Once cleared, only Chrysler can retrieve codes that were stored previously so they can prove which ones were set and repaired. That is only done, very rarely, to prove a problem existed previously under warranty, or in the case of air bag computers, they can prove you were within the speed limit when the bag deployed. Every computer can store diagnostic fault codes. Engine codes can be displayed in the odometer readout. All other computers need a scanner that can communicate with them to get their codes. Codes can be stored in the ABS, Air Bag, Body, Transmission, and most other computers.
No one has ever asked before how many codes could be stored. On very early rear-wheel anti-lock brakes, only one code out of the nine possible could be stored at once. When that problem was repaired and the codes was erased, a new code could pop up if there was a second problem. On newer computers you'll rarely get more than two or three codes at once unless the cause affects multiple things. A broken common feed or ground wire to multiple sensors is a good example. Disconnecting things while the ignition switch is on will also set multiple codes.
It's very important to not disconnect the battery or let it run dead until those codes have been read and recorded, otherwise that valuable diagnostic information will have been lost.
One strategy of determining when to set a code has to do with reconciling sensor readings with each other. For example, the computer knows that when the engine has been off for six hours, the coolant temperature sensor and the intake air temperature sensor had better be reading the same temperature. If there is a fault code stored for one of them, it will not store a code for the other one because it has nothing reliable to compare it to. That's a generalization though. There are multiple tests each sensor must continually pass. A defective sensor can give a wrong reading but as long as it's within the acceptable range, no code will be set. That acceptable range is typically 0.5 to 4.5 volts. Unplugging a sensor will usually send the signal volts to 0.0 volts or 5.0 volts. Those are definite problems that will set a code even when there are already codes stored for the things the computer compares them too.
I doubt any computer ever stored 99 codes at once. There are hundreds of possible codes that can be set, but many of them require the engine to be running for the defects to show up. If there was so much wrong that it wanted to store 99 codes, there would be way too many problems for the engine to run. Ten codes is well within the realm of possibility, but that is a lot.
Codes don't stay in memory either once the problem is fixed or doesn't act up again. When the problem is no longer detected, most codes will erase automatically after 50 engine starts. Here again, for intermittent problems, it's important to have the codes read and recorded before they erase.
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Sunday, February 5th, 2012 AT 7:19 PM