Used car cold start: why you should see the engine start cold

Why it matters

On a cold start the engine runs differently from when it is warm. The oxygen sensors are not yet at working temperature, so the engine computer runs richer, from a pre-programmed table, instead of correcting against what they report.

Some faults belong to exactly that phase. A failing glow plug on a diesel shows as hard starting and rough running for the first few minutes, and on some direct-injection petrol engines, intake-valve deposits cause a rough cold idle and misfire codes that clear when warm. Once the engine is warm those symptoms can be gone.

What a cold start can reveal

• A rough or hesitant start, which can be informative on any engine
• Diesel: hard starting, white smoke or rough running for the first few minutes, which points at the glow plugs
• Diesel: hard cold starting can also come from low compression
• Direct-injection petrol engine: a rough cold idle, or misfire codes on cold start that clear when warm
• A rattle on start-up, which can come from the variable valve timing solenoid
• A squealing noise on startup, a sign the belts are due for inspection
• A warning light that stays on, or one that never comes on with the ignition

Step by step

1. Ask to start the car from cold yourself, before anyone has driven it.
2. If the engine is already warm, ask to switch it off and start it again later.
3. Turn the ignition on first and check that the warning lights come on and then go out.
4. Start the engine and listen through the first minutes for a rattle, a squeal, or running that is rough or hesitant.
5. Watch the exhaust and note the color of any smoke.
6. Let it warm up and notice whether anything you heard or felt fades.

Normal, or worth asking about?

• Diesel, the glow plug symbol lights for a few seconds before starting: normal. The car is saying wait a moment.
• Diesel, the glow plug symbol flashes after start-up: the control unit has logged a fault in the circuit.
• Diesel, hard starting, white smoke or rough running for the first few minutes: points at the glow plugs.
• White smoke that persists after warmup: can indicate coolant entering the combustion chamber. This is urgent.
• Blue exhaust smoke: usually oil being burned.
• Black exhaust smoke: usually the engine running rich.
• Electric car on a cold morning, coasting further than usual for the first few miles: normal. A cold battery pack limits regeneration.

What people get wrong

Accepting a car that is already warm and running when you arrive. A cold start is often when a real problem shows itself. If the engine is warm, ask to switch it off and start it again later, and don't let anyone rush you past it.

When to get a professional inspection

If the start was rough, smoky or noisy, or a symptom faded as the engine warmed up, tell the mechanic doing the pre-purchase inspection exactly what you saw and when. Anything that needs tools belongs at a workshop.

Read next

• Exhaust smoke color: white, blue or black explained
• Glow plug light on a diesel: wait until it goes out
• Rough idle: why the engine runs unevenly when stopped

This page is general information and does not replace a professional assessment. Symbols, codes and behaviour vary between makes. Always check the handbook for your own car.