You are not going to be able to use diagnostic fault codes to tell you which circuit to diagnose further. The engine computer uses numerous sensor inputs and operating conditions and compares them to each other to determine when something is wrong. When one of those things used for comparison is known to have a problem, as in your codes 420 and 430, it suspends any tests that need that information for reference. In fact, there is always a real long list of conditions that must be met for a fault code to set, and one of those conditions is that certain other codes are not already set.
The computer will at times force an artificial lean condition or rich condition, then watch that the oxygen sensors respond appropriately. It also expects to see a temporary rich condition when it commands the charcoal canister purge valve to open. Since it knows it cannot use the oxygen sensor readings, there is no point in running those tests that need that information. The result is there can be all kinds of defects that will never get detected. When they are not detected, there is no fault code to direct you to the circuit or system that needs further diagnosis. You have to figure that out on your own, and in some cases that can be very time-consuming.
The proper way to fix this obviously is to put catalytic converters and oxygen sensors back on, but for this problem, the crank/no-start condition already exists. The computer cannot resume its tests until the engine is running. You did not say how you tested the fuel pump or what the results were. Changing random parts, like the ignition coil, is the most expensive and least effective way to diagnose a problem. The fact these are two relatively new parts suggests you have been having problems leading up to this current one, and it suggests you do not have a good understanding of how the systems work. Either of those parts could cause this no-start, but they would normally be way down on the list of suspects.
The first thing you need to do is determine if the fuel pump is running during engine cranking, not just for the first one second when you turn on the ignition switch, and if you have spark. About ninety five percent of crank/no-starts are caused by a loss of both systems, not just one of them, so you have to look for what they have in common. You cannot simply look at fuel pressure because the fuel pump should run for one second each time you turn on the ignition switch, as I mentioned. That will keep the fuel pressure up to near normal even when the pump does not resume running during cranking. If you have a scanner, you can view "inputs/outputs" under one of the live data screens to see if the fuel pump relay is being commanded on. You can check for spark the same way it has been done since forever. If both spark and fuel pump are missing, injector pulses will be too. The best suspects for that are the crankshaft position sensor and the camshaft position sensor. Some scanners will display those with a "no" or "present", or something similar, to indicate whether their signals are showing up at the computer.
You can also use the scanner to command the computer to turn on the fuel pump and to fire the ignition coil, to test each of them along with their wiring. Be aware too that some GM engines are extremely fussy about correct fuel pressure, and will not run right or even start if pressure is just a few pounds low. Along with a weak pump, a weak battery can cause that too. The clue is the engine may start with a jump-start even though it seems to be cranking okay.
A failing mass air flow sensor is another suspect. If it measures an incorrect amount of air entering the engine, the computer will not command enough fuel to go with it.
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Tuesday, July 25th, 2017 AT 3:45 PM
