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Vague question for a problem that could have a lot of different causes.  You could have a burnt or bent valve, bad valve seat, broken or worn rings, cylinder wall wear or damage, leaking head gasket, or a warped head.  How about a little more info:  like in what, what kind of engine, and why you think the compression is low.

THAT WHICH DOESN'T KILL YOU MAKES YOU STRONGER. Bull. Reality is, it will usually hurt like hell and leave you with a permanent limp.

Check all maintenance items needed to control the right amount of fuels and spark entering cylinder. Than to sat timing for proper firing inside of cylinder will equal the right compression.

If you have a cylinder in your car with decreased compression, you have a serious problem unless it's only about 10% less than the other cylinders. Can you be more specific? Is this what the mechanic told you?

Not a Jennyus.

I know a few things about cars, so of the things I have asked my mechanic about have been looked at but for the replacement of the head gasket or to see if its warpt. One of the cylinders the compression flexuwaites, some times it runs normal and others times its wentas low as 40%. I have a 1998 Nissan Altima, I have changed the two oxygen sensors and spark plugs and wires plus distributor. I had three other mechanics look at it and they arestumped. I put some value cleaning agent in the gas tank and oil crankcase and now the service engine light is staying off and I am not feeling the inconsistencies I got around the 35 to 40 speed. I did not pass emissions so I was looking for a solution to pass. I was told that compression issue could be a few things the report was coming up fuel injectors but I have had a Nissan in the past with that problem it had different symtoms, they have been checked and cleared. Any other suggestions

If I understand you correctly, you're saying that sometimes you do a compression check by cranking on the engine with the spark plugs removed (typical method), and getting a compression guage reading showing that all the cylinders are about the same compression and other times you check and one cylinder is much less than the others. If this is what you are saying, I would say that the cause is cylinder compression is being allowed around at least one exhaust valve. Since a valve rotates in it's guide (by design, to reduce same-spot running), the valve is coming around to a spot in it's rotation where the seat has a low spot and the valve also has a low spot. When this happens a certain amount of cylinder gasses are allowed thru this gap. This is often evidenced by an intermittent "tooting" at the tailpipe when the engine is at idle. It's the aligning of the low spots on the valve and the valveseat that causes the "miss" in the engine when idling. It isn't noticed when the engine is running at higher rpms unless the gap is considerable. The unburned/partially burned gasses being sent into the exhaust may be why you're getting emissions above the state's maximum allowable. Eventually, you're going to have to do a valve-regrind (or valve job) to cure the problem. If the engine has, say, more than 125,000 miles, I would figure on an engine rebuild since grinding the valves and seats increases the cylinder pressure considerably and puts sudden, new strain on worn crankshaft bearings and will lead to early failure of those components. Since engine rebuilds are expensive you need to decide if the car's worth it. This is my interpretation of your problem. Hopefully, it's something cheaper. I really don't think a blown headgasket is going to give you an "intermittent" clue. The headgasket failure would be pretty much the same regardless of how many times you did a compression check. If this is not urgent, you should wait to see what someone else suggests since, with me, I always look at the least expensive possibilities first.

Not a Jennyus.

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