I had the little oil can symbol light up on my dash this morning, and after the car sitting outside my house for a few hours I've noticed a patch of oil on the road under the front of the car. Is this an easy fix or is it a garage job? If it's a garage will my car be safe to drive to the garage?
The "little oil can symbol light" is connected to a pressure monitoring device (oil pressure switch) screwed into the main oil system of the engine. It really is a very simple thing with an electrical contact, a diaphragm and spring inside a small hollow casing. The spring pushes the diaphragm towards the engine which makes the electric circuit so the little oil can light on the dashboard lights up when it gets a current from turning the ignition switch on. The other side of the diaphragm is open to the influence of the oil in the main oil gallery so, when the engine starts up and the oil pump pumps oil round the engine, oil pressure in the gallery acts directly on the diaphragm and pushes it away against the spring. When the diaphragm moves away the electrical contacts break the circuit and the little oil light goes out.
Engine oil pressure when running varies from manufacturer to manufacturer - somewhere around 40 to 80 psi hot in many I've seen - The oil pressure switch activation pressure will be much less, maybe 8 to 10 psi. If the oil light comes on when the engine is running (and it shouldn't) there are a number of main "suspects" to check out.
The immediate most likely suspects are: 1) The switch has failed. Often, but not always, it will leak oil from where the electrical contact/s exit the switch (because often the switch will have failed because the diaphragm has ruptured). Renewing the switch is usually a complete cure. 2) lack of oil in the engine. The level in the sump drops to such an extent that the pickup pipe in the sump is uncovered and can suck air so allowing the spring to return the diaphragm in the switch to the position where the circuit is completed for the little oil can light so it lights up. Just a brief period of running like this will mean the engine has been running starved of oil and usually proves terminal for the engine if not immediately then in the very near future. 3) Wear in the engine has opened up bearing (and other) clearances to such an extent that the oil pump cannot supply a great enough flow of oil to "balance" that leaking past the worn components. An engine this "sick" will often be making knocking noises from it's big ends and have other wear related symptoms.
Having grown up in an environment where interest in vehicles and machinery was my main preoccupation, then going on to train for a life in the motor trade, it came as a big surprise and a bit of a shock when early workshop experience opened my eyes to how some people had absolutely no interest in that very expensive piece of machinery they were driving around in! Simple maintenance checks? not on your nelly! Still to this day I meet people who don't know how to open their bonnet! (I live in hope the new driving test requirements will improve this). All my children, especially my daughter, (now fully grown and fled the nest of course) know - and have had practice in -how to change a wheel SAFELY. (I also make sure all their vehicles have either full size (preferably) or space saver (if space precludes a full size) spare wheels. They also know how to do the simple under bonnet checks - Coolant level, brake fluid level, engine oil level, washer fluid, steering fluid level if it's hydraulic, visual hose check for leaks, battery terminals for corrosion and tightness. Lastly tyre condition and pressures including the spare. I stress to them how the inside shoulder of the tread can wear without you noticing so you need to be thorough in your inspection and, because tyres are often abused against kerbs, the sidewall needs a good look too. Inner sidewalls are very difficult to check without getting "mucky" so that usually falls to me a couple of times a year as I service the cars myself.
Ideally I suppose a weekly check is to be recommended as that way you are most likely to catch a problem before it develops. I know my kids sometimes leave it for a fortnight or even a month but always know to check before a journey of any length. So far following this regime has avoided nearly all potential disasters!