Emissions Diagnostic Checklist

If the problem involves drivability, test-drive the vehicle first, if at all possible. Perform a preliminary visual and/or functional check. Look for damaged or disconnected wires, connections, and vacuum lines. Make sure the vehicle is adjusted to manufacturer specs regarding timing, dwell, idle speed, etc. Don't forget to check for correct timing also at cruise. Access any TSBs or recall notices. Follow the diagnostic flow chart.

If the vehicle is warmed up and the temperature is still cold, you may have a defective gauge, bad thermostat, the incorrect stat for that vehicle, or a defective ECT sensor.

If they're dirty and clogged, replace them. High CO can result from a restricted air intake.

Is it sticking open or sticking closed? Are the passages clogged or is the hose itself collapsing? A defective PCV can cause high CO or HC readings. A high CO reading will result if the crankcase oil is contaminated with fuel. A high HC reading will result if the PCV valve does not meter the airflow properly, if the incorrect valve is installed of if the vacuum side of the system is leaking air (such as through a cracked or broken hose, loose fitting, etc).

Is there a vacuum leak at the baseplate? Is the choke stuck open? Are there any leaking fuel lines or injectors? On fuel-injected vehicles it is crucial to check for proper fuel pressure; pressure that's too high or too low causes high emissions and drivability problems.

Look for holes, cracks, disconnected, misconnected, or missing lines, or lines plugged off with screws or BBs. All the high-tech diagnosis in the world won't help if you've got uncorrected vacuum leaks.

Another condition to be corrected or eliminated is catalytic converter failure or a blocked exhaust system. To check for proper operation, first block the wheels, put the vehicle in Neutral or Park, disconnect the air injection system and check that all fluid levels are satisfactory for safe operation. (Caution: do not use these procedures for BMW, Peugeot, or Volvos with ZF automatic transmissions!)

A) Disconnect the evaporative canister purge line.

B) Measure the HC, CO, CO2, and O2 tailpipe readings with the engine at idle and at normal operating temperature.

C) If CO and HC are above specs but CO2 is low and O2 is between 1% and 2%, the problem could be either temporary saturation of the catalytic converter or a defective cat.

D) To check for temporary saturation of the cat, run the engine at 2500 rpm for three minutes and observe the HC, CO, and CO2 readings. If the cat is functioning properly, CO and HC readings should decrease and CO2 should increase. If CO and HC do not decrease but CO2 does, repeat the procedure. If CO and HC still do not decrease, go to step 6 above.

E) Check the O2 level. If it is below 1%, the engine may be running too rich. Investigate and repair. If the O2 rises above 3 or 4%, the engine may go into a lean misfire, causing HC to rise. Again, investigate and repair. Note: the use of oxygenated fuels may push the O2 up to 3 or 4% even though nothing is wrong.

F) Connect a vacuum gauge up to a manifold vacuum source.

G) Disconnect and plug the EGR valve.

H) Observe manifold vacuum at idle. Most vehicles will produce 17 to 21 incehs of vacuum. Some will produce only as much as 14 to 16 inches, but this must be suspect if the reading is this low. The vacuum gauge needle must be holding fairly steady without wild swings back and forth.

I) Accelerate the engine to 2500 rpm and observe manifold vacuum for at least 15 seconds. If system vacuum is high and steady, the exhaust system is probably ok. If the vacuum drops, check for damage to the exhaust system. If there is no damage apparent, remove and inspect the cat for internal damage.

The most important point to all of this is to remember the basics. When you see a doctor, he takes your temperature and blood pressure, and asks you to explain your symptoms before he starts testing. Rule out or correct all the obvious problems first before spending time on electronic diagnosis. If you've done everything you can think of and a problem persists, call a tech hotline. Be prepared to give the model, engine size and other details about the vehicle and what you have done, in that order.