The MIL only comes on when a malfunction occurs that directly effects emissions and on the systems it was designed to check. Most Diagnostic Trouble Codes (DTC) will only show up with a code reader or scan tool. In addition, it is not uncommon for many "No Code" problems to arise.

If there is a DTC, you should check the whole circuit to locate the problem. If you get a DTC "P0185 Fuel Temperature Sensor B Circuit Malfunction" don't assume it means a bad Fuel Temperature Sensor. While eight times out of ten it is, there is a 20% chance it is a burnt wire or bad connector terminal. While it's not too bad to gamble with a $20.00 part, it is an entirely different matter when a $900.00 distributor is indicated.

Another thing to be careful of is mis-interpreting a DTC. A DTC of "P1130 Heated O2 Sensor (HO2S) 11 At Adaptive Limit" may sound like a bad HO2S, but what it's really saying is "Hey, I've corrected as much as I can. This problem's worse than I can compensate for." So you need to look for a problem in the fuel delivery system because the HO2S is doing its job correctly and replacing it is just throwing $100.00 out the window. It's like your wife telling you your car is on fire and you go out and get a new wife.

Okay, so you checked for DTCs and there were none. Now what do you do? The first tool to use is your eyes. Look under the hood for damaged or disconnected vacuum lines, loose or dirty connections, cracks in the large air intake hoses or a loose, burnt or disconnected ignition wire.

You can still do some basic checks of the engine controls and sensors. If you engine has timing marks, you can check the ignition timing with a timing light to see if it is within specifications. Always refer to the under hood emissions label for the proper procedure for checking ignition timing. If there are no timing marks, you can use a scan tool to check the ignition timing.

Set your scan tool to Timing Check and read the timing. Gradually open the throttle to about mid-range and look for a smooth, steady increase in ignition timing. If it is not smooth and steady, it may indicate a loose timing belt or timing chain.

If the timing is good, you can check the Throttle Position Sensor (TPS) and Mass Airflow Sensor (MAF). These, too, should show a smooth, steady increase as the throttle is increased. If there are any voltage spikes or dips, the sensor is bad. You can also tap the TPS and MAF to see if there are any voltage spikes shown on the scan tool or hiccups. If there are, then the sensor is bad.

Another common problem is the Coolant Temperature Sensor (CTS). With the engine cold use the scan tool to see what temperature it is reporting. It should be within four or five degrees of ambient air temperature. Now start the engine and watch the temperature reading. It should climb smoothly and steadily up to normal engine operating temperature, usually in the 1900 to 2300 range. The Manifold Absolute Pressure (MAP) sensor would be the next thing to look at. Disconnect the vacuum line from the MAP sensor and hook up a hand vacuum pump. Now slowly apply vacuum to the MAP sensor.

� When manifold pressure is low (high vacuum), sensor output voltage is low, under 1 volt.
� When manifold pressure is high (low vacuum), sensor output voltage is high, 4.5 volts.

Again, it should show a smooth, steady increase in the voltage readings.

Okay, now you've checked all these things and you still can't find a problem. The next thing to do is to locate and expose the PCM. In some vehicles it's located under the dashboard behind the glove compartment. In others it's under the hood. But most often it is located behind the right hand kick panel.

Once you have it out, and with someone slowly increasing engine rpm, tap the PCM and see if there is any effect on the engine. Also move the wires going into the PCM back and forth. If there is an effect on the engine, you have a bad wire and/or connection at the PCM.