car computer reset

jemdandy

17 years ago

Maybe there was trouble code set when the old alternator went out, and the code was not cleared when the new alternator was installed. I would have thought that the old code would have been cleared when the problem was resolved, but maybe not. Some codes require up to 50 starts to clear.

Why not have the codes read and if there is one associated with a failed alternator, clear it. A number of simple code readers have the capability to clear some, but not all codes.

In reading the codes, you may find the one the testing station is objecting to. Did they say which code was set, or that only a code was set and did not identify it?

john_g

17 years ago

Your truck should pass the emissions test with two monitors still not set. Plus, your truck should easily pass all but one of the monitors in a single road test of fifteen to twenty miles that is performed under varying driving conditions.

The "drive cycle" is the routine that makes you operate the truck in each of the specific conditions that allow the individual tests which make up the monitors to run. You can google "drive cycle" and your trucks make, model, and year, and print out the routine yourself to follow. A monitor can consist of anywhere from one to a group of individual tests. If one of the individual tests does not run, then the monitor does not complete, and that's when you end up in the situation you find yourself in. Each individual test has its own operating criteria. Engine speed, engine temperature, vehicle speed, are all important details. But additionally, certain vehicle problems can cause specific tests to not run. These are known as "Blocking Conditions". An example would be if the engine temperature isn't high enough, then the fuel control monitor won't run. An engine missfire when detected could block the fuel control monitor, the O2 sensor performance test of the O2 monitor, and the catalytic converter monitor.

With all of this being said, the first thing we need to see now is what monitors are completing, and which ones are not. Then with the next key on we need to see if the completed monitors cleared, which can happen if a problem with the PCM's keep alive memory has occurred. If monitors that did complete did not clear, then the tech simply needs to concentrate on the monitors that did not complete and see if there is a blocking condition occurring, and if so repair that problem so the vehicle's computer can complete its self testing. Its not uncommon to find that once we get the blocking condition if one exists solved we often find the vehicle finally run all of its monitors, and fail one of them and turn the check engine light on and set a code.

Be glad you have a 96, take it from me this system is very forgiving and relatively easy for a properly trained and equipped technician. Today's cars are much, much harder to solve these kinds of problems with.

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The OBD (On Board Diagnostic) port only works as well as the cars software at reporting problems. In many cases the diagnostic tree starts with getting an error code, and then making sure the sensor itself is operating correctly. If the sensor is bad, any input it supplies is suspect, and the computer may create other problems that appear only vaguely related to the sensor problem. A bad throttle position sensor can cause the conptuer to alter the engine air-fuel mixture, or a bad oxygen sensor can result in changes in air-fuel also (producing all the symptoms of that problem). It is not like the computer always identifies the defect, it just might say that the oxygen sensor is 'out of expected range.'

jemdandy

17 years ago

John_G:

The reason I suggeested reading the trouble codes is that I did not know which state that truck was in, and that states have different testing requirements, futhermore, it would be good to know which trouble codes were set. My state has reduced the test requirement to first, reading the OBD2 codes and if one is set, they say "Go away, fix the problem, and then come back." And they do not necessarily tell you which code was set. Its an exesperating situation.

However, in this case, I do agree that it is a good idea to take this vehicle to a qualified diagnostic technican.

john_g

17 years ago

Hi Jem.

Cindip did not specify that the check engine light has come back on indicating that there is a trouble code setting. That suggests that there is a condition that is either blocking a test, or a group of tests from completing which could potentially result in a code setting if they did, or the computers memory is simply being cleared each time the truck is turned off.

Every trouble code that can set actually relates to a monitor in the diagnostic system. Think of each monitor as a test, and the generation of a trouble code is the score that the vehicle got on a particular test.

car computer reset

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