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Unread 06-07-2009, 09:55 AM   #1
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Default How good are fault code readers?

I've heard that all readers do is tell you what IS happenning and not WHY.

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Unread 06-07-2009, 10:14 AM   #2
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Originally Posted by soccerboss View Post
I've heard that all readers do is tell you what IS happenning and not WHY.
Yes but once you have the code you can go online and find a list of possible causes, what the code does is gives you a general area to start looking for the problem.

Some codes are more specific than others. If you get a code that says your throttle positioning sensor is bad, it is probably bad. Another code might tell you that your fuel system has an air leak somewhere. That sort of code could be any number of problems in the fuel system from a bad fuel cap, to an actual rupture in a line.

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Unread 06-07-2009, 04:29 PM   #3
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Any code reader can only tell you what the code is that is set, not WHY it was set.

If you get a code for a (insert desired device here) isn't a 100% indication that it has failed. The code will be set when the reading is out of range. Devices are expected to produce readings that make sense to the ECM/PCM, and if it doesn't see it, it sets a code for that device or system.

If the normal reading for a particular sensor is say, 100 - 1200 ohms, and the the wire to the sensor has rubbed through and is "open" the sensor's reading is "infinity" and thus out of range. If the wire is shorted to ground, the sensor's reading is "zero" and out of range. Also if the sensor is reading 1500 ohms it's out of range.

So if your scanner simply says "TPS out of range" what do you replace? The correct action is to confirm the readings by using a Volt Ohm Meter (VOM) by checking the wires from the device to the ECM/PCM. Yes you can just by a TPS and plug it in, but if you are wrong you are likely stuck with the part, and still have the same problem. The ECM/PCM can't tell you if the device has an open or the wires going to it have an open, it's up to you to figure that out.

Also some devices may be out of range do to something else. For example, a leak in an air intake boot may cause a code for a MAF out of range. The ECM/PCM has determined that the air going through the MAF is less than it should be, based on calculations of RPM & throttle position, and throws a code for MAF out of range. So what do you replace in that case?

If your scanner has data stream reading reading abilities you can sometimes watch other reading to see if you can tie in another system that's suspicious.

For example, a coolant temp sensor (CTS) that always reads 60* and never changes may "seem" to be correct according to the scanner, but the engine is always running rich because the ECM/PCM thinks it's still warming up. Of course, you know that this is not the case.

I believe that you need a scanner that reads data stream and not simple codes. You also need to understand that the scanner CANNOT tell you what to replace.
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Unread 06-07-2009, 04:42 PM   #4
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Yeah, the scanner is a tool in your toolbox, not a portable mechanic. It simply gives you an idea of what to look for by giving you fairly specific symptoms. (i.e. you think it's running rough, you hook up your scanner, and it tells you that your first cylinder is misfiring. You still don't know the problem, but you know exactly what to look for - a spark plug, wire, coil, etc to the first cylinder.)
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Unread 06-07-2009, 04:56 PM   #5
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DTC DIagnostic Trouble Code not what's broken from the code you perform a flow chart to crack tjhe problem back to the source true sometimes you can guess and get lucky but you can also be wrong.
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