A.G.S. said:
3. Get an exhaust sample. SOP for this was a 55 gallon Hefty bag. Wrap it around the exhaust pipe tightly, and try to get the first few seconds flow for an accurate reading. Tie off the bag as best you can, and take it to your local service shop to have it analyzed. If ya can, try and find a veteran run one, as they are more likely to know how to handle it.
Did you use a clear bag?!? Don't you know the sunlight will ruin the sample. Do it again and use a black bag this time.
Did you use a black bag?!? Don't you know it absorbs more heat and throws off the reading. Go get a clear bag and do it again.