Why doesn't it start?

First off it's never a good idea to start an engine on brake cleaner so I would get it taken to a real mechanic shop. Second it could be alot of things from fuel pump to a mass air flow sensor. More than likely its a fuel issue. Check all fuses and If blown replace them with same size fuse. Check again to see if it starts if that doesn't work Check the rela for fuel pump you may be able to swap it out with another one in different location in panel see I it works. After this I it still won't start you need a scan tool. Good luck

You mean the starter doesn't spin the engine?

Sorry ,but you need a better mechanic . This wouldn't be the relay .if it was the relay it would do it all the time . Hooking up a professional scan tool an checking sensor input data to the PCM- engine computer is what should be done . What are the fuel trims at ? short & long trims , are the O2 sensors switching normally , what's the mass airflow sensor data ? any codes stored . It just so happens the relay on your vehicle is a PCB relay . Printed circuit board relay . Which means it's inside the under hood fuse , soldered to the circuit board . Do yourself a favor an take it to a qualified repair shop.

Vehicle fault lights is caused by fuel emissions system more commonly, you at the time of fault lights to start vehicles, car, start, out after three times in the launch vehicle would you see if the trouble light or light, or light up you need in a timely manner to the store check the fault code, is a problem of engine or the sensor fault, or light may be occasional failure can need not in time to the shop to check the sohu auto q&a platforms,auto diagnostic tool.

Step 1 - Anytime you have a problem with electronically controlled components such as an engine, transmission, ABS brake, or SRS (supplemental restraint system, Air Bag) inspect all fuses using a test light and check the under hood power distribution center and under dash fuse panels. If all fuses test okay continue to the next step.
Step 2 - To check for problems with electronically controlled components such as an engine, transmission, ABS brake, or SRS (supplemental restraint system, Air Bag) and the fuses test okay a trouble code scan is needed to identify any system trouble. Use a simple scanner tool to retrieve trouble codes and see if they relate to the specific problem, like a crank angle sensor failure code. If the trouble code present does not pertain to the immediate problem like an EVAP code ignore it until a later time, after the car is running.
The reason we repair non-related codes after the engine is running is because sometime false codes can be triggered by the engine not running. Once the engine is running again the code present might cycle and turn itself off. You might say "if the engine doesn't run shouldn't it have a trouble code?" Sometimes conditions occur that will not be detected by the computer, example: if the fuel pump fails the computer cannot detect the failure, so the engine doesn't start and the computer thinks everything is okay with no codes. If no trouble codes are present proceed to the next step.
Step 3 - The spark plugs in your engine are used to ignite the compressed fuel air mixture. If the condition of the spark plugs are fouled by excessive fuel or carbon the engine will not start, backfire or run rough. Remove all spark plugs to inspect their condition. Please use this spark plug condition reference guide to see how the spark plugs are operating.
Step 4 - Determine if the engine has compression, this can be done a number of ways but the most complete method is to perform a compression check. Remove the spark plugs and perform a compression test on one cylinder. If one cylinder has compression then the remaining cylinders usually will be close to the same. Crank the engine over about 5 seconds, normal compression readings should be between 125 psi and 160 psi on each cylinder. If no or little compression exists additional tests will be needed. The most common reason for an engine to lose compression is a timing belt or timing chain failure.
If low or no compression exists remove the oil fill cap and observe camshaft rotation when the engine is cranked over. If no rotation exists the timing belt or chain has failed. If your engine has a timing belt and you cannot see the camshaft easily remove the upper bolts to the timing cover and gain visual access to the belt, recheck cam rotation by cranking the engine over. Sometimes a timing belt or chain can jump causing the camshaft to lose correlation with the crankshaft and therefore causing low compression. The best test for this condition is to remove the timing belt/chain cover and inspect timing marks. If the compression is ok proceed to next step.
Step 5 - Test the ignition system output, ignition systems can vary in configuration but operate on the same principal. Ignition systems can consist of a coil, pick up coil, crank angle sensor, cam angle sensor, spark plugs, spark plug wires, distributor cap, ignition rotor and a distributor and any variations of these components. An ignition coil is a voltage stepper coil that transforms a low voltage (12 volts) signal into tens of thousands of volts needed to jump the gap of the spark plug.
This coil is activated by an ignition module triggered by using the camshaft/crankshaft angle sensor; timing is adjusted by the PCM (computer).This primary electrical signal is generated by the PCM which calculates spark timing by using a variety of sensors including coolant temperature, mass air flow, and oxygen sensors. To perform a basic ignition output test you must have a test light and follow the ignition system output test video.
If the ignition system test is weak or non-existent test the car fuses, both under hood power distribution center and the fuse panel under dash. This test is performed with a test light tool. The test light should illuminate on both sides of the fuse, if not the fuse has failed and needs to be replaced. If the fuses are ok a manufacturer specific repair procedure is required and an online auto repair manual is needed to continue. If the ignition system tests ok proceed to the next step.
Step 6 - Test fuel system pressure, test for proper fuel pressure with a test gauge on the fuel rail or in line somewhere in the system, most throttle body injection cars (TBI) are between 13 psi and 17 psi. Most (DPI) direct port inject systems are between 40 psi and 55 psi. If good fuel pressure is present continue to next step. If no or little fuel pressure is present check the fuel pump fuse and fuel pump control relay located in the fuse panel, you can find this fuse and relay by checking your owner's manual, back of the fuse panel cover diagram, or an online auto repair manual, if the fuse or relay has failed replace it a new unit and re-test.
(Note: some Ford cars have an inertia switch designed to cut off the fuel pump in the event of an accident. Sometimes this switch can accidentally be triggered causing the engine to not start. If the car is exposed to a random bump either in the road or by another car this switch can be triggered. To check for this condition locate the inertia switch, if the cut off switch has been active it will have a white or red indicator at the top of the switch. Push this indicator down to disarm the cut off switch, if the indicator does not move down it is not activated and is not the problem.)
Have a helper crank over the engine while you place your fingers over the relay, does the relay click under your fingers? If so the relay could be working, there is a chance the relay has burned contacts inside causing the problem but we will get back to that. Next, access the fuel pump power feed wire, there are a few ways to do this, first you need a wiring schematic to find the color wire needed for testing, the best way to do this is with an online auto repair manual. Once you have found the color wire it should be located in the wiring harness near the fuel tank were the pump is located.
Ground the test light and probe (pierce the wire's outer coating with the test light point) the wire, have a helper crank the engine over. If the test light illuminates and you have no fuel pressure the fuel pump had failed and needs to be replaced. If the test light doesn't illuminate the fuel pump control relay has probably failed, replace it with a new unit and re-test, in most cases this relay is under thirty dollars. There is an outside chance the power feed to the relay has failed but it doesn't happen very often. If this is the case use an auto repair manual to trace the power source to the relay.
Step 7 - Test fuel injector pulse and supply voltage output (test is used for most cars). This test will tell you if the computer system has operating voltage and injector trigger signal. Remove an electrical connector from a fuel injector (it doesn't matter which injector) probe both sides of the connector with a grounded test light (there are only two terminals). Have a helper turn the key to the "on" position without cranking the engine and observe the test light. The test light should illuminate one side of the connector only.
Next, switch the test light lead to the positive side of the battery to test the system ground injector trigger, probe the side of the connector that did not light up, have a helper crank the engine over and observe the test light, it should blink on and off. If this test checks ok continue to next step. (Note: if no injector pulse is present try disconnecting the remainder of injectors and re-test, if a fuel injector is shorted it can shut down the injector driver causing no injector pulse. If injector pulse returns plug injectors electrical connectors in one at a time until the pulse fails and replace that injector)
If this test revealed that there was no pulse but system has power the PCM is not generating a fuel injector trigger. If there is no trigger to the fuel injector it will not allow fuel to enter into the engine. Some of the most popular reasons that can cause this condition include a shorted crankshaft angle sensor, shorted camshaft position sensor or shorted PCM. (When a system trouble code scan is performed it does not always catch a crankshaft angle sensor, camshaft position sensor failure).
Tip: try disconnecting all non-essential sensors, example: oxygen sensor, coolant sensor, throttle position sensor, air intake temperature sensor, mass air flow or map sensor and EGR valve pressure differential sensor. Crank the engine over, if the injector pulse returns, one of the sensors is shorted causing the system to not operate. Plug the sensors in one at a time until the injector pulse fails then replace that sensor and reassemble.
(Note: Some Ford cars have an EGR valve pressure differential sensor that when the catalytic converter becomes slightly plugged will melt the sensor causing the system to shut down. Inspect sensor for melting at the electrical connector then repair or replace as needed and recheck).
If the test reveals that the connector has no power on either side at any time the system power has been disrupted. Some of the most common reasons for this is condition are the main PCM fuse, main PCM power relay and main PCM power feed wire failure. (Some vehicle PCM feed wires are located near the battery and corrosion can stop the voltage feed). If all power sources check out the system ground needs to be checked, this is done by reversing the test light lead and installing it on the positive side of the battery.
Now the test light will illuminate when grounded. Use the test light to check main system grounds to the PCM, most system ground wires are black but to be sure you will need an online auto repair manual. If repairs have recently been made a system ground lead could have been left off of the engine causing the system not to power up, so double check all engine wiring harness grounds.
Step 8 - If the engine has compression, ignition and fuel injector pulse and the engine still doesn't run it could have a plugged exhaust system. Disconnect the exhaust system before the catalytic converter and crank over, if the engine starts the car has a plugged converter or exhaust system. Disassemble the exhaust system to inspect to replace the exhaust component that has failed and reassemble to recheck.

It makes me wonder if the new MAF sensor did not fix the problem in that circuit? Were the codes cleared after replacing it? And engine light stays off? Only way to know is have them or someone check for new codes again. The codes direct you to a circuit malfunction. That's why a new sensor doesn't always fix the problem.
It may be something else, but one place to start is have the codes read again. If, as you say, may be an air fuel mixture, you could have your fuel pressure checked to see if it is up to specs. At least eliminate that possibility. Good luck.

Check the codes, you should find that you need to replace the crankshaft position sensor and/or the camshaft position sensor. You can take it to most auto parts stores and they will put it on a meter and run the code test for you. If you need more help, please do not hesitate to ask.

first of all check the fuses
check to see if there is any spark, no spark check ignition system
check fuel pressure, no pressure then check fuel pump
if there is pressure and spark then check to see if fuel injectors work
if no spark and no fuel injector check crank sensor, ignitor and wirings

When was the last time you changed plugs, wires, and cap/rotor? Also check the fuel filter.

I have a 97 gmc sierra .This truck is the same as the chevy ok.Its a 350 5.7 liter 4x4 Z71.I took it to auto zone which was my first mistake.They put it the scanner and said bad O2 sensor.Ok so which one is it i said.They dont know.Well there four (4) O2 sensors in this truck i hope you know that.I had to replace all four of them before i had to take it to a real shop.It seems the fuel pump i replaced a week earlier with an auto zone fuel pump was bad.Ive changed my feul pump 4 times now and have 235000 miles on still running.You need to get your fuel pressure checked.It should be 65 psi to start it and 55 psi to run it.If the pressures are good it could be a different O2 sensor or maybe the mass air flow senor.My last option would be the fuel pressure regulator because it seems to be a waste of time and money.I would get that fuel pressue checked first.Good luck.