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The degree of how stringent the test depends a lot on what state and region you are in. My two 240's are veterans of VA and CA emissions testing, among the toughest. For these two states, each has two sets of passing requirements, one more difficult for states' high traffic density regions (e.g. LA County and Northern Virginia). The tougher test requires a 15 and 25 mph dyno "load" on the car and includes NOX and well as CO and HC. The easier test is for non-urban areas, tests for CO and HC only (no NOX) at Idle RPM and 2500 rpm w/no dyno load.
In general however, prep your car as best you can:
New filters
Recent complete ignition tune up
Operating O2 sensor
Operating Catalytic Converter
Many 240s suffer from intake vacuum leaks which will drive up the NOX number and also may cause the O2 sensor to overcompensate and affect the HC and CO numbers. Most common source of vacuum leaks are fuel injector seals and cracked, broken or disconnected rubber vacuum lines.
If CAT is not good, there is no way the car will pass.
You are right, pre-heat hose needs to be intact and air box thermostat needs to work, or at least be in the open (not pre-heated) position. Emissions test also includes fuel tank cap test; make sure it has a good seal. Your PCV (flame trap) system should be maintained.
All above from my own experiences. Many shops offer a pretest service so you can determine what shape your car is in.
Good luck
Robert
84 245Ti
88 245DL
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