sensor

Crankshaft Position Sensor

The engine's master timing reference. It reports exactly where the crankshaft is and how fast it is turning. On most engines, if this signal disappears the engine will crank but never start — and if it fails while you are driving, the engine can shut off at speed.

Typical cost: $150–$550
Wear item: No
Service life: No service interval; replaced on failure. Heat and oil are what kill it, so sensors mounted low near the exhaust or in the path of a chronic oil leak fail sooner than average. If oil has soaked the sensor or wicked into its connector, fixing the leak is part of the repair, otherwise the replacement will fail the same way.

What it does

Everything the engine computer does with spark and fuel is timed against crankshaft angle. This sensor supplies that angle and the engine speed. It is also how the computer detects misfires: it watches for the tiny slowdown in crank rotation that happens when a cylinder fails to contribute, and identifies which one. On most vehicles, if this signal is lost the computer stops firing coils and injectors entirely — the starter turns the engine over and nothing happens. That is why this part deserves more urgency than a typical sensor. Its signature failure is not a check engine light; it is the engine shutting off without warning, sometimes at road speed. If that happens to you, the steering and brakes still work mechanically, but both get much heavier immediately: power steering assist is gone as soon as the engine stops, and on many vehicles with electric power steering the assist drops out too. Vacuum brake boost is usually good for only one or two pedal applications, after which the pedal goes hard. Keep both hands on the wheel. Do not turn a mechanical key to OFF or LOCK, and do not hold the stop button on a push-button car while the vehicle is moving — the steering column can lock. Signal, coast toward the shoulder, and brake with one hard steady push rather than pumping the pedal away. Then restart if it will restart, or call for a tow. A vehicle that has stalled once at speed should not be driven in traffic, on a highway, or with passengers until the fault has been found. An intermittent crank sensor gives no warning before it does it again, and no clue about when.

A toothed reluctor wheel — mounted on the crankshaft itself, on the flywheel or flexplate, or behind the harmonic balancer — spins past the sensor. The wheel is deliberately imperfect: a common pattern is 60 teeth with 2 removed, or 36 with 1 removed, so once per revolution the computer sees a gap in the rhythm. That gap is the reference mark that converts a stream of pulses into absolute crank position rather than just speed. Tooth counts and patterns vary by manufacturer. The sensor uses one of the same two technologies as the cam sensor. A variable reluctance type has two wires and generates its own alternating voltage, with amplitude that grows as engine speed rises. A Hall-effect type has three wires — power, ground, signal — and produces a square wave that is just as clean at slow cranking speed as at redline. The computer combines this with the camshaft signal to know which stroke each cylinder is on. Some engines can start on the cam signal alone with a delay if the crank signal is lost; most cannot. After replacing this sensor, the harmonic balancer, or the computer, many vehicles require a crank relearn — sometimes called a crankshaft variation relearn — so the misfire monitor can compensate for tiny manufacturing variations in the reluctor. Whether it is required and how it is performed is vehicle-specific.

How it fails

Internal breakdown with heat, which produces the most recognizable pattern in the whole trade: the engine runs normally, stalls once thoroughly hot, refuses to restart, and then starts fine 30 to 60 minutes later. Because that stall arrives with no warning and often at speed, this failure mode should be treated as a stop-driving-it fault, not as something to nurse along until it strands you. A cracked sensor body or a hardened O-ring letting oil inside. Iron debris clinging to the magnetic tip and blurring the signal. A damaged, rusted, or cracked reluctor ring. A harmonic balancer whose rubber isolator has separated, letting the outer ring and its timing marks shift relative to the crank — this looks like a sensor or timing fault and is actually a mechanical failure that must be caught before you chase your tail. Chafed, oil-soaked, or heat-damaged wiring and corroded connectors, which are a frequent cause and cost far less than a sensor. A poor engine ground or interference from badly routed aftermarket wiring.

Symptoms when it is failing

Engine cranks but will not start, with no spark and no injector pulse
Stalls without warning while driving, then restarts after cooling for half an hour or so — this is the urgent one; get off the road and stop driving the vehicle until the fault is identified
Tachometer stays at zero or jumps erratically while cranking
Hard starting specifically when the engine is hot
Intermittent misfire, stumble, or hesitation
Random or multiple-cylinder misfire codes with no mechanical cause found
Sudden loss of power with the engine simply shutting off, including at highway speed

These symptoms overlap with several other faults. They are a reason to test this component, not evidence that it has failed.

How to test it before replacing it

On a crank-no-start, the single fastest useful check is engine speed on a scan tool while the starter is turning the engine. If the computer shows zero RPM or wildly erratic RPM during cranking, the crank signal is not reaching it — and the fault is the sensor, its wiring, or the reluctor. If RPM reads normally and steadily during cranking, the crank sensor is doing its job and you should be looking at fuel, spark, security system, compression, or cam timing instead. Do not rely on the dash tachometer for this on every vehicle; use scan data. From there: 1. On a three-wire sensor, verify the reference voltage feed and a good ground at the connector with the key on. On a two-wire reluctance sensor there is no feed; measure its resistance and its AC output while cranking, and compare both to the vehicle's own specification. Those values vary by manufacturer, so a number from a generic source proves nothing. 2. Scope the signal. This is the proper test. Look for consistent amplitude, clean square or sine transitions, and the missing-tooth gap appearing exactly once per crank revolution. Capture it during cranking on a no-start, not just at idle. 3. For an intermittent hot stall, you have to catch it failing — but do it safely. Heating a suspect sensor with a heat gun in the bay while monitoring the signal is the first choice, because it reproduces the fault with the vehicle parked; keep the gun off fuel lines, the fuel rail, plastic intake parts, and wiring insulation. If you must log RPM on the road, have a second person run the scan tool, use a quiet road or a closed lot rather than a highway, and stop road testing once the vehicle has actually stalled at speed — at that point diagnose it in the bay or trailer it. 4. Remove the sensor and inspect the tip for a beard of metal fuzz, the O-ring for hardening, and the mounting boss for debris that would change the air gap. 5. Verify the harmonic balancer has not slipped by checking the timing mark against known top dead center. A separated balancer will make a good sensor look bad. 6. Replace the sensor only once one of the above confirms it — no RPM at the module with good wiring, a scope trace with dropouts or missing events, an out-of-spec output, or the fault reproduced with heat. A crank position code is a detection that the signal was wrong, not proof the sensor is the part that was wrong. 7. Perform any required relearn after replacement, or the misfire monitor may not function correctly and may set codes on its own. Safety in the bay: keep hands, tools, sleeves, and test leads clear of the belt, the fan, and the balancer while anyone is cranking, and make sure the person at the key can see you and knows not to crank until you say so. The transmission must be in Park or Neutral with the parking brake set and the wheels chocked before anyone cranks the engine. If the vehicle is raised, it must be on stands or a lift, never a jack alone. Exhaust components near a low-mounted sensor stay hot long after shutdown, so let them cool. Safety on the road: if the engine quits while you are moving, see the guidance above — assist steering and brake boost both go away within a moment, so keep both hands on the wheel, leave the key out of OFF/LOCK, and coast to the shoulder before doing anything else.

Specification values differ between manufacturers and model years. Where a figure is not genuinely standard across OBD-II vehicles, check it against service information for your specific vehicle rather than a generic number.

Where it sits

Usually low on the engine block near the crankshaft snout, at the bell housing looking at the flexplate teeth, or behind the harmonic balancer in the timing cover. Access ranges from a two-minute reach to removing the starter, the exhaust, an engine mount, or the balancer itself. Location, connector style, air gap requirements, and whether a relearn procedure is required all vary by vehicle.

What replacement costs

Typically $150–$550 at a US independent shop.

US independent shop, parts and labor. Accessible sensors sit at the low end. Sensors requiring removal of the starter, exhaust, harmonic balancer, or intake, and vehicles needing a scan-tool relearn afterward, move toward and past the top of the range. If the actual fault turns out to be a separated harmonic balancer or a damaged reluctor ring, the repair is larger and more expensive. Add a tow to the budget if the vehicle has already stalled at speed, since driving it to the shop is not the safe option.

Ranges are wide because access varies enormously between vehicles — the same part can be a twenty-minute job on one engine and a half-day on another.

Trouble codes involving this component