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The basics of no-start diagnosis

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For a seasoned veteran, or a backyard novice, no start diagnostics can be a frustrating experience. Whether you live here in the Northeast and it’s the middle of January, or if you live further south where the weather is balmy, a no start condition can cause a person to wonder why they wanted to work on cars in the first place. No-start conditions can at first seem simple, but then quickly become frustrating. With the advent of computer control systems, no-start conditions have become more complicated.

Years ago the mantra was “air, fuel, spark, compression”. Assuming you’re still driving a conventional automobile, this mantra holds true. However, once you decide which of the magic four ingredients you’re missing, the inspection list gets longer…or shorter depending on how you view things. No matter what though, no-start diagnostics begin with the absolute basics...always!

Sometimes the biggest challenge is still in determining which of the four ingredients you’re missing. Air is generally an easy one. Is the air filter clear? Is the intake boot cracked? Does the throttle actually open? With the advent of electronic throttle control, you’ll want to make sure the throttle is at least in the default position and not plugged with carbon. Most ETCS systems allow for approximately 20% throttle with no electronic control, with the system pulling the throttle closed for idle speed. Carbon build up will mess this all up though!

Fuel can be a bit of a challenge these days. If you’ve got a fuel pressure gauge, and the appropriate connection, or if the manufacturer has allowed you easy access to the fuel system, you can still measure fuel pressure to determine whether you’ve got fuel. On some vehicles, measuring fuel pressure can be such a pain that you don’t want to do it! At some point however you’ll need to measure fuel pressure. A good guideline for operating fuel pressure on port injected vehicles is 35 to 45psi. Most fuel pressure specifications fall into this range. Fuel pressure below this spec points to a bad fuel pump, a pressure regulator problem, or sometimes a restricted fuel filter. Pressure above this specification generally means a bad pressure regulator. If you have no fuel pressure at all, then your next step is to check the electrical operation of the fuel pump and its associated control devices such as fuses, relays, and modules. Don’t forget to make sure there’s actually fuel in the tank (no matter what the gauge says)!

Spark, on many vehicles is easier to check than fuel pressure may be. The use of a good quality spark tester (I prefer an adjustable spark tester) is essential. In a pinch you can use a spark plug with the gap widened just a bit. You want to see a nice, consistent, solid blue spark. A yellow spark may indicate poor coil operation, and of course no spark is a big problem! If you have a no spark condition, the next step is to determine the type of system your vehicle is using (distributor, distributorless, or coil-on-plug). From here you need an understanding of what signals are involved in determining spark production and delivery. If the entire engine is missing spark, potential problem areas are the crank sensor, powertrain control module, or wiring harness problems. If you are only missing spark on individual cylinders, problem areas include the coil packs themselves, spark plug wires (if equipped), wiring harness between the PCM and coil pack, ignition module (if equipped), or the PCM itself. Always inspect spark plugs for wear, cracks, or missing electrodes!

Compression is determined using a compression gauge. Each engine will have its own cylinder compression specification. Although compression checking is commonplace there are a couple of reminders regarding accurate compression checking. Use a good quality gauge, block the throttle completely open (if not electronic throttle control), and make sure you’ve got a good battery. A bad battery will slow the engine crank speed and will affect the maximum compression that can be generated. Once you’ve established that low compression may be contributing to your no-start condition, you’ll need to do a cylinder leak down check to determine if you’re losing compression through the valves, or past the piston rings.

Determining if you’re missing air, fuel, spark, or compression is the first step in diagnosing a no-start condition. I’ve listed the basics of the basics here. There is, of course, a lot more to no-start diagnostics on modern vehicles. I’ll be addressing some of the other aspects in future articles.

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