Doesn't matter. Per government mandate, the light must turn on when the detected problem, (and fault code), could adversely affect tail pipe emissions. Can't have excessive emissions if the engine doesn't run.
Specific conditions must be met to set a fault code. Those conditions usually include that certain other codes are not set in memory. Here's just one example. The Engine Computer needs a strategy to determine if the speed sensor is not working vs. The car is standing still. It knows the car is coasting by a high vacuum reading for a lengthy period of time. You can snap the throttle open real quickly. When it snaps closed, the vacuum will go very high, but just for a second or two. The only way to get the high vacuum reading for a longer period of time is by coasting. If the computer knows you're coasting, it knows you're moving so it had better be getting a reading from the speed sensor. If there is a stored fault code related to the MAP sensor, there is almost no chance it will store a code for the speed sensor.
Fault codes also have levels of severity. The lowest level codes never cause the Check Engine light to turn on. Number 17, "Engine running cold too long" is a perfect example. Every Chrysler product up north will have that code all through the winter because when started and left to idle, the engine will not get up to normal temperature within 6 minutes.
For a slightly more severe problem, the light will turn on only while the problem is occurring and it will go off when the problem goes away. The code will stay in memory. Still more severe and the light will "latch" on. Even if the problem goes away, the light will stay on until the ignition switch is cycled off and back on. Then the light will stay off until the problem occurs again.
Really severe and the light will always be on whenever the engine is running even if the problem hasn't occurred recently. In the worst case, the light will be flashing. That means "bail out"! Excessive raw fuel is entering the catalytic converter causing it to overheat. It will be permanently damaged if you don't stop the engine.
When you have a no-start condition, have the fault codes read first. If there are no related codes, use the "live sensor data" screen to view the cam and crank sensors. Aftermarket scanners usually read "yes" when pulses are being generated by that sensor. Chrysler scanners read "present" when the pulses are showing up. If you find the pulses missing from either sensor, the automatic shutdown relay won't turn on and the engine won't run.
Wednesday, January 5th, 2011 AT 7:58 AM