This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
Fatal System Error 0911 and Self-Diagnostic Test 09010001 mean the same thing: You are, very likely, experiencing one of the following malfunctions: o- Temperature-sensitive ADB IC This problem was first identified in a 1988 article in issue #58 of Computist. If your IIgs is a true ROM-01 (produced starting in mid-late 1987) or a ROM 3, you are very unlikely to have an ADB IC with this defect. Otherwise ... A guess would be that you are running a ROM-00 machine which has been upgraded to ROM-01. If this is the first time you've noticed the '0911 problem, it is likely that this is the first summer you've owned and used this particular machine. Many early GS's come with an ADB IC which malfunctions over a narrow range of relatively low temperatures. Rooms are normally cooler during summer; so, this is when the error pops up most frequently. Some users first notice a plague of '0911 crashes after adding a System Saver-GS (which increases cooling). Note: Some reports have attributed '0911 crashes to overheating of one of the two main ADB IC's. So far, it looks like these reports are cases of misinterpreting the actual failure syndrome-- i.e. warming up a cool IC enough to get into its failure region. '0911 bombs can occur 'any time' but they usually happen at startup and when doing OpenApple-CTRL-ESC accesses to the Desk Accessories (CDA's, Control Panel, ...) menu. As the machine warms up, '0911 crashes tend to become less likely. The bad news is that there is no 100% fix except to replace the temp sensitive ADB IC-- hard to do since it is soldered to the motherboard and, in any case, known-good replacements are difficult to find. As to which ADB IC-- there are two, the ADB Controller and the ADB GLU- - our notes say the ADB Controller; but, we could have easily misidentified the function back then and the ADB GLU IC 'clicks' better with memories of the fix. (The ADB GLU IC is a square IC near the right front of the motherboard.) Note: The easiest way to identify the temp sensitive IC is to apply the fix (below) and see if it works. If it does, fine. If not, it is easy to move the fix to the other IC. For now, my suggestion is to try the ADB GLU IC first. Since the problem is coolness, a decent cure is to tape a small 12V bulb (e.g. a 20-30 ma. panel light bulb) to the top of the ADB IC. Use duct tape and try to enclose the bulb and IC in a kind of mini-oven. Run the leads from the bulb to the +12V Fan power pins near the back left area of the motherboard. The idea is to quickly warm up the IC. I used a scheme like this on our early GS and 0911 bombs dropped from 4-5 per day to 2-3 per week. If you can safely power the bulb via an external power module (e.g. a calculator or radio 'AC adapter') so that the bulb can be ON at least a few minutes before powering up the computer, 0911 bombs might disappear entirely. o- Defective keyboard, loose connector on KB, and/or a bad KB cable Some users report that swapping in another keyboard cured their '0911 crashes. In the same vein, a loose connector socket in the KB or a bad KB cable would be worth checking for. o- System noise and/or lowered motherboard voltages If '0911 crashes suddenly appear after an accelerator or other power- sucking board is added, it is fairly likely that the added load has increased system noise and, possibly, also lowered motherboard voltages. One possibility is that your power supply needs to have heavier leads swapped in. Also, you may need to jumper some of the power traces feeding Slots on the motherboard. For more discussion about this see Q&A 005-007 in Csa2POWER.txt. A defective power supply on the way to failing is another possibility. A failing power supply is more likely to be the culprit if '0911 crashes appeared 'from nowhere'-- i.e. nothing was changed, no new board was added, etc.. o- Poorly socketed Expansion Memory card With power turned OFF, try removing and replacing your Expansion Memory card. By: Rubywand, David Kopper, Guenther Unger, Gabriel Hawkins, Michael Mahon