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No need to get under the car - the problem is under the hood or with the gauge system itself. Check coolant level as others say. Then start the engine from cold and check the temperature of the hose from the 'stat housing to the rad. With a working thermostat, it will start to get warm very slowly, because the stat is not a perfect shutoff valve, and leaks some coolant into the hose. But, after some minutes, the hose should suddenly become too hot to hold. That's the stat opening up.
If the hose just consistently gains temperature with no detectable "step" upward, the stat has failed open - as they usually do. Checking heater output is a good additional test. Replacing the stat every few years is a worthwhile idea. A non-turbo 940 uses a 92C unit. I prefer the gen Volvo piece...a few more bucks, a bit better quality.
My "bad gauge" experience matched that of some other postings here: it would start to climb, then bounce around, then die. This went on for many cold starts until it quit completely. A multimeter showed that the gauge sender was dropping in resistance properly as the engine warmed up, until it leveled off at about 200 ohms. I removed the instrument cluster, tightened the screws which hold the gauges and which also conduct the electricity to make them operate, and everything has worked fine for two years now.
Gotta love a 0$ fix.
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Bob (son's 81-244GL B21F/M46, dtr's 83-244DL B23F/M46, my 94-944 B230FD and 89 745 (LT-1 V8); hobbycar 77 MGB, and a few old motorcycles)
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