Troubleshooting Why a Car / Truck / SUV's Engine Won't Start

There are many things that prevent an engine from starting.  There is a way to isolate the reason, and the most common issues aren’t difficult to take care of.

These are the most common causes seen.  

1) Loose or corroded battery terminals.

2) Bad battery
3) Faulty Ignition Switch
4) Bad Starter
5) Fuel Pump Failure / Wiring Issues to the Fuel Pump

6) Battery Cables (look for swelling)
7) Bad Fuse(s) or Relay(s)
8) Valve Cover Gaskets (can leak oil into spark plug wells)
9) Bad Spark Issues - Distributor, Cap, Rotor, Coil, or Sensors (Camshaft / Crankshaft Position Sensors)

Section 1.
Diagnosing the Problem - Vehicle’s Engine is not Starting
When an auto mechanic / technician first approaches a vehicle that won’t start, we go ahead and open the hood.  Locate the battery.  How do the battery terminals look?    At that point, it’s best to test if we have any power.  This is easily done with a volt meter if handy - looking for 12.0 volts at the least.  If less, we need a jump.  If a meter is not handy, we can move on to the next step anyways.  

Let’s get in the vehicle and put the key in.  Does the dash light up?  You should also hear a small hum for a few seconds (the fuel pump powering up) .  

If we get past that step, we have some power, we know.  

Try to start the vehicle.  Is the engine cranking over? If the engine is turning over just fine, skip this next section, as the starter circuit is just fine.  If the engine is not cranking over, this next part should help you isolate why we can’t get the starter to crank.

Section 2.
Why Won’t the Starter Work or Turn Over the Engine?

Try turning the headlights on bright for a few seconds?  Are they too bright to look into?  If so, you have plenty of battery juice. If not, try getting a jump start - see here.  The next thing you need to see for is if the ignition switch is sending a ‘signal’ to our starter relay.  Have a friend help you.  If you can hear or feel the starter relay clicking, the ignition switch is fine.  If not, the following things can also inhibit the ignition switch, before you assume it’s bad.  

- Neutral safety switch - Under the clutch pedal on manual transmissions, located on the transmission at the linkage for automatics.  Check to also make sure the vehicle is in park.
- Bad Starter Relay - Try switching out with another one that’s identical on the vehicle.
Manufacturer Passlock Systems - Check for security lights lighting up or flashing when attempting to start the car.  Try another key if available.  General Motor vehicles commonly have this issue.
- Weak Ground / Battery Connections - We always look for this anyway, but sometimes when trying to get the starter to draw a LOT of current, it will antagonize a short.  This is especially true if the problem seems intermittent.  Look for a loss of all power if this occurs, and check if the battery terminals are loose.  Trace the negative cable as well.

Section 3.
The Engine Cranks Over, but Will Not Start

The best thing to do in this instance is crank the engine over for a good 10 seconds.  Next step, get a buddy to help you.  Pull out a spark plug that is easily accessible.  You want to do this immediately after trying to crank it over.  Does the plug smell of gasoline?  If so, we can rule that out.  If it does not smell of gasoline, begin diagnosing fuel issues.  At this point, take a spark plug hooked into the wire and touch the tip of it on the engine somewhere metal.  Have your buddy crank the engine over for a good 2 seconds.  Check for nice bright sparks.  This can be done with a spark tester tool also, if you have it. If you cannot find spark, diagnose the ignition components.  If we can confirm fuel and spark, we are now onto the third main ingredient; compression.  With a compression tester, check each cylinder for at least 100 PSI - ideally over 125 PSI.  A good engine averages between 150-175.  If you’re missing compression, check for mechanical timing issues.  If the timing is fine, a cylinder head issue could be an issue.  At this point, a professional mechanic is highly advised.  Cylinder head work should be done at a machine shop with the proper heavy equipment.  New head gaskets should also be re-installed, and we recommend to replace the cylinder head bolts as well to make sure the proper torques get applied while seating the head(s).  All more the reason to make sure the work is done by a professional.  

Section 4.
Other Causes to Look For

- Bad ECM or wiring
- Power Control Module shorts / malfunction
- Bad fuel pump or fuel pump power circuit issues

- Check the OBDII system.  Fault codes may point to a malfunctioning sensor.  
- Vacuum Lines broken, unhooked, broken wires, or anything obviously out of place in the engine   bay.
- Check that all spark plug wires are securely seated on the plugs and their corresponding coils.  

When all else fails, consult a trained auto repair professional who has access to automotive manufacturer specific fault trends and diagnostic procedures.  Vehicles these days can get very complicated.  This article is designed to cover a large majority of the common issues that we see everyday as auto repair professionals.  Then and again, there are issues so out of the ordinary that it can even stump highly trained technicians.  Hope this info helps.