The oil pressure light is on for my 1994 Toyota Camry LE - 4-Cylinder engine, built in Japan. Oil is full, so I took it to my mechanic to get sensor replaced. Turns out oil pressure really is low. Mechanic suggested filter screen in oil pan was clogged or oil pump was bad. Both repairs are expensive.
Any other ideas as to why the oil is apparently not flowing? What is best plan of attack to diagnose this problem? Car is in pretty good shape - A/C and Transmission still work - so I'd like to fix this. Thanks!

Since you did not indicate that the car was using or leaking oil it sounds like the mechanic is telling you the truth.

Valves and camshafts are last on the oil train and usually the first to indicate oil-related problems. Did you hear unusual valve noise? When you remove the oil filler cap, and look inside, do the cams look worn or highly polished? The highest pressures in an engine are on cam lobes. If you suspect the possibility of oil failure, have a used oil analysis (UOA) performed ($15). The results may indicate abnormal metal levels and the possibility of excessive journal clearances, which would account for the low oil pressure.

Basically, I would look for:

. Oil pump worn or damaged.

. Clogged oil filter. Was the current filter left on too long and did it start to disintegrate?

. Clogged oil strainer.

. Indications of sludging and blocked oil passages.

. Excessive journal (worn engine bearing) clearances.

. Engine overheating.

After repairs are made, I recommend using 5W-30 full-synthetic engine oil (e.g. Mobil 1, Amsoil, Valvoline SynPower, Castrol Edge, or Pennzoil Platinum) and using a premium oil filter (e.g. Purolator PureOne, Amsoil Ea, or Mobil 1 EP). Change oil and filter every 6,000 miles or 6 months, whichever comes first. These products offer a much greater "margin of safety" than conventional oil and filters.

Advance Auto Parts is currently offering 5 qts. Of Castrol Edge and a PureOne oil filter for $24.99. The cost, after the rebate below, is $10.00:

http://www.castrol.com/liveassets/bp_internet/castrol/castrol_usa/STAGING/local_assets/downloads/e/Castrol_Edge_D_web_all_other.pdf

Do you hear any heavy knocking, especially when under load? That would indicate a rod bearing failing. In that case the pump may just not be able to maintain pressure.

I've also seen mechanical failure in the oil pump. I had a new filter fail so part of the check valve got caught in the oil pump and damaged the gears, strange things can happen.

Good luck

Not much I can ad to this. Your mech is probably right. It's about a 8 hour job.
The general routine is timing belt, front seals, pull pan, r&r oil pump, clean screen, and reseal pan.
This will give a good look at metal in the pan to see if it's worth going further.
I had one once that looked so clean, I took a chance and removed the oil pressure switch and thought I saw a clog. I removed the oil filter and blew compressed air in the sensor hole while covering the oil filter housing with a rag.
All kinds of crud came out of there. The oil light never came back on!
This was rare, but maybe worth a try to start with.
Good luck.

Update: Since the oil flow was blocked anyway, my mech suggested an engine oil flush. I guess miracles do still happen, because apparently it worked and I have oil pressure.
Probably one cause of the problem is that I only drive the car 3-4K miles a year, mostly on short trips. My mech recommended changing oil at least every three months, which I'll do.

Glad you got it fixed :)

That's probably good advice, more oil changes for all of us. It's cheap insurance.