P0337 Crankshaft Position Sensor "A" Circuit Low Input

P0337 means the engine control module (ECM/PCM) has detected an electrical signal from the Crankshaft Position Sensor “A” circuit that is lower than the expected range.

The important part of this code is “Low Input.” It refers to the electrical signal received by the ECM. It does not mean low engine RPM, low crankshaft speed, low engine oil pressure, or low mechanical compression.

The crankshaft position sensor is critical because the ECM uses its signal to determine engine speed and crankshaft position. If the signal becomes too weak or falls below the expected threshold, the ECM can lose its ability to accurately synchronize fuel injection and ignition.

Unlike P0339, which indicates an intermittent circuit problem, P0337 specifically tells you that the ECM is detecting a low electrical input.

What Does the Crankshaft Position Sensor Do?

The crankshaft position sensor monitors the rotation of the crankshaft and sends a signal to the ECM.

This signal allows the ECM to determine:

• Engine RPM

• Crankshaft position

• Piston position

• Ignition timing

• Fuel injection timing

• Engine synchronization

• Misfire information

The crankshaft signal is one of the primary timing references used by the engine management system.

If that signal becomes too weak or disappears, the ECM may not know precisely where the crankshaft is.

That can lead to difficult starting, engine stalling, misfires, or a complete no-start condition.

What Does “Sensor A” Mean?

“Sensor A” is a manufacturer-specific designation.

Many engines have only one crankshaft position sensor, in which case the distinction is straightforward. Some applications may have more complex speed or position sensing arrangements.

Therefore, the exact meaning and location of Sensor A should be confirmed using the vehicle's wiring diagram and service information.

What Does “Low Input” Actually Mean?

P0337 means that the ECM is seeing a crankshaft sensor signal below the expected electrical threshold.

This can happen for several reasons.

The sensor may be producing a weak signal.

The sensor may not be receiving the correct power or ground.

The signal wire may have excessive resistance.

The wiring may be shorted to ground.

The sensor-to-reluctor gap may be incorrect.

The reluctor may be damaged.

The signal may be weak because of a mechanical or electrical problem.

Therefore, “low input” does not automatically mean:

Replace the crankshaft sensor.

The code identifies what the ECM is seeing electrically, not necessarily which component is responsible.

Common Causes of P0337

Faulty crankshaft position sensor

A failing sensor may produce a signal that is too weak for the ECM to reliably recognize.

Sensor performance can also change with engine temperature. A sensor may work normally when cold and produce a weak signal after the engine becomes hot.

Damaged signal wiring

The signal wire may have:

• Excessive resistance

• Partial breakage

• Corrosion

• Chafed insulation

• Poor previous repair

• Intermittent connection

Even when the wire is not completely open, excessive resistance can reduce the signal reaching the ECM.

Signal wire shorted to ground

A short to ground can pull the signal voltage down and cause the ECM to interpret the input as too low.

This is particularly relevant when the wiring has damaged insulation.

Poor connector contact

A loose terminal, corroded pin, moisture contamination, or damaged connector can increase resistance in the circuit.

The sensor may be functioning correctly, but the ECM receives an inadequate signal.

Sensor power supply problem

Some crankshaft sensors require an external power supply.

If the supply voltage is missing or too low, the sensor may not be able to generate the expected output.

The exact electrical configuration depends on whether the engine uses a Hall-effect, variable-reluctance, or another sensor design.

Poor sensor ground

A poor ground can interfere with sensor operation and cause an abnormal or weak output.

A ground that looks acceptable with no electrical load may still have excessive resistance under operating conditions.

Incorrect sensor air gap

On applications where the sensor operates in conjunction with a toothed reluctor, the distance between the sensor and target is important.

If the sensor is too far away, the signal can become weak.

A damaged mounting surface, incorrect sensor installation, or mechanical movement can alter the gap.

Damaged or contaminated reluctor

The sensor needs a correct target pattern to generate a valid signal.

Problems with the reluctor can include:

• Damaged teeth

• Missing teeth

• Excessive contamination

• Cracks

• Incorrect positioning

• Physical damage

A damaged target can make a good sensor appear to have a signal problem.

Metal debris

On magnetic sensor designs, metal particles can accumulate around the sensing element and interfere with signal generation.

This should be considered particularly when the sensor is located in an area exposed to engine or transmission debris.

Crankshaft mechanical movement

Excessive crankshaft end play or another mechanical problem can alter the sensor-to-target relationship.

If the distance between the sensor and reluctor changes unexpectedly, the signal can become weak or unstable.

ECM/PCM fault

An internal ECM input problem is possible, but it should be considered only after the sensor, wiring, connector, power, ground and reluctor have been tested.

Symptoms of P0337

P0337 can cause symptoms ranging from a slight starting problem to complete engine failure.

Possible symptoms include:

• Check Engine Light

• Hard starting

• Extended cranking

• Intermittent no-start

• Engine stalling

• Rough running

• Misfires

• Hesitation

• Poor acceleration

• Sudden loss of power

• Tachometer behaving abnormally

• Reduced engine performance

In some vehicles, the engine may continue running despite P0337.

In others, even a weak crankshaft signal can prevent the ECM from maintaining synchronization.

Why Can P0337 Cause a No-Start?

The ECM needs to know that the engine is rotating and where the crankshaft is positioned.

During cranking, the ECM looks for the crankshaft sensor signal.

If the signal is too weak to cross the required detection threshold, the ECM may effectively see:

“I cannot reliably determine crankshaft position.”

Without that information, the ECM may be unable to correctly control ignition and fuel injection.

The result can be an engine that cranks normally with the starter but does not start.

This is why crankshaft sensor faults can sometimes produce a crank-no-start condition.

Why Can the Engine Run Normally With P0337?

A low signal does not necessarily mean the signal is completely absent.

If the sensor produces a signal that is just strong enough for the ECM to recognize under some conditions, the engine may continue running.

The problem may become apparent only:

• During starting

• At low RPM

• At high RPM

• When the engine becomes hot

• During acceleration

• During vibration

• Under specific electrical conditions

This can make P0337 an intermittent real-world problem even though the diagnostic code describes a low input.

Hall-Effect vs Variable-Reluctance Sensors

Correct diagnosis requires knowing what type of crankshaft sensor the engine uses.

A Hall-effect sensor generally uses an external power supply and produces a digital-style switching signal.

A variable-reluctance (VR) sensor generates an AC signal as the reluctor passes the sensing element. Its signal characteristics change with engine speed.

This distinction is important because there is no universal voltage or resistance specification for P0337.

A measurement that would be normal for one sensor design may be completely wrong for another.

How to Diagnose P0337 Correctly

The best approach is to determine why the ECM is receiving a low signal.

1. Scan all codes

Do not diagnose P0337 in isolation.

Look for:

• Other crankshaft sensor codes

• Camshaft sensor codes

• Cam/crank correlation codes

• VVT codes

• Misfire codes

• Low-voltage codes

• Communication codes

The combination can provide valuable clues.

Freeze-frame data can also show engine RPM, coolant temperature, battery voltage and other conditions when the fault was recorded.

2. Inspect the sensor and connector

Check the crankshaft sensor carefully.

Look for:

• Oil contamination

• Metal debris

• Water intrusion

• Corrosion

• Bent terminals

• Loose terminals

• Damaged connector housing

• Broken locking tabs

• Harness damage

Because many crankshaft sensors are mounted low on the engine, they can be exposed to contamination and physical damage.

3. Identify the sensor type

Determine whether the engine uses a Hall-effect, VR, or another sensor design.

This determines how the sensor should be tested.

Do not apply a generic resistance or voltage test without knowing the circuit architecture.

4. Verify power and ground

If the sensor requires an external power supply, check that the correct voltage is present.

Check the ground circuit as specified by the manufacturer.

A voltage-drop test can be particularly useful for finding resistance in power or ground circuits.

5. Test the signal circuit

Check for:

• Open circuits

• Excessive resistance

• Short to ground

• Short to voltage

• Poor terminal contact

• Intermittent wiring faults

With a P0337, a signal wire that is being pulled toward ground deserves particular attention.

6. Check the actual signal

An oscilloscope is often the best tool for diagnosing a difficult P0337.

Instead of simply asking whether voltage exists, the technician can see the actual waveform.

Look for:

• Low signal amplitude

• Missing pulses

• Distorted waveform

• Irregular pulse spacing

• Electrical noise

• Signal dropout

• Changes with RPM

This can help distinguish a sensor problem from a wiring or mechanical problem.

Why an Oscilloscope Can Be Important

A multimeter may show that a sensor has some voltage, but that does not necessarily mean the ECM is receiving a usable crankshaft signal.

For example, a VR sensor may generate a reasonable signal at one engine speed but become too weak at another.

An oscilloscope can show how the signal changes as the engine accelerates.

It can also reveal whether the signal is being pulled down electrically or whether the sensor simply cannot generate sufficient amplitude.

Check the Reluctor and Sensor Gap

If the electrical circuit appears correct, inspect the mechanical relationship between the sensor and its target.

Check for:

• Incorrect sensor installation

• Excessive air gap

• Damaged reluctor teeth

• Contamination

• Loose reluctor

• Physical damage

• Abnormal crankshaft movement

A weak signal is not always an electrical wiring problem.

The sensor needs the correct physical relationship with the rotating target to generate the expected signal.

Can a Bad Crankshaft Sensor Cause P0337?

Yes.

A failing crankshaft sensor is a common possibility.

But the correct diagnosis should determine whether the sensor is actually producing an inadequate signal.

If the sensor produces a healthy waveform at its connector but the ECM receives a low signal, the problem is more likely to be somewhere between the sensor and ECM.

Can Wiring Cause P0337?

Absolutely.

A wiring fault can pull the signal down or prevent the ECM from receiving the signal correctly.

Common examples include:

• Signal wire shorted to ground

• High resistance

• Partially broken conductor

• Corroded terminal

• Loose connector

• Damaged harness

This is why replacing the sensor without checking the wiring can result in the same P0337 returning immediately.

Can a Weak Ground Cause P0337?

Yes, depending on the sensor design.

A poor ground can change the electrical reference used by the sensor and interfere with its output.

The correct test is a voltage-drop test or the manufacturer's specified ground test rather than simply checking continuity with an ohmmeter.

A ground circuit can show continuity while still having enough resistance under load to cause a sensor signal problem.

Can the Timing Chain Cause P0337?

A timing-chain problem can produce crankshaft/camshaft synchronization faults, but it does not normally explain a simple low electrical input by itself.

If the sensor waveform is electrically weak, investigate the sensor, wiring, power, ground and sensor/reluctor relationship first.

If the crankshaft signal is clean but the camshaft and crankshaft signals are incorrectly synchronized, then timing-chain or VVT problems become more relevant.

Additional correlation codes are particularly useful in making this distinction.

Can Low Battery Voltage Cause P0337?

Low battery voltage is not the direct definition of P0337.

However, battery voltage is important during cranking.

If system voltage drops excessively while the starter is operating, sensors and the ECM may not receive the electrical conditions they require.

If several unrelated electrical codes appear at the same time, check:

• Battery condition

• Charging system

• Battery terminals

• Engine grounds

• ECM power supply

• Cranking voltage

A healthy battery does not eliminate P0337, but an unstable electrical system should be corrected before drawing conclusions about individual sensors.

P0337 vs P0335, P0336, P0338 and P0339

The crankshaft sensor “A” codes describe different types of problems.

P0335 generally indicates a Crankshaft Position Sensor “A” circuit malfunction.

P0336 generally indicates a range/performance problem.

P0337 indicates a low input.

P0338 indicates a high input.

P0339 indicates an intermittent circuit problem.

This distinction is diagnostically useful.

With P0337, the technician should concentrate particularly on why the signal reaching the ECM is electrically too low.

With P0339, the investigation shifts more toward identifying when and why the signal intermittently disappears or becomes abnormal.

What Should Be Replaced?

There is no automatic replacement for P0337.

If testing confirms that the crankshaft sensor is producing an inadequate signal, replace the sensor.

If the signal wire is damaged, repair the wiring.

If the connector has poor terminal contact, repair or replace the connector.

If the sensor power or ground is faulty, repair the corresponding circuit.

If the sensor gap or installation is incorrect, correct it.

If the reluctor is damaged, repair the mechanical problem.

If the crankshaft has excessive mechanical movement affecting the sensor gap, the underlying mechanical problem must be addressed.

The ECM should be considered only after the rest of the system has been proven functional.

Final Diagnostic Approach

P0337 should be approached as a low electrical input from the Crankshaft Position Sensor “A” circuit.

A sensible diagnostic path is:

Check all codes → inspect the sensor and connector → identify the sensor type → verify power and ground → test the signal circuit for resistance and shorts → observe the actual waveform → check sensor-to-reluctor relationship → inspect the reluctor → compare crankshaft and camshaft synchronization if necessary → investigate mechanical problems → consider the ECM last.

The most important point is that “low input” describes what the ECM is receiving, not necessarily what the sensor itself is producing.

A defective sensor can certainly cause P0337, but so can a poor connector, high-resistance wire, short to ground, inadequate power or ground, incorrect sensor gap, or damaged reluctor.