Dashboard warning lights

DTCs (Diagnostic Trouble Codes) - Most often are ranked by a level of "perceived severity" at design time. Those that are considered more severe often have to be cleared manually.

For the less severe ones, if the fault does not occur again over a number of ignition cycles (i.e. power up/down cycles and possibly even taking the power up time into account), the fault *may* clear and whatever associated warning light (or warning message) *may* go out. There's usually a DTC history kept anyway, even if the fault has "cleared by itself". This history *might* also "leak away" over time.

Another scenario is where a fault is detected by the ECU, but not immediately flagged. A DTC may be stored, but no warning given. If the fault persists over time or over a number of ignition cycles, it may then give a visible warning.

i.e. what I'm saying is, there's no hard rule (or often no immediately obvious one to the end user) that says a DTC will clear by itself or that it has to be cleared. The only sure way is to connect a diagnostic tool, make a note of what DTCs are stored and/or the DTC history and then clear the codes. Then, see if the code returns. Many "hard" (rather than "intermittent") faults, will often set the DTC (or DTCs) almost immediately again, after being cleared.

So why do you get things like "4x4 fault" when there is nothing obviously wrong with the 4x4 system?

Something like an engine management system fault could mean that there is no guarantee that the requested (not necessarily from the driver) torque/power can be provided. Things like the vehicle stability control depend on a combination of things like braking force to each wheel, drive split front to back (in the case of a control-able 4WD system) and engine torque control.

If the engine management system (EMS) throws out a CAN message on the vehicle bus saying it has a problem, or even stops sending certain messages altogether (messages that other ECUs use), stability control and/or 4WD will then very likely decide all bets are off and indicate "faults" of their own. i.e. EMS is off sick, so I'm warning you, that because of EMS, I can't do my job properly either. So when Toyota say the problem might not necessarily be related to some of the fault codes, there is more than an element of truth in that.

As an example of the above and my increasing senility, when I serviced my UC a few months ago, before restarting the engine, I forgot to reconnect the MAF sensor. This resulted in VSC and 4WD warnings immediately, as well as EMS, even before the vehicle had moved. Easily cleared though.

Another thing to bear in mind, is that the fault detection parts of the software in an ECU can often outweigh the actual functionality.....and all that fault detection has to be tested and verified (usually called fault insertion testing). It can get quite long and complex to do and there can be some unforeseen interactions, so sometimes a few oddities at "DTC setting time" can crop up.

So, look at stored codes, clear the codes, look for what returns and also remember they don't always get it right. They are a guide, a reasonably good guide, but a guide.

Oh, it's nowhere near as fraught as Windows.