer you will get a moderate to severe hesitation.

Most shops and dealers will charge up to $150.00 to hook up a scan tool and run a diagnostic. This makes buying a code reader or scan tool a viable option for the DIY. You will need to learn how to use your scan tool, there is a definite learning curve, but now that scan tools have become more affordable and easier to use, this is a good time to start using them yourself.

» Part 1       » Part 2       » Part 3       » Part 4
Additional Information provided courtesy of
ALLDATAdiy.com and Warranty Direct
© 2000-2007 Vincent T. Ciulla

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