Could someone please explain how the "freeze frame"(I hope the is the correct term) works? I was under the impression that if the "Check engine lite" came on, a snapshot of engine parameters that existed at the time of the fault, was stored as a freeze frame and could be looked at later. The dealer's service desk said that the freeze frame is lost when the engine is shut down. Is this correct? If so, when the check engine lite comes on, must the engine not be shut off until a reading is made?
Freeze frame data supplies very little information..... mostly fuel trim status, vehicle speed and engine load.... for misfires, there is a little more information stored. Freeze frame data is cleared when continuous memory codes are cleared. Depending on the DTC(s) that are stored in your PCM, the freeze frame data is, quite possibly, useless to you and can be hard to form an accurate diagnosis of any problem - there are way to many things that can affect fuel trims.
I rarely find any need (or use) in consulting freeze frame data in the course of diagnostics and, quite frankly, paying shop time to record freeze frame data doesn't sound like a wise financial decision.
For most DTCs, you are much further ahead to perform pinpoint testing for the code(s) stored. Don't forget that CMDTCs should be noted and cleared and then the On Demand codes should be checked..... if there are no ODDTCs, an engine running self test should be performed (codes generated here would be a form of On Demand code). On Demand codes get fixed first, then engine running codes would be rechecked.
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