A P0016 trouble code usually means the engine control module has found a mismatch between crankshaft position and camshaft position. In plain terms, the engine is no longer seeing the two signals in the relationship it expects, which is why this fault often points to timing, not just a faulty sensor. This article breaks down what the code means, the symptoms that matter, the most likely causes, and the diagnostic order I would use in a real workshop.
What this fault usually means and how to approach it
- P0016 is a correlation fault, so the ECU sees the crankshaft and camshaft signals out of sync.
- The problem is often mechanical timing or VVT-related, not just an electrical sensor failure.
- Common symptoms include rough running, long cranking, reduced power, and a rattling noise on start-up.
- In the UK, a basic diagnostic scan is often around £60-£120, while timing repairs can move into four figures.
- Clearing the code rarely solves it unless the underlying timing or signal issue is fixed.
- If the engine misfires badly or rattles, I would treat it as a do not ignore fault.
What the fault actually means
P0016 is a correlation code. The engine computer is comparing the crankshaft sensor signal with the camshaft sensor signal and deciding they do not line up the way they should. On a healthy engine, those signals stay locked in a predictable relationship because the camshaft and crankshaft are mechanically synchronised by a timing belt or timing chain.
That is why this fault is different from a simple sensor circuit code. A bad cam sensor or crank sensor can trigger it, but so can stretched timing chain, a slipped belt, a sticking cam phaser, or even an engine that was assembled slightly out of time after recent work. On many engines, “Bank 1” means the side with cylinder one, and “Sensor A” usually refers to the camshaft signal the ECU is using as its primary reference, although exact naming varies by manufacturer.
I treat this fault as a timing problem first and an electrical problem second. Once that distinction is clear, the symptoms make a lot more sense.
The symptoms that usually show up first
Some cars only light the check engine lamp and keep driving. Others feel noticeably off straight away. The difference usually depends on how far the timing is out and whether the engine is also dealing with VVT or oil pressure issues.
| Symptom | What it usually feels like | What it suggests |
|---|---|---|
| Check engine light | The warning lamp comes on, sometimes with no obvious driving change at first | The ECU has detected a timing correlation fault |
| Hard starting | The engine cranks longer than normal or stumbles when it fires | Cam and crank timing may be far enough out to affect starting |
| Rough idle or misfire | Shaking, unstable idle, hesitation, or uneven power delivery | The valve timing may be wrong under light load |
| Rattling on start-up | A brief metallic rattle from the top or front of the engine | Chain stretch, tensioner wear, or phaser delay becomes more likely |
| Reduced power | The car feels lazy, especially when accelerating or climbing hills | The ECU may be limiting performance to protect the engine |
| Poor fuel economy | More fuel used for the same journey | Incorrect valve timing can hurt combustion efficiency |
If the car only has a light on the dash and drives normally, the fault is still real. But if there is rattling, misfiring, or a hard start, I would stop treating it as a minor warning and start treating it as a timing issue that can get worse quickly. That leads directly to the part most owners want to know: what actually causes it.
What usually causes the correlation error
The cause list is broader than most people expect, but the pattern is usually clear once you sort it by likelihood.
| Likely cause | Why it matters | Typical clue |
|---|---|---|
| Stretched timing chain or worn belt | The camshaft is no longer tracking the crankshaft precisely | Often worse on higher-mileage engines or after long oil change intervals |
| Timing installed incorrectly | Even one tooth out can be enough to trigger the code | The fault appears after recent belt, chain, head, or cam work |
| Faulty camshaft or crankshaft sensor | The ECU is being given a bad signal | Intermittent code, rough running, or signal dropout on live data |
| Wiring or connector damage | Corrosion, oil intrusion, or heat damage can distort the signal | Code appears and disappears, often with other electrical faults |
| Sticking VVT solenoid or cam phaser | The camshaft is not moving to the commanded position quickly enough | Oil-related problems, sluggish response, or noise on cold start |
| Low oil level or dirty oil | Variable valve timing depends on clean oil and decent pressure | Service history is poor or the oil looks thick, dark, or diluted |
| Reluctor wheel or mechanical timing component issue | The signal reference itself is no longer trustworthy | Less common, but important if the fault appeared suddenly after repair |
The biggest mistake I see is starting with parts replacement. A new cam sensor sometimes fixes the issue, but just as often it only empties the wallet while the real fault stays hidden in the timing gear or oil control system. The next section is the order I would actually use to diagnose it.
How I would diagnose it step by step
A cheap code reader can confirm the fault. It usually cannot prove the cause. For that, I want live data, timing verification, and a proper visual inspection.
- Read all codes and freeze-frame data. I look for related faults such as P0017, P0018, P0019, cam sensor codes, misfire codes, or VVT actuator faults. If several are present, the diagnosis shifts quickly toward timing or control problems.
- Check oil level and oil condition. If the oil is low, dirty, or the wrong grade, I fix that before chasing anything else. Variable valve timing is very sensitive to oil quality.
- Inspect connectors and wiring. I check the cam sensor, crank sensor, and VVT solenoid harnesses for oil contamination, broken clips, rubbed insulation, and loose pins.
- Look at live data. A scan tool that can graph cam and crank signals is much more useful than a basic reader. I want to see whether the cam angle is tracking the command the ECU is asking for.
- Verify mechanical timing marks. If the engine uses a belt or chain, I check the physical alignment. A stretched chain, weak tensioner, or mis-installed belt is a common root cause.
- Test the VVT system. If the engine has a cam phaser or oil control solenoid, I check whether it responds properly to command changes. A stuck phaser can mimic a timing problem.
- Use a scope if the diagnosis is still unclear. A lab oscilloscope shows the sensor waveforms directly, which helps confirm whether the issue is electrical or mechanical.
- Perform any relearn procedure required by the manufacturer. Some engines need a cam/crank relearn after repairs. Skip that step and the fault may return even though the hardware is fine.
If the code appeared immediately after a timing belt, timing chain, head gasket, or cam sensor job, I would put the burden of proof on the recent repair first. That is often where the problem is hiding. Once diagnosis is solid, the repair choice becomes much easier to judge.
What the repair usually costs in the UK
There is no single price for this fault because the fix depends on whether the problem is electrical, oil-related, or mechanical. In many UK workshops, the diagnostic bill is the first line item, and that is money well spent if it avoids unnecessary parts.
| Repair path | When it is usually the answer | Typical UK cost |
|---|---|---|
| Diagnostic scan and correlation check | First step for almost every case | £60-£120 |
| Oil and filter service | Oil is low, dirty, or the wrong grade | £70-£180 |
| Sensor or wiring repair | Clear electrical fault, damaged connector, or signal dropout | £90-£250 |
| VVT solenoid or cam phaser replacement | The cam timing control system is sticking or slow to respond | £150-£600 |
| Timing belt or chain correction | The engine is mechanically out of time | £450-£1,500+ |
| Internal engine repair | Timing damage has led to bent valves, compression loss, or major wear | £1,500-£4,000+ |
Those figures are rough workshop ranges, not fixed quotes. The engine layout matters a lot, and labour can rise sharply if the timing components sit deep inside the engine bay. That is also why a simple reset rarely solves the real problem for long.
Why clearing the code usually does not fix it
Resetting the warning light can make the dashboard look better for a short time, but it does not restore mechanical synchronisation. If the crank and cam signals are genuinely out of phase, the ECU will see the same mismatch again as soon as the test conditions are met.
That is why a fault that returns after clearing is useful information, not bad luck. It tells me the issue is still present under load, during cold start, or at a particular cam angle. If the code comes back quickly, I move away from guesswork and back toward timing verification, oil pressure checks, and waveform testing.
I also pay close attention to any recent work on the car. A code that starts after a belt change or chain service is not proof of a bad new part, but it is a strong reason to inspect the installation before replacing anything else. From there, the most useful next step is not another reset. It is a sensible plan for the next workshop visit.
What I would tell a driver before the next start-up
If the engine is quiet, starts cleanly, and the only symptom is the warning light, the car may be drivable for a short trip to a garage. If it rattles, misfires, struggles to start, or feels weak enough to affect normal traffic, I would not keep using it as though nothing is wrong.
The most useful information to bring to the garage is simple: when the light came on, whether the car had recent timing or oil work, what the engine feels like when cold, and whether any other codes are stored alongside the cam/crank fault. That gives a technician a proper starting point instead of a parts-swapping exercise.
My rule with this fault is straightforward: check the oil, scan the related data, verify the timing, and only then decide what to replace. That approach protects the engine, avoids unnecessary cost, and usually gets you to the real cause faster than chasing the sensor itself.
