Ive got soot trouble. need to adjust air-fuel mixture. the underside of the unit is exposed in garage. how do i clean and adjust oxygen/ fuel mixture?

it is normally the HO2S ( heated oxygen sensor) in the exhaust manifold before the cat converters
That unit reports the exhaust gas composition to the ECM which adjusts the mixture according to the MAF/VAF sensor readings
Add to all of those the ambient temp sensor and the BARO sensor and you have the system that adjusts the air/fuel mixture
how to fix
have the fault codes read and replace the faulty sensors

Might be an injector.

Unless your engine has a carburetor, you can't adjust your fuel mixture. That is done by your cars computer. It analyzes your exhaust emissions and adjusts your fuel/air ratio.
Your oxygen sensor(s) may need to be replaced, or there may be a computer glitch causing the problem.

Due to air polution standards, the adjustment screws on engines are concealed by blocking devices. Have you thought to check your oxygen sensor? A faulty oxygen sensor will sometimes cause the symptoms you describe. It is probably best to get this checked at the service dept of the dealer. If an adjustment needs to actually made, their certified techs are allowed to perform such adjustments. Best Wishes

The control module receives information from the heated oxygen sensor (HO2S) about the fuel/air mixture when idling at both the lower and upper part-load range - If the fuel/air mixture deviates from [lambda]=1 the short-term fuel trim will compensate for this by adjusting injection time so that [lambda]=1 is achieved. When the short-term fuel trim makes an adjustment, the integrator median must be adjusted by the long-term trim. Diagnostic trouble code (DTC) ECM-261A will be stored when the long-term trim must be adjusted to maximum in the lower part of the load area.

Black smoke is caused when the mix of fuel and air becomes un-balanced. Normal mixture is 14.5 parts air to1 part fuel. When the fuel to air mixtures change because of a malfunction the mixture can go as high as 14.5 to 2 or 3, two to three times the proper amount. The black smoke is the excess fuel generated from the rich mixture and can be cause by one of the following:

• plugged air filter
• Shorted or stuck fuel injector
• Failed fuel pressure regulator
• Vacuum leak
• Shorted ECM Sensor , scan for any problem codes if possible.

try cleaning the maf (mass air flow sensor) it is located just out side the air cleaner box just before the large air hose heading to Intake use electrical parts cleaner and toothbrush or a cotton swab to clean this, but be carefull part is fragile and expensive to replace, look for two exposed small wires located on the bottom of the sensor that would be exposed to ingoing intake air this is what needs cleaned, soot or dirt will cause a improper air fuel mixture generally lean causing power loss good luck

If your check engine light is on when the car is running, then that means there is probably a trouble code stored in your vehicles computer. With a code reader, you can find out what the code is. That will direct you to the problem. You can post your code here and we will tell you what it means.
Regarding the 'gas' smell, first check to see if your car is running 'rich'. You can rub your finger inside the tailpipe and see if there is a lot of black soot there, which would indicate its running too rich (too high of a fuel to air mixture). If this is inconclusive to you, remove the spark plugs and check if they are with black soot on them where the pin is. This will tell you how the engine is running in terms of fuel mixture. If they are black, then you have found the problem (fuel injection problem or Oxygen sensor, most likely). The trouble code might also point to this. Other than that, sometimes an exhaust leak can cause fumes to leak up into the car when running. My car had the gas smell problem and it was both a hole in the exhaust under the car and an overly rich fuel mixture. I had black soot inside the tailpipe and black sooty spark plugs and the final problem ended up being a bad oxygen sensor (located on the intake manifold), that was sending an improper signal to the vehicle's computer, thus the reason for the problem.

The pre cat sensors running lean does not mean they need to be replaced. They are are detecting a symptom. You need to know why.

If you are using a basic scantool (code only) you need to look up the P0000 code.
I thought this an odd code. I looked it up in my Autotap unit and it does not exist. The lowest # P (powertrain) code I have listed is P0016.

A regular scan tool, won't scan transmission or body codes, mu Auto tap doesn't either. You need a Tech 2 from the dealer for transmission codes.

First some history. The first O2 sensor was introduced in 1976 on a Volvo. California vehicles got them next in 1980, then federal emission laws made O2 sensors virtually mandatory on all cars and light trucks built since 1981. And now that OBD-II regulations are here (1996 and newer vehicles), most vehicles now have multiple O2 sensors, some as many as four!
The O2 sensor is mounted in the exhaust manifold to monitor how much unburned oxygen is in the exhaust. The signal from the O2 sensor tells the computer if the fuel mixture is burning rich (less oxygen) or lean (more oxygen).
A lot of factors affect the richness or leanness of the fuel mixture, including air temperature, engine coolant temperature, barometric pressure, throttle position, air flow and engine load. Other sensors monitor these factors too, but the O2 sensor is the master monitor for what's happening with the fuel mixture. Problems with the O2 sensor can throw the whole system out of whack.
The computer uses the oxygen sensor's input to fine tune the fuel mixture for the best balance of power, economy and emissions. The engineering term for this type of operation is "closed loop" because the computer is using the O2 sensor's input to adjust the fuel mixture. The result is a constant flip-flop back and forth from rich to lean which helps the catalytic converter operate at its best and keeps the average fuel mixture in proper balance to minimize emissions. It's a complicated setup but it works.
If no signal is received from the O2 sensor, like when a cold engine is first started (more on that in a minute) or the 02 sensor fails, the computer orders a steady, rich fuel mixture. This is referred to as "open loop" operation because no input is used from the O2 sensor to fine tune the fuel mixture. If the engine fails to go into closed loop when the O2 sensor reaches operating temperature, or drops out of closed loop because the O2 sensor's signal is lost, the engine will run too rich causing an increase in fuel consumption and emissions. As you might have guessed, that will set a code and turn on your check engine light.
How does it work? The O2 sensor produces a voltage once it gets hot. The sensor compares how much oxygen is in the exhaust to the oxygen in outside air. The greater the difference, the higher the voltage reading.
If you ever replace an O2 sensor (and if you're a DIY'er this is something you will do eventually), its important to remember that the O2 sensor needs to "breath" outside air to work. So don't put any grease on the sensor because it could block this air flow.
An oxygen sensor will typically generate up to about 0.9 volts when the fuel mixture is rich and there is little unburned oxygen left in the exhaust. When the mixture is lean, the sensor's output voltage will drop down to about 0.1 volts. When the air/fuel mixture is balanced or at the equilibrium point of about 14.7 to 1, the sensor will read around 0.45 volts.
When the computer reads a rich signal from the O2 sensor it leans the fuel mixture to reduce the sensor's reading. When the O2 sensor reading goes lean the computer reverses again making the fuel mixture go rich. This constant flip-flopping back and forth of the fuel mixture occurs anywhere from 2 to 7 times a second at 2500 rpm on OBDII vehicles, depending on what type of fuel injection system they have.
The oxygen sensor must be hot (about 600 degrees or higher) before it will start to generate a voltage signal. Many oxygen sensors have a small heating element inside to help them reach operating temperature more quickly.
Ok – that was a lot of info on what they do and how they work. The next thing to know is that trouble codes relating to O2 sensors are very common. But you really need investigate further before replacing an O2 sensor just because you got that trouble code. Armed with the information above on how often the O2 sensor "flips" back and forth and AutoTap or another scantool that allows you to monitor O2 sensor voltage, you can be certain whether the O2 sensor itself is really the problem. These sensors can be pricey, so don't just replace them the first time you see that trouble code!

The O2 sensors are expensive, diagnose what really is going on.