P0340: camshaft position sensor — why the car only starts on the second try

P0340 “Camshaft Position Sensor Circuit”, together with P0341–P0343, is one of the most common causes behind “it starts every other time, then drives fine”.

What the sensor does

The control module needs to know not only where the crankshaft is but which stroke is happening in a given cylinder. The crankshaft turns twice per combustion cycle, so its position alone cannot distinguish compression from exhaust.

The camshaft sensor supplies that. Without it the module does not know which cylinder to inject into right now.

Why the car still drives

Here is the interesting part. Having lost the camshaft signal, most modules fall back on a backup strategy: inject into all cylinders simultaneously, or determine phasing by trial over the first revolutions.

The consequences:

• Longer cranking — the module needs a few revolutions to decide. The classic “cranks for a while, then catches”.
• Sometimes it will not start first time at all.
• Slightly higher consumption.
• Worse response, especially at low RPM.
• It may stall while driving and restart immediately.

Which is why people run with this code for months: the car works, it just annoys them.

Causes by probability

1. Wiring and connector. The sensor lives in a hot area on the cylinder head. Insulation ages, contacts corrode. The most common cause and the cheapest.

2. Sensor contamination. It is magnetic, and metal debris from the oil sticks to it. Typical on high-mileage engines. Remove, clean, carry on.

3. The sensor itself. Killed by heat. The signature is distinctive: works cold, disappears warm. The most treacherous variant, because a workshop testing it cold finds nothing.

4. The reluctor ring or pulley. Damaged teeth produce an uneven signal.

5. The timing chain. If P0016 appeared alongside P0340, the sensor is no longer the issue — see the P0016 guide.

How to check

1. Read every code. P0016 alongside redirects the search to the chain.
2. Watch live data warm. If the code only appears hot, it is almost always the sensor itself.
3. Inspect the connector for corrosion and heat damage.
4. Remove the sensor and look at its tip — a metal “beard” means contamination.
5. Compare with the crankshaft sensor. Codes for both usually mean power or ground, not two failed sensors.

How this differs from the crankshaft sensor

People confuse them constantly, and the difference is radical:

	Camshaft	Crankshaft
Code	P0340	P0335
Without it the engine	Starts slowly, runs	Will not start at all
Symptom	Hard starting, rough running	Dead stall

The crankshaft sensor is critical and covered separately. The camshaft sensor is not.

What it risks

The engine, essentially nothing. The risk is practical: the car may refuse to start at an inconvenient moment or stall at a junction.

One nuance though: on engines with cam phasers, the module controls them using this same sensor. So P0340 frequently arrives with P0011 or P0014 — and the cause is the sensor, not the phasers. Start with the sensor: the cam phaser guide explains how not to confuse them.

In short

• The sensor tells the module which stroke is happening; without it the module works by trial.
• The main symptom is long cranking, not total failure.
• The most common cause is the connector and wiring, then metal contamination.
• Disappearing when hot almost always means the sensor itself.
• Alongside P0016 it is the timing chain, not the sensor.

Download the app — codes and sensor live data are read with an ELM327 adapter over Bluetooth SPP on Android.