Volvo 850 Starting Problem
Many of these cars suffer difficulties starting, especially when the engine is only run for a short time then switched off, and an attempt is then made to restart it. It is inadviseable to start any car with a catalytic converter in order to move it a short distance. The ECU's cold running sequence (which is designed to protect the catalytic converter from unburned petrol) cannot cope with this and having detected too much raw fuel in the xhaust, will shut the engine down.
Another cause, especially on the 850 models, is that the hydraulic tappets do not close properly when the engine is switched off. Oil gets behind them stopping them from closing fully. This means there is no or little compression in the cylinder when there is an attempt to start the engine again. One cure is to remove the plugs and squirt engine oil into each cylinder then replace the plugs and start the engine. The oil helps seal any gaps under the valves and give adequate compression in the cylinders.
Another cure is to use Mobil 1 engine oil all the time. This is thinner than other oils and will allow the tappets to close properly. If you change to Mobil 1 oil be sure to run the engine at 3,000rpm for about 20 minutes to ensure the oil gets into all the small channels in the engine.