Check engine light on '98 pontiac montana comes on then goes off again several times. Is this common? One place said it is likely an oxygen sensor, but why woiuldn't it stay on?

That code is actually due to the fuel pressure being too low and causing a lean condition that the oxygen sensor picks up. It is common in that vehicle. Check the fuel pump pressure, filters etc.

ciny,

If your car is 1996 or newer, you have 2 oxygen sensors. They are screwed into your exhaust pipe, one before the catalytic converter and one after the cat or cats.
The upstream sensor is the important one that controls your fuel mixture. If it fails, it will turn on your Check Engine Light, or if badly failed, it will flash your CEL light. If flashing, take action quickly to avoid destroying your cats.
The downstream sensor just monitors what the oxygen content is after the cats, and it reports this info to your engine computer. It bascially is a tattle tail on the front sensor if it's not working. Hope this helps you. Buy them cheap on ebay.

this light can come on for many different reasons. the way to find out what the problem is to read the codes stored in the computer. most auto parts store such as autozone or advance auto will read the codes for free and give you probable causes for the error. most commonly if the light comes on and the car appears to be running properly it is most likely a oxygen sensor gone bad, but to be certain get the codes read.

It's an oxygen sensor issue. Most cars these days have 2 oxygen sensors. 1 "upstream" of converter and 1 downstream. They compare exhaust before and after the converter. When their measurements equal each other, means converter is not functioning (exhaust is same in & out) and not always triggering dash lights. Either you have a bad sensor or your converter is and, is why you didn't pass inspection.

ignition control module, throttle position sensor and other sensors need to be tested to establish problem, any codes from OBDII ? check engine light staying on ?

Check the pcv hose for leaks or collapsing while it's running. A common problem.

P1133 .......Oxygen sensor bank 1 sensor 1.

How do you know the thermostat is ok? If in doubt replace it. Water pumps are a even bigger what if. Did he do a radiator flush? Cooling system flush of the engine? Cooling fans come on at what temp and go off when? Replace the engine temperature sensor? 6 years old is bound to have sensors and such go bad.

You must be looking at a different sensor as the oxygen sensor is mounted in the exhaust system and wouldn't be exposed to the oil in the engine. The oil pressure sensor is likely what you see, it is not uncommon at all for these to fail internally and leak oil to the outside of the engine. Sometimes you need a special socket to change them. Both the sensor and the socket should be available at any auto parts store.

A code P0420 may mean that one or more of the following has happened:

• Leaded fuel was used where unleaded was called for
• An oxygen sensor is not reading (functioning) properly
• The engine coolant temperature sensor is not working properly
• Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
• Retarded spark timing
• The oxygen sensors in front and behind the converter are reporting too similar of readings
• That should get you going.