Could my low oil pressure warning light be caused by the oil filler cap not fitting properly?

No.
The oil pump in the engine produces the oil flow and the clearances between moving parts in the engine is what creates the "oil pressure" (which is why a worn engine will typically display low oil pressure).

The cap on the oil filler opening is totally irrelevant to the oil pressure being generated in the engine.

If you have low oil pressure you need to have it investigated immediately. Driving the vehicle with low oil pressure will cause engine damage because insufficient lubrication will be getting to all moving parts. If your 2004 engine is not very high mileage, in good condition and if well maintained with adequate oil changes and so on(which most are not) then the problem may be that the oil pressure relief valve is jammed open or the oil pump itself might be failing and is unable to produce sufficient flow volume.

If you have a worn engine, replacing the oil pump will not fix a low oil pressure issue solely caused by excessive clearances between moving parts due to years of normal wear or years of higher wear due to maintenance neglect.

you should be OK if the oil is on the low mark, I would complain to the dealer big time no excuse for leaving oil cap off, demand a new filler cap and free oil change and filter. Then find a new shop.

Low oil pressure.
Causes
1. Warn oil pump
2. Warn engine bearings
3. Wrong grade of engine oil
Could also be a bad oil pressure sender or electrical connector.
Remove oil pressure sender and and fit a manual gauge to check the true oil pressure.

For your help, there are few Car Engine oil related solved problem listed below. You can go through it.
High Oil Level on Dipstick http://whatisbyme.blogspot.in/2012/06/high-oil-level-on-dipstick.html Oil light came on and the car died? http://schematicsdiagram.blogspot.in/2012/03/oil-light-came-on-and-car-died.html How to replace Oil filler cap or breather cap? http://repairhelpcenter.blogspot.in/2012/02/how-to-replace-oil-filler-cap-or.html Oil pouring out from left front of engine? http://schematicsdiagram.blogspot.com/2012/01/oil-pouring-out-from-left-front-of.html Common Causes of Car Oil Leaks? http://schematicsdiagram.blogspot.com/2011/12/common-causes-of-car-oil-leaks.html Troubleshoot Car Oil leak? http://schematicsdiagram.blogspot.com/2011/12/troubleshoot-car-oil-leak.html 2004 Dodge Stratus: Windage Tray Problem http://howtobyme.blogspot.in/2012/07/2004-dodge-stratus-windage-tray-problem.html Toyota Highlander Sport Oil change details? http://repairhelpcenter.blogspot.com/2012/01/toyota-highlander-sport-oil-change.html Low oil pressure light on Dodge Magnum?
http://howtobyme.blogspot.com/2011/11/low-oil-pressure-light-on-dodge-magnum.html

Kia Sportage leaking oil from oil pan?
http://howtobyme.blogspot.com/2011/11/kia-sportage-leaking-oil-from-oil-pan.html
Car Turns OFF due to low oil pressure warning light?

http://schematicsdiagram.blogspot.com/2011/12/car-turns-off-due-to-low-oil-pressure.html
Toyota Corolla leaking oil from timing cover? http://technoanswers.blogspot.in/2012/02/toyota-corolla-leaking-oil-from-timing.html

Overfilling oil in engine?

http://whatisbyme.blogspot.in/2012/06/overfilling-oil-in-engine.html
2000 Dodge Ram 3500 Cummins Diesel: Oil Leak issues? http://schematicsdiagram.blogspot.in/2012/06/2000-dodge-ram-3500-cummins-diesel-oil.html-----------
These will help.Thanks.

I don't want to worry you but have you checked to see if the oil pump is ok, as if it has failed the oil pressure will be the problem not the switch which operates the warning light. ? Have you tried taking off the oil filler cap whilst the engine is running as if you do that you can sometimes be able to see ( with the aid of a torch/flashlight ) if the oil is getting pumped up to the timing gear, i.e. the camshaft and the valve gear, followers etc. If the oil is sluishing about at the top of the engine ( spitting out even ) then that is some good indication that the oil is being pumped up by the oil pump from the bottom of the engine ( where it is usually situated above the oil sump at the base of the engine) Furthermore...The oil pump 'pick up' pipe usually has a gauze filter attached to it, to stop any ' foreign bodies' entering the lubrication system if it has become blocked the oil flow into the pump will be restricted and could cause a low pressure warning, Unfortunately the only way I know to check this is by 'dropping' i.e. removing the sump which can be a big job depending on the access to it.I am not familiar with your model of car nor am I a qualified mechanic but know quite a bit about how engines work and how to fix problems. Also if there is insufficient oil in the engine this will cause the warning light to come on. Have you tried changing the oil as old worn out oil might also give you problems including wrecking your engine. I hope something I have said may be helpful, Good Luck...... Stephen

For oil pump,oil pump pressure,oil warning light,oil low,oil ewplacing and other car engine oil problems,the help links are provided below.Click the link to go through the procedure :-----Oil light came on and the car died?http://schematicsdiagram.blogspot.in/2012/03/oil-light-came-on-and-car-died.html--------How to replace Oil filler cap or breather cap?http://repairhelpcenter.blogspot.in/2012/02/how-to-replace-oil-filler-cap-or.html---------Oil pouring out from left front of engine?http://schematicsdiagram.blogspot.com/2012/01/oil-pouring-out-from-left-front-of.html------Common Causes of Car Oil Leaks?http://schematicsdiagram.blogspot.com/2011/12/common-causes-of-car-oil-leaks.html--------Troubleshoot Car Oil leak?http://schematicsdiagram.blogspot.com/2011/12/troubleshoot-car-oil-leak.html--------Oil change details?http://repairhelpcenter.blogspot.com/2012/01/toyota-highlander-sport-oil-change.html---------Low oil pressure light?
http://howtobyme.blogspot.com/2011/11/low-oil-pressure-light-on-dodge-magnum.html
----------
leaking oil from oil pan?
http://howtobyme.blogspot.com/2011/11/kia-sportage-leaking-oil-from-oil-pan.html ----------
Car Turns OFF due to low oil pressure warning light?

http://schematicsdiagram.blogspot.com/2011/12/car-turns-off-due-to-low-oil-pressure.html

------------
leaking oil from timing cover?

http://technoanswers.blogspot.in/2012/02/toyota-corolla-leaking-oil-from-timing.html

----------
For more oil and oil pump details check out the direct links below: ---

How to Replace Oil Pan on car?
http://repairhelpcenter.blogspot.com/2011/11/how-to-replace-oil-pan-on-audi-a4-and.html
--------
How to Replace an Oil Pan Gasket?
http://schematicsdiagram.blogspot.com/2011/07/how-to-replace-oil-pan-gasket.html
-----------
HOW TO REPLACE AN OIL PUMP
http://schematicsdiagram.blogspot.com/2011/07/how-to-replace-oil-pump.html
----------
HOW TO CHANGE ENGINE OIL AND FILTER
http://schematicsdiagram.blogspot.com/2011/07/how-to-change-engine-oil-and-filter.html
----------
HOW TO REPLACE AN OIL PRESSURE SWITCH (SENSOR)
http://schematicsdiagram.blogspot.com/2011/07/how-to-replace-oil-pressure-switch.html
-----------
How to Check Differential Oil Leaks and Replace Oil
http://schematicsdiagram.blogspot.com/2011/07/how-to-check-differential-oil-leaks-and.html
----------
HOW TO PERFORM A BASIC ENGINE TUNE UP-OIL CHANGE
http://schematicsdiagram.blogspot.com/2011/08/how-to-perform-basic-engine-tune-up-oil.html

-------

Oil pan Gasket on car?
http://howtobyme.blogspot.com/2011/11/oil-pan-gasket-on-1998-toyota-tercel.html

---------

Oil pressure for car?

http://schematicsdiagram.blogspot.com/2011/12/oil-pressure-for-2001-gmc-jimmy-v6.html

----------
How to replace Oil Pan on car?

http://schematicsdiagram.blogspot.com/2011/12/how-to-replace-oil-pan-on-2008-honda.html

---------
How to replace Oil Pressure Sensor on car?

http://schematicsdiagram.blogspot.com/2011/12/how-to-replace-oil-pressure-sensor-on.html

-----------
How to replace Oil Pump on car?

http://howtobyme.blogspot.com/2011/12/how-to-replace-oil-pump-on-2008-honda.html

----------
Car oil leak?

http://schematicsdiagram.blogspot.com/2011/12/car-oil-leak.html

----------
Car oil leaks from valve cover gasket?

http://schematicsdiagram.blogspot.com/2011/12/car-oil-leaks-from-valve-cover-gasket.html

----------
Oil light blinks on dashboard?

http://schematicsdiagram.blogspot.com/2011/12/oil-light-blinks-on-dashboard.html

-----------
Oil pressure issues on car?

http://repairhelpcenter.blogspot.com/2012/01/oil-pressure-issuers-on-toyota-tundra.html

-----------
Oil leaks from air filter on ford?

http://technoanswers.blogspot.com/2012/01/oil-leaks-from-air-filter-on-ford.html

---------
Wire running from oil pan to alternator?

http://technoanswers.blogspot.in/2012/02/wire-running-from-oil-pan-to-alternator.html

--------------
Why car leaks oil from oil filter?

http://schematicsdiagram.blogspot.in/2012/02/toyota-camry-leaks-oil-from-oil-filter.html

------------
Why engine is consuming lots of oil?

http://schematicsdiagram.blogspot.in/2012/02/2008-scion-xb-24l-engine-is-consuming.html

-----------
Maint REQD message on car dash?

http://schematicsdiagram.blogspot.in/2012/02/maint-reqd-message-on-2008-lexus-rx350.html

------
Remove Sump oil pan?

http://repairhelpcenter.blogspot.in/2012/03/remove-volkswagen-sump-oil-pan.html

--------
Sump not coming OFF?

http://repairhelpcenter.blogspot.in/2012/03/sump-not-coming-off.html

-----------
How to remove the sump on car?http://repairhelpcenter.blogspot.in/2012/03/how-to-remove-sump-on-vw-beetle.html----------Car oil sump removal diagram?

http://repairhelpcenter.blogspot.in/2012/03/volkswagen-beetle-oil-sump-removal.html

---------

This will help.

Thanks.

For oil pump,oil pump pressure,oil warning light,oil low,oil ewplacing and other car engine oil problems,the help links are provided below.Click the link to go through the procedure :-----Oil light came on and the car died?http://schematicsdiagram.blogspot.in/2012/03/oil-light-came-on-and-car-died.html--------How to replace Oil filler cap or breather cap?http://repairhelpcenter.blogspot.in/2012/02/how-to-replace-oil-filler-cap-or.html---------Oil pouring out from left front of engine?http://schematicsdiagram.blogspot.com/2012/01/oil-pouring-out-from-left-front-of.html------Common Causes of Car Oil Leaks?http://schematicsdiagram.blogspot.com/2011/12/common-causes-of-car-oil-leaks.html--------Troubleshoot Car Oil leak?http://schematicsdiagram.blogspot.com/2011/12/troubleshoot-car-oil-leak.html--------Oil change details?http://repairhelpcenter.blogspot.com/2012/01/toyota-highlander-sport-oil-change.html---------Low oil pressure light?
http://howtobyme.blogspot.com/2011/11/low-oil-pressure-light-on-dodge-magnum.html
----------
leaking oil from oil pan?
http://howtobyme.blogspot.com/2011/11/kia-sportage-leaking-oil-from-oil-pan.html ----------
Car Turns OFF due to low oil pressure warning light?

http://schematicsdiagram.blogspot.com/2011/12/car-turns-off-due-to-low-oil-pressure.html

------------
leaking oil from timing cover?

http://technoanswers.blogspot.in/2012/02/toyota-corolla-leaking-oil-from-timing.html

----------
For more oil and oil pump details check out the direct links below: ---

How to Replace Oil Pan on car?
http://repairhelpcenter.blogspot.com/2011/11/how-to-replace-oil-pan-on-audi-a4-and.html
--------
How to Replace an Oil Pan Gasket?
http://schematicsdiagram.blogspot.com/2011/07/how-to-replace-oil-pan-gasket.html
-----------
HOW TO REPLACE AN OIL PUMP
http://schematicsdiagram.blogspot.com/2011/07/how-to-replace-oil-pump.html
----------
HOW TO CHANGE ENGINE OIL AND FILTER
http://schematicsdiagram.blogspot.com/2011/07/how-to-change-engine-oil-and-filter.html
----------
HOW TO REPLACE AN OIL PRESSURE SWITCH (SENSOR)
http://schematicsdiagram.blogspot.com/2011/07/how-to-replace-oil-pressure-switch.html
-----------
How to Check Differential Oil Leaks and Replace Oil
http://schematicsdiagram.blogspot.com/2011/07/how-to-check-differential-oil-leaks-and.html
----------
HOW TO PERFORM A BASIC ENGINE TUNE UP-OIL CHANGE
http://schematicsdiagram.blogspot.com/2011/08/how-to-perform-basic-engine-tune-up-oil.html

-------

Oil pan Gasket on car?
http://howtobyme.blogspot.com/2011/11/oil-pan-gasket-on-1998-toyota-tercel.html

---------

Oil pressure for car?

http://schematicsdiagram.blogspot.com/2011/12/oil-pressure-for-2001-gmc-jimmy-v6.html

----------
How to replace Oil Pan on car?

http://schematicsdiagram.blogspot.com/2011/12/how-to-replace-oil-pan-on-2008-honda.html

---------
How to replace Oil Pressure Sensor on car?

http://schematicsdiagram.blogspot.com/2011/12/how-to-replace-oil-pressure-sensor-on.html

-----------
How to replace Oil Pump on car?

http://howtobyme.blogspot.com/2011/12/how-to-replace-oil-pump-on-2008-honda.html

----------
Car oil leak?

http://schematicsdiagram.blogspot.com/2011/12/car-oil-leak.html

----------
Car oil leaks from valve cover gasket?

http://schematicsdiagram.blogspot.com/2011/12/car-oil-leaks-from-valve-cover-gasket.html

----------
Oil light blinks on dashboard?

http://schematicsdiagram.blogspot.com/2011/12/oil-light-blinks-on-dashboard.html

-----------
Oil pressure issues on car?

http://repairhelpcenter.blogspot.com/2012/01/oil-pressure-issuers-on-toyota-tundra.html

-----------
Oil leaks from air filter on ford?

http://technoanswers.blogspot.com/2012/01/oil-leaks-from-air-filter-on-ford.html

---------
Wire running from oil pan to alternator?

http://technoanswers.blogspot.in/2012/02/wire-running-from-oil-pan-to-alternator.html

--------------
Why car leaks oil from oil filter?

http://schematicsdiagram.blogspot.in/2012/02/toyota-camry-leaks-oil-from-oil-filter.html

------------
Why engine is consuming lots of oil?

http://schematicsdiagram.blogspot.in/2012/02/2008-scion-xb-24l-engine-is-consuming.html

-----------
Maint REQD message on car dash?

http://schematicsdiagram.blogspot.in/2012/02/maint-reqd-message-on-2008-lexus-rx350.html

------
Remove Sump oil pan?

http://repairhelpcenter.blogspot.in/2012/03/remove-volkswagen-sump-oil-pan.html

--------
Sump not coming OFF?

http://repairhelpcenter.blogspot.in/2012/03/sump-not-coming-off.html

-----------
How to remove the sump on car?http://repairhelpcenter.blogspot.in/2012/03/how-to-remove-sump-on-vw-beetle.html----------Car oil sump removal diagram?

http://repairhelpcenter.blogspot.in/2012/03/volkswagen-beetle-oil-sump-removal.html

---------

This will help.

Thanks.

For oil pump,oil pump pressure,oil warning light,oil low,oil ewplacing and other car engine oil problems,the help links are provided below.Click the link to go through the procedure :-----Oil light came on and the car died?http://schematicsdiagram.blogspot.in/2012/03/oil-light-came-on-and-car-died.html--------How to replace Oil filler cap or breather cap?http://repairhelpcenter.blogspot.in/2012/02/how-to-replace-oil-filler-cap-or.html---------Oil pouring out from left front of engine?http://schematicsdiagram.blogspot.com/2012/01/oil-pouring-out-from-left-front-of.html------Common Causes of Car Oil Leaks?http://schematicsdiagram.blogspot.com/2011/12/common-causes-of-car-oil-leaks.html--------Troubleshoot Car Oil leak?http://schematicsdiagram.blogspot.com/2011/12/troubleshoot-car-oil-leak.html--------Oil change details?http://repairhelpcenter.blogspot.com/2012/01/toyota-highlander-sport-oil-change.html---------Low oil pressure light?
http://howtobyme.blogspot.com/2011/11/low-oil-pressure-light-on-dodge-magnum.html
----------
leaking oil from oil pan?
http://howtobyme.blogspot.com/2011/11/kia-sportage-leaking-oil-from-oil-pan.html ----------
Car Turns OFF due to low oil pressure warning light?

http://schematicsdiagram.blogspot.com/2011/12/car-turns-off-due-to-low-oil-pressure.html

------------
leaking oil from timing cover?

http://technoanswers.blogspot.in/2012/02/toyota-corolla-leaking-oil-from-timing.html

----------
For more oil and oil pump details check out the direct links below: ---

How to Replace Oil Pan on car?
http://repairhelpcenter.blogspot.com/2011/11/how-to-replace-oil-pan-on-audi-a4-and.html
--------
How to Replace an Oil Pan Gasket?
http://schematicsdiagram.blogspot.com/2011/07/how-to-replace-oil-pan-gasket.html
-----------
HOW TO REPLACE AN OIL PUMP
http://schematicsdiagram.blogspot.com/2011/07/how-to-replace-oil-pump.html
----------
HOW TO CHANGE ENGINE OIL AND FILTER
http://schematicsdiagram.blogspot.com/2011/07/how-to-change-engine-oil-and-filter.html
----------
HOW TO REPLACE AN OIL PRESSURE SWITCH (SENSOR)
http://schematicsdiagram.blogspot.com/2011/07/how-to-replace-oil-pressure-switch.html
-----------
How to Check Differential Oil Leaks and Replace Oil
http://schematicsdiagram.blogspot.com/2011/07/how-to-check-differential-oil-leaks-and.html
----------
HOW TO PERFORM A BASIC ENGINE TUNE UP-OIL CHANGE
http://schematicsdiagram.blogspot.com/2011/08/how-to-perform-basic-engine-tune-up-oil.html

-------

Oil pan Gasket on car?
http://howtobyme.blogspot.com/2011/11/oil-pan-gasket-on-1998-toyota-tercel.html

---------

Oil pressure for car?

http://schematicsdiagram.blogspot.com/2011/12/oil-pressure-for-2001-gmc-jimmy-v6.html

----------
How to replace Oil Pan on car?

http://schematicsdiagram.blogspot.com/2011/12/how-to-replace-oil-pan-on-2008-honda.html

---------
How to replace Oil Pressure Sensor on car?

http://schematicsdiagram.blogspot.com/2011/12/how-to-replace-oil-pressure-sensor-on.html

-----------
How to replace Oil Pump on car?

http://howtobyme.blogspot.com/2011/12/how-to-replace-oil-pump-on-2008-honda.html

----------
Car oil leak?

http://schematicsdiagram.blogspot.com/2011/12/car-oil-leak.html

----------
Car oil leaks from valve cover gasket?

http://schematicsdiagram.blogspot.com/2011/12/car-oil-leaks-from-valve-cover-gasket.html

----------
Oil light blinks on dashboard?

http://schematicsdiagram.blogspot.com/2011/12/oil-light-blinks-on-dashboard.html

-----------
Oil pressure issues on car?

http://repairhelpcenter.blogspot.com/2012/01/oil-pressure-issuers-on-toyota-tundra.html

-----------
Oil leaks from air filter on ford?

http://technoanswers.blogspot.com/2012/01/oil-leaks-from-air-filter-on-ford.html

---------
Wire running from oil pan to alternator?

http://technoanswers.blogspot.in/2012/02/wire-running-from-oil-pan-to-alternator.html

--------------
Why car leaks oil from oil filter?

http://schematicsdiagram.blogspot.in/2012/02/toyota-camry-leaks-oil-from-oil-filter.html

------------
Why engine is consuming lots of oil?

http://schematicsdiagram.blogspot.in/2012/02/2008-scion-xb-24l-engine-is-consuming.html

-----------
Maint REQD message on car dash?

http://schematicsdiagram.blogspot.in/2012/02/maint-reqd-message-on-2008-lexus-rx350.html

------
Remove Sump oil pan?

http://repairhelpcenter.blogspot.in/2012/03/remove-volkswagen-sump-oil-pan.html

--------
Sump not coming OFF?

http://repairhelpcenter.blogspot.in/2012/03/sump-not-coming-off.html

-----------
How to remove the sump on car?http://repairhelpcenter.blogspot.in/2012/03/how-to-remove-sump-on-vw-beetle.html----------Car oil sump removal diagram?

http://repairhelpcenter.blogspot.in/2012/03/volkswagen-beetle-oil-sump-removal.html

---------

This will help.

Thanks.

The engine oil 'warnig' light being ON indicates that you have a BAD problem with your engine oil system: this indicates that your engine is not being lubricated properly (due to low pressure) and you should get it checked immediately!
Oil warning light ON can be caused by:
- Low Engine Oil level
- Blocked oil filter
- Leaking oil filter
- Leaking oil seals
- Faulty oil pressure sensor (pretty rare) or loose sensor connector
- Also heck the obvious like loose Oil Filler cap and dip stick seal.
Hope it heps and do not drive your car without resolving this issue first as you could easily blow the engine!

You're low on oil - the Audi dipstick isn't the most accurate thing out there. Add a quart and see what happens.