Will not start in the cold: five causes in the order you should check them

Frost rarely creates a new fault. It does something else: it finishes off whatever was already marginal. The battery that still cranked in summer, the glow plug the engine started without in October, the weak fuel delivery invisible in warm weather.

So diagnosis starts not with causes but with the symptom — it halves the list immediately.

Step zero: what exactly is happening

Symptom	Where to look
Does not crank, clicks	Battery or starter
Cranks slowly	Battery or thick oil
Cranks strongly, does not catch	Fuel, spark, crankshaft sensor
Catches then stalls immediately	Fuel, immobiliser
Diesel cranks and does not catch	Glow plugs, fuel

Now each cause in order of probability.

1. The battery

The most common cause, by a wide margin.

Why in winter specifically: capacity falls 20–40 % in the cold while the current needed to crank rises — the oil is thick and the engine turns harder. Two factors working against each other.

What to check: voltage after standing overnight. 12.4–12.7 V is normal; below 12.2 means partly discharged already.

Important: a battery showing 12.5 V at rest can collapse to 9 V under cranking load. So resting voltage is a necessary but not sufficient test.

If it is flat: a charger, not a jump start, when voltage is below 11 V. The correct procedure is in the jump-start article.

If it goes flat regularly — hunt a parasitic drain; procedure in its own guide.

2. Glow plugs (diesel)

The second most common cause on diesels, and practically seasonal.

Why it goes unnoticed earlier: in summer a diesel starts without them. The problem appears precisely on the first freezing night, when the car is needed.

Codes: P0670–P0674. Covered in their own article.

Practical takeaway: check them in autumn, not in the frost. Same logic as the winter preparation checklist.

3. Fuel

On diesel — waxing. Summer fuel thickens below freezing and the pump cannot move it. The signature: even cranking, no catch at all, and it starts after warming in a heated garage.

Condensation in the filter freezes — the same effect. Which is why the fuel filter is replaced before winter regardless of mileage.

On petrol the problem is milder: old fuel vaporises poorly when cold. The signs of old fuel are in the bad fuel guide.

4. Crankshaft position sensor

The cause that takes longest to find, because it is not seasonal by nature — frost simply pushes the sensor over the edge.

The sign that settles it: crank the engine and watch the tachometer. A motionless needle means no signal. Without that signal the module supplies neither spark nor fuel.

Code P0335, covered in its own article. There may be no code at all, because the engine never ran long enough.

5. Immobiliser and key

In winter the key’s battery dies faster and a cold antenna reads the transponder less reliably.

The signature: the engine catches and stalls immediately, with a key or car symbol flashing on the dashboard.

What to try: hold the key against the marked spot on the steering column or in the cupholder — most cars have a backup reading zone. Covered in the immobiliser article.

What not to do

• Crank for more than 10 seconds at a stretch. The starter overheats and the battery goes flat for good. Pause at least 30 seconds between attempts.
• Spray ether into a diesel intake without understanding the consequences — that is shock loading on the pistons.
• Jump-start a deeply discharged battery — both alternators end up working at their limits.
• Give up after three attempts and call recovery without looking at the tachometer and voltage. Two free checks close most cases.

A ten-minute sequence

1. Switch on the ignition and look at the dashboard. Are the lamps bright or dim?
2. Crank and watch the tachometer. A dead needle means the crankshaft sensor.
3. Measure voltage — before starting and during cranking if you can.
4. On a diesel, listen for the glow plug relay and watch the indicator.
5. Read codes if an adapter is to hand.
6. Recall when the battery and fuel filter were last replaced.

In short

• Frost does not create faults, it finishes off marginal ones.
• The symptom halves the search: no crank means power, cranking without catching means fuel or spark.
• The battery is cause number one; resting voltage is necessary but not sufficient.
• A dead tachometer while cranking means the crankshaft sensor, and the check is free.
• Check glow plugs in autumn, not on the first freezing night.

Download the app — voltage, codes, and live RPM while cranking are read with an ELM327 adapter over Bluetooth SPP on Android.