P0021
- Powertrain
- Generic (SAE)
- Emissions
Diagnose soon
Valve timing stuck advanced hurts driveability, fuel economy and emissions, and can cause rough idle or stalling, but it is not an immediate mechanical failure on its own. It moves up sharply if the underlying cause turns out to be low oil pressure or a chain out of position, both of which threaten the engine.
- Safe to drive
- Usually drivable short-term if it idles and runs cleanly. Stop driving if oil pressure is low, or if it stalls or misfires.
"A" Camshaft Position - Timing Over-Advanced (Bank 2)
The engine computer detected that the bank 2 'A' camshaft is sitting more advanced than it asked for, or will not return to its rest position when told to. This code is also the general VVT performance code for that camshaft on many vehicles — it can set for a phaser that will not reach or hold commanded timing at all, not only one sitting advanced. It has detected a valve timing position error, not identified the part that caused it.
The ECM controls camshaft phasing by pulse-width-modulating an oil control valve that routes pressurized oil into the advance or retard chamber of the phaser. It reads back actual cam angle from the bank 2 camshaft position sensor referenced to the crank signal, and compares it to the commanded angle. P0021 sets when the actual position exceeds the commanded position in the advance direction by more than the allowed error for a calibrated period. That happens when the valve leaks or sticks in the advance position, when the phaser cannot bleed off oil, or when the cam is mechanically advanced by a chain sitting a tooth off. Because the SAE title is "'A' Camshaft Position — Timing Over-Advanced or System Performance (Bank 2)", many manufacturers also set P0021 for any failure to achieve or hold the commanded angle, so do not test only for over-advance.
How the car detected itThe module runs a closed-loop position check whenever enable conditions are met — typically engine running, oil temperature above a threshold (measured or modelled, since many engines infer it from coolant temperature and run time), and a stable rpm and load band. It commands a target cam angle, waits a settling time, then compares actual to target. If actual is advanced beyond target by more than the calibrated error window, and stays there through the confirmation period, the code sets. Many strategies also flag P0021 when the phaser fails to return to park with the solenoid de-energized, or when it cannot reach the commanded angle at all. The threshold and the settling time are vehicle-specific.
A trouble code records what a control module measured. It does not identify which part failed. Test before replacing anything.
- Safe to drive
- Usually drivable short-term if it idles and runs cleanly. Stop driving if oil pressure is low, or if it stalls or misfires.
- Urgency
- Diagnose soon
- Will it pass emissions
- No
- If ignored
- Fuel economy and mid-range power stay degraded and the vehicle will not pass an emissions test. If the cause is sludge or low oil pressure, the same restriction that is holding the phaser advanced is starving other oil-fed parts — chain tensioners and bearings included. Persistent rough running loads unburnt fuel into the catalytic converter.
- Check engine light
- Solid
- Clearing the light
- A completed drive cycle is normally required before the monitor re-runs and the light can clear.
- Power loss especially in mid-rpm range
- Possible rough idle or stalling
- Increased fuel consumption
Many vehicles show no symptom at all beyond the warning light. The absence of a symptom does not mean the fault is not real.
These are the causes this code can have. For the order to work through them in, see what to check first.
| # | Cause | How often | Component |
|---|---|---|---|
| 1 | Oil control valve stuck (advance) | Very common | — |
| 2 | Sludge in VVT passages | Common | — |
Ordering reflects how often each cause is responsible in general, not a probability for your vehicle. Confirm by testing.
In order. Each of these is cheaper or faster than what follows it, and each one can make the rest unnecessary.
Engine oil level, condition and specified viscosity
The phaser is hydraulic. Contaminated oil or the wrong grade is the single most common reason a phaser will not release, and it costs almost nothing to rule out.
Oil change history and interval
Extended intervals are the usual road to varnish in the narrow VVT feed passages and the solenoid screen.
Recent timing chain or phaser work
A chain reassembled one tooth advanced sets this code with a perfectly healthy solenoid, and no amount of electrical testing will find it.
Freeze frame conditions
A fault that appears only when the oil is cold and thick points at flow restriction; one that appears hot at all loads points at the valve or phaser itself.
Which camshaft 'A' is on this engine
'A' is usually the intake or front camshaft, but the assignment is manufacturer-defined — checking service information before pulling a part avoids replacing the wrong solenoid.
Confirm oil level, condition and grade
Tools Dipstick, shop light
Check level cold on level ground, inspect for sludge or varnish on the dipstick and under the oil cap, and confirm the fill matches the specified viscosity. If the oil is overdue or the wrong grade, change it and the filter before any further testing — it is both a likely cause and a prerequisite for meaningful results.
ExpectedOil at full, clean amber to brown, correct specified grade
Watch commanded versus actual cam angle at warm idle
Tools Scan tool with live data graphing
Graph desired and actual bank 2 cam position live. With the solenoid at rest the actual angle should sit at that engine's park value — usually full retard for an intake phaser, but some engines use an intermediate lock position and exhaust phasers commonly park advanced, so confirm the park value in service information rather than assuming zero. A resting angle away from the specified park value tells you the phaser is not returning; a value that overshoots on command tells you the valve is passing oil it should not.
ExpectedActual at the engine's specified park value with no command, tracking desired within the manufacturer's allowed error when commanded
Sweep the phaser with bidirectional control
Tools Scan tool with bidirectional controls
Command the bank 2 'A' camshaft phaser through its range at a steady warm idle. Note whether it advances, whether it releases, and whether idle quality changes. A phaser that advances but will not release points at a stuck-open oil control valve or a phaser that cannot bleed.
ExpectedSmooth movement in both directions with a prompt return to the specified park position
Test the oil control valve electrically and mechanically
Tools Digital multimeter, hand tools, 12 V bench supply, service information
Measure the solenoid winding resistance and check the control circuit for shorts and opens. Then remove the valve and inspect its filter screen for debris and varnish. Work the spool by hand or with a bench 12 V supply — it must move freely and seal, not sit part open. Winding resistance specifications are vehicle-specific.
ExpectedSpool moves freely and returns fully; screen clean
Verify oil pressure and supply to the phaser
Tools Mechanical oil pressure gauge and adapter, shop light or borescope
Fit a mechanical gauge and read pressure at idle and raised rpm at operating temperature. With the valve out, confirm the feed passage in the head or cover is clear — varnish there mimics every other fault in this list.
Check the camshaft position sensor signal and its reluctor
Tools Two-channel lab scope, reference waveforms
Scope the bank 2 cam sensor against the crank sensor. A cam signal with the wrong phase or with missing edges makes the module calculate an advanced angle when the cam is actually where it should be. Inspect the sensor tip and the target wheel for metal debris.
ExpectedClean pulses in a fixed phase relationship with the crank signal
Verify mechanical cam timing if everything above passes
Tools Cam locking tools, service information
Bring the engine to the manufacturer's reference position and compare timing marks, or use the specified locking tools. A chain installed or jumped one tooth in the advance direction produces exactly this code with healthy hydraulics. Procedures and tools are engine-specific.
| Repair | Only after you confirm | Typical cost | Who |
|---|---|---|---|
| Oil and filter change with the correct specified viscosity, plus a shortened service interval going forward | Oil found degraded, sludged, low, or the wrong grade, and cam angle tracks command normally afterwards | $60–$180 | DIY |
| Bank 2 'A' camshaft oil control valve and screen replacement | Solenoid spool found sticking or leaking on bench test, screen blocked, or the phaser will not release under bidirectional command with oil pressure verified good | $150–$480 | DIY |
| Cleaning of the VVT oil feed passages | Varnish or debris found in the feed passage with the solenoid removed, and a clean solenoid still failing to move the phaser | $200–$700 | Hard |
| Bank 2 'A' camshaft phaser replacement | Oil pressure, feed passage and solenoid all verified good, but the phaser still will not hold or release commanded position | $700–$2,500 | Shop |
| Timing chain re-index or chain, guide and tensioner replacement | Timing marks or locking-tool check show the cam mechanically advanced against the crank reference | $1,100–$3,500 | Shop |
| Camshaft position sensor or wiring repair | Scope capture shows a distorted or wrongly phased sensor signal with mechanical timing verified correct | $130–$420 | DIY |
Solenoid access on bank 2 of a transverse V engine is often the rear bank against the firewall, which can turn a 30-minute job into three hours — the same part number can mean $180 on one car and $500 on another. Budget $100 to $200 for diagnosis, and ask for the oil service to be done first if the oil is overdue, since it changes the result.
- Replacing the oil control valve without inspecting the screen or the oil. A new valve in contaminated oil can stick again.
- Cleaning a varnished solenoid with brake cleaner and reinstalling it without addressing the oil. The varnish is still in the feed passages, not just the valve, so the fault often returns.
- Assuming 'A' camshaft means intake on every engine. The assignment is defined by the manufacturer and needs confirming before ordering parts.
- Testing only for over-advance. On many vehicles P0021 is the general VVT performance code and will set for a phaser that cannot reach or hold the commanded angle at all.
- Chasing the electrical side when the code appeared right after timing chain work. Check mechanical timing first in that case.
- Treating a fault that only sets on cold start as intermittent noise. Cold-only faults are a real and specific symptom of restricted oil flow.
- Camshaft Position Actuator
- Oil Control Valve
- Definition
- Generic code set (SAE J2012 / ISO 15031-6) via the OBDexopen database, dedicated to the public domain under CC0-1.0.
- Diagnostic guidance
- Written and reviewed in-house. See our editorial policy.
- Review status
- Reviewed
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