High Carbon Monoxide levels can be caused for a variety of reasons, but in general high CO levels from the exhaust can be attributed to and is a by-product of poor or incomplete combustion. This can be the result of too much fuel or not enough air going into the engine. All fuel delivery is control by the on-board computer hidden under your dashboard, but several sensors can lead the computer to delivering too much gas. Keep in mind failing an emissions test can seem really bad, but it doesnt necessarily mean there is something seriously wrong with your car; it just means its producing a higher level of emissions than the state you reside in has deemed acceptable. Either way if you want to keep driving your 93 Chevy Lumina regardless of the engine configuration you will have to lower the carbon monoxide your vehicle is producing.
Even something as simple as skipping the basic maintenance and failing to replace your air filter can lead to higher than acceptable CO levels. That clogged air filter is restricting air to the engine while the ECM is still pushing the same amount of fuel into the engine; creating a rich mixture. This rich mixture causes an incomplete burn resulting in a higher carbon monoxide production during the combustion process. Change out the air filter with a new one, and make sure there are no other restrictions in the air tubing or throttle body. Small animals can crawl into your engines filter housing and build nests; causing a restriction and raising your air to fuel mixture too high. If you live in a lightly populated area or your ride sits for days at a time you can attach a small piece of wide space screen to the inlet of the intake box.
The manifold absolute pressure sensor or MAP sensor reads the pressure or vacuum inside the intake manifold to help the ECM know how much fuel to air is actually needed. If the MAP sensor is faulty or relaying a bad reading, the computer may begin to dump extra gas that isnt needed raising your CO levels. The same concept goes for the throttle position sensor. You can find this opposite of where the throttle linkage connects to the throttle body. If the TPS is sending a high signal, the computer may mistake 50% throttle for an actual input of 35% apply a large amount of unneeded gas.
Coolant Temperature Sensor
The coolant temperature sensor is the sensor that relays your engine temperature to your temperature gauge and to the computer. If this sensor relays a lower than accurate temperature the computer will deliver more fuel to the combustion chamber because a lower temperature requires more fuel for proper operation. Without being able to determine an accurate temperature reading the computer will naturally lean towards a richer mixture to avoid the higher temperatures related to a lean mixture. On the 3.1-liter engine the sensor is on the rear of the engine by the upper plenum, on the 3.4-liter it is on the front left side of the engine to the left of the exhaust manifold and on the 2.2-liter engine it is on the left hand side of the engine attached to the coolant outlet housing. You can remove the sensor and visually inspect it for any build up and to clean as necessary. If there is a lot of buildup on the sensor head you should also perform a coolant flush before your next emissions test.
The oxygen sensors attached to your exhaust piping relay how much oxygen content is left over after the combustion process. The on-board computer will use this information to help adjust how much fuel to deliver. If the oxygen sensors are wrongly reporting a lean condition the computer will compensate with more fuel to make up the difference and raise the air to fuel ratio. A faulty sensor will likely cause the computer to illuminate the check engine light, but in some cases the computer may not catch the faulty reading. In the end your elevated level of carbon monoxide may come from any one of these sensors or a number of them combined. Your best bet is to retrieve any trouble codes stored in the computer as the first step to finding the main source of your elevated CO levels. You may be able to find a local parts store that has an emissions sniffer for rent to help you detect your emissions levels as you complete any repairs.