A diesel that won't start is a different beast to a petrol no-start. The fuel system works differently, the ignition system works differently, and the list of suspects is its own thing. Working through it in the right order saves you time and money.
First question: is it cranking at all?
That single question splits the diagnosis into two completely separate paths.
Not Cranking at All
You turn the key, get lights on the dash, maybe a click, but the engine doesn't turn over. This is a power or mechanical problem, not a fuel or glow plug problem.
Battery first. A diesel engine needs significantly more cranking amps than a petrol equivalent. A 2.0-litre petrol might crank fine on a battery putting out 300 cold cranking amps. A 2.2-litre diesel often needs 600+. An old or partially flat battery will power your dash lights and radio just fine but fail the moment it has to spin that starter. Get the battery load tested, not just voltage checked. A battery showing 12.4 volts can still be knackered.
If you've got a multimeter, check battery voltage while someone cranks the engine. Anything dropping below 9.5 volts under load usually means the battery is on its way out.
Earth connections. Diesels are sensitive to poor grounds. There are typically two earth straps: one from the battery negative to the body, and one from the body or battery to the engine block. Corrosion or a loose strap creates resistance that causes all sorts of weird electrical behaviour, including a no-crank. These straps are cheap and easy to check. Look for green or white corrosion at the terminals.
Starter motor and solenoid. A single loud click when you turn the key usually points to the starter solenoid engaging but the starter motor failing to spin. A rapid series of clicks (click-click-click-click) usually points to the battery or connections. No click at all might mean the neutral safety switch or clutch switch isn't seeing the car as parked/neutral.
Immobiliser. Modern diesels have immobilisers just like petrols. If the key fob battery dies, the transponder chip in the key sometimes can't communicate with the ECU. Try the spare key if you have one. Some older European diesels, like the Mercedes W202 C-Class CDI, are also known for faulty immobiliser relay issues causing no-crank symptoms.
Cranking But Not Starting
The starter is spinning, the engine is turning over, but it just won't fire. This is where diesel gets interesting.
Glow plugs. Diesel engines don't have spark plugs. They compress air until it gets hot enough to ignite fuel on contact. Glow plugs are heating elements in each cylinder that pre-warm the air during cold starts, giving compression enough help to get combustion going. When glow plugs fail, the engine might start fine in summer and refuse completely in winter, or crank for ages before reluctantly firing.
Most modern diesels have a glow plug light on the dash. You wait for it to go out, then start. If the light doesn't come on at all, or the car cranks but won't fire in cold conditions, glow plugs are a very likely culprit.
Testing them is straightforward with a multimeter: each plug should show low resistance (typically under 1 ohm). High resistance or open circuit means a dead plug.
Cost to replace glow plugs: $150-400 USD ($220-600 AUD) for a full set including labour, depending on accessibility. Some engines make glow plug access a nightmare.
Air in the fuel system. Diesel fuel systems are closed and pressurised. If air gets in, through a leak in the fuel lines, a cracked fuel filter housing, or a loose connection, the system loses prime and the engine won't start. You'll usually hear the engine crank normally but never fire.
Many diesel vehicles have a manual primer pump on or near the fuel filter. Pumping it until you feel resistance can clear air and get the system primed again. If air keeps getting in, you've got a leak somewhere that needs finding.
Older diesels (pre-2000s) are particularly prone to this. Fuel line O-rings harden with age and start letting air in overnight, causing hard morning starts.
Fuel delivery and injector problems. If the engine cranks well, glow plugs are fine, but nothing happens, fuel delivery is the next suspect. The lift pump (low-pressure pump that pulls fuel from the tank to the injection pump) can fail quietly. So can a clogged fuel filter, which on a diesel should be changed every 20,000-30,000 km, not left for years.
On common rail diesels, the high-pressure pump generates 1,000-2,000 bar of pressure to fire the injectors. If it's struggling, you might get hard starts, rough running, or no start. Injectors themselves can fail, either not opening (no fuel) or being stuck open (flooding, white/black smoke).
Testing fuel pressure at the rail requires specialist equipment. This one generally needs a diesel-specific workshop.
Cold Weather Makes Everything Worse
Diesel fuel can gel in very cold temperatures. The waxy components in regular diesel crystallise and block filters and fuel lines. If you're somewhere that gets below about -10°C, or you've accidentally been sold summer-blend diesel heading into winter, this can cause a complete no-start.
Diesel anti-gel additives (around $10-20 USD / $15-30 AUD a bottle) mixed into the tank can prevent this. If you're already gelled up, the car needs to be warmed before it'll move.
What to Check First: Quick Summary
Start with battery voltage and connections before pulling things apart. A flat or weak battery is the most common cause of a diesel no-start, and it's free to check.
If the battery is good and it won't crank: check earth straps, test the starter solenoid, check for immobiliser faults.
If it cranks but won't fire: check glow plugs, bleed any air from the fuel system, replace the fuel filter if it's overdue, then move to fuel pressure testing.
Diesel systems aren't as DIY-friendly as petrol once you're past the basics. Fuel injection testing, injector coding, and high-pressure system work really do need someone with the right diagnostic equipment. But the battery, grounds, glow plugs, and fuel filter are all things you can check or replace yourself and will solve the problem more often than not.