The dreaded MIL light has returned on SWMBO's Yaris :-(. It's a 2002 1.5 T Sport. There are no drivability issues whatsoever; the car runs fine until one day she will start it up and the MIL light remains on. It then continues to drive flawlessly, without any effect on performance or economy, but the MIL light remains on with every subsequent use of the car, until I disconnect the battery.
Resetting the ECU gets rid of the light for a number of days, sometimes weeks, but the problem always seems to return. I have read the fault code stored and it is 94. I believe this is the code for a small fuel system leak or loss of pressure.
I posted about it a while ago and it was suggested that she tighten the fuel cap more thoroughly (to prevent pressure loss). This seemed to help for a while but has failed to eradicate the problem. The cap looks fine and I'm sure it provides a good seal because there is a noticable rush of air when removing it.
Any ideas regarding what to investigate next would be most appreciated. I'm reluctant to resort to having to pay for diagnostics in order to solve a problem that is merely an annoyance, but I don't want to continue ignoring the MIL in case a more serious fault occurs!