P0337

What Does Code P0337 Mean?

DTC P0337 signifies a detected condition of “Crankshaft Position Sensor ‘A’ Circuit Low Input.” This code is set by the Engine Control Module (ECM), often referred to as the Powertrain Control Module (PCM), when it receives a voltage signal from the Crankshaft Position (CKP) sensor circuit ‘A’ that falls below its calibrated lower threshold or expected operational range for a specified period. The CKP sensor is a critical input device that monitors the rotational speed and precise angular position of the crankshaft relative to Top Dead Center (TDC) for cylinder #1. This information is vital for the ECM to accurately determine engine RPM, synchronize fuel injection timing, ignition timing, and enable misfire detection.

Most CKP sensors are either inductive (magnetic reluctance) or Hall effect type. An inductive sensor generates an AC voltage signal whose amplitude and frequency increase with engine speed. A Hall effect sensor typically outputs a digital square wave signal with varying frequency. When the ECM detects a signal from this sensor that is either too weak in amplitude (for inductive sensors), too low in voltage (for Hall effect sensors), or intermittently drops out to a level indistinguishable from a circuit short to ground, it interprets this as a “low input” and sets P0337. This condition directly impacts the ECM’s ability to precisely control engine operation, often leading to performance issues or a no-start condition due to insufficient or inaccurate crankshaft position data.

Common Symptoms

• Hard Start or No-Start Condition: The engine may crank but fail to start, as the ECM cannot accurately determine engine position for spark and fuel timing.
• Intermittent Engine Stalling: Particularly common during deceleration, coasting, or at idle, as the unreliable signal disrupts engine management.
• Rough Idling or Erratic Engine Operation: Inaccurate crankshaft position data can lead to inconsistent spark and fuel delivery, causing the engine to run poorly.
• Reduced Engine Performance: The vehicle may experience a noticeable lack of power or sluggish acceleration.
• Illuminated Malfunction Indicator Lamp (MIL): The “Check Engine” light will be illuminated on the dashboard.
• Engine Cranks but Does Not Fire: Similar to a no-start, but specifically indicates the starter motor is operating, but combustion is not initiated.
• Transmission Shifting Issues: In some vehicles, the ECM uses CKP data for transmission shift points, and a faulty signal can cause erratic shifting.

What Causes the Code P0337?

• Faulty Crankshaft Position Sensor: The most common cause. The sensor itself may have an internal short, open circuit, a weakened magnet (inductive type), or a failed Hall effect element, resulting in a low or absent output signal.
• Wiring Harness Issues:
  • Short to Ground: The CKP sensor signal wire or its reference voltage wire may be chafed and shorting directly to the vehicle’s chassis or engine block.
  • Open Circuit: A broken wire within the CKP sensor circuit (signal, reference, or ground) can prevent the signal from reaching the ECM.
  • High Resistance: Corroded terminals, a partially severed wire, or loose connections can cause excessive voltage drop, leading to a “low input” reading.
• Corroded or Damaged Electrical Connectors: Poor connectivity at the CKP sensor connector or at the ECM/PCM connector due to corrosion, bent pins, or poor terminal tension can impede signal transmission.
• ECM/PCM Failure: While less common, an internal fault within the Engine Control Module preventing it from correctly interpreting the CKP sensor signal, or a failure to supply proper reference voltage/ground to the sensor, can trigger this code.
• Foreign Material or Debris on Sensor Tip: Metallic debris adhering to the tip of an inductive CKP sensor can interfere with its magnetic field and distort the generated signal.
• Incorrect Sensor Air Gap or Damaged Reluctor Wheel: An excessive gap between the sensor and the reluctor wheel (tone wheel) on the crankshaft, or damage to the reluctor wheel itself (e.g., bent, missing teeth), can result in a weak or erratic signal.

How to Diagnose and Troubleshoot

Diagnosing P0337 requires a systematic approach, often leveraging a digital multimeter (DMM), an OBD-II scanner, and ideally a digital storage oscilloscope (DSO).

1. Initial Scan and Freeze Frame Data: Connect an OBD-II scanner to record the P0337 code. Crucially, retrieve and analyze freeze frame data, which captures engine conditions (RPM, engine load, coolant temperature, vehicle speed) at the moment the code was set. This information can provide valuable clues about when the fault typically occurs. Check for any other related codes, such as P0335 or P0336.
2. Visual Inspection:
   • Thoroughly inspect the CKP sensor and its entire wiring harness for any visible signs of damage, fraying, chafing, or melting, particularly where it routes near hot engine components or moving parts.
   • Examine the CKP sensor electrical connector for corrosion, bent pins, pushed-out terminals, or a loose connection. Perform the same inspection on the ECM/PCM connector pins corresponding to the CKP circuit.
   • If accessible, inspect the crankshaft reluctor ring (tone wheel) for physical damage, missing teeth, or excessive runout.
3. Circuit Integrity Test (Using DMM):
   • Reference Voltage (Power) Check: With the ignition ON and the CKP sensor connector disconnected, back-probe the reference voltage pin at the sensor connector. Verify the presence of the specified reference voltage (typically 5V or 12V, depending on the sensor type and vehicle manufacturer). Compare to manufacturer specifications. If absent or low, trace the circuit back to the ECM.
   • Ground Circuit Check: With the ignition OFF, check for continuity between the ground pin at the CKP sensor connector and a known good chassis ground. Resistance should be less than 5 ohms. Alternatively, with ignition ON, measure voltage drop between the sensor’s ground pin and battery negative; it should be less than 0.2V.
   • Signal Wire Continuity and Shorts: Disconnect both the CKP sensor and the ECM/PCM connectors. Measure resistance between the signal wire terminal at the CKP sensor connector and its corresponding terminal at the ECM connector. Resistance should be very low (less than 1 ohm). Then, measure resistance between the signal wire and chassis ground (should be infinite) and between the signal wire and battery positive (should be infinite) to check for shorts.
4. CKP Sensor Resistance Test (Inductive Type): For inductive-type sensors only, disconnect the sensor and measure its internal resistance across its two signal terminals. Compare the reading to the manufacturer’s specifications. An open circuit (infinite resistance) or a resistance significantly outside the specified range indicates a faulty sensor. (Hall effect sensors typically cannot be tested this way).
5. CKP Sensor Signal Test (Oscilloscope Recommended):
   • Using a DSO: This is the most definitive test. Back-probe the CKP sensor signal wire with the sensor connected and the engine cranking or running. Observe the waveform. For an inductive sensor, expect an AC sine wave whose amplitude and frequency increase with RPM. For a Hall effect sensor, expect a clean digital square wave. A “low input” will manifest as a significantly lower amplitude than expected, severe signal distortion, or intermittent signal dropouts.
   • Using a DMM (Less Definitive): While cranking the engine, an inductive sensor should produce a small, fluctuating AC voltage (typically 0.2-1.0V AC, increasing with cranking speed). A Hall effect sensor may show a fluctuating DC voltage. This is a basic test and may not reveal intermittent issues or signal quality problems.
6. Wiggle Test: With the engine running (if possible) and monitoring the CKP sensor signal via oscilloscope or live data on the scanner, gently wiggle and manipulate the CKP sensor wiring harness and connectors. If the signal drops out or becomes erratic, it indicates a poor connection or internal wiring fault.

Recommended Repairs and Solutions

Once the diagnostic steps have pinpointed the root cause, the following repairs are typically recommended:

1. Replace the Crankshaft Position Sensor: If diagnostic tests confirm the CKP sensor itself is faulty (e.g., incorrect resistance, no output, or a demonstrably weak/distorted waveform), replace it with a new, high-quality OEM or equivalent aftermarket unit. Ensure the new sensor is properly seated and its O-ring (if applicable) is intact to prevent oil leaks.
2. Repair or Replace Wiring Harness: If damaged or corroded wiring or connectors are identified, perform precise repairs. For wiring, use appropriate automotive-grade wire, crimp connectors, and sealed heat-shrink tubing. If the damage is extensive, replace the affected section of the harness or the entire harness, if necessary.
3. Clean Electrical Connectors: If corrosion is the primary issue, thoroughly clean the sensor and ECM connector terminals using electrical contact cleaner and a small brush. Apply a thin layer of dielectric grease to prevent future corrosion and improve conductivity.
4. Inspect and Clean Reluctor Ring: If accessible, carefully inspect the crankshaft reluctor ring for any metallic debris, damage, or excessive wear. Clean any debris and address any physical damage; severely damaged reluctor rings may require more extensive engine work to replace.
5. ECM/PCM Replacement: This should only be considered as a last resort, after all other components (sensor, wiring, connectors) have been rigorously tested and confirmed to be functioning correctly. ECM replacement typically requires reprogramming to the vehicle’s VIN and immobilizer system.

Important Mechanics’ Tips:

• Always disconnect the negative battery terminal before performing any electrical repairs to prevent accidental shorts or damage to vehicle electronics.
• Allow the engine to cool down before working on the CKP sensor, as it is often located near hot engine components.
• When replacing the CKP sensor, ensure any mounting bolts are tightened to the manufacturer’s specified torque. Some sensors may require a specific air gap adjustment, consult the factory service manual.
• After completing any repair, clear the DTCs from the ECM using an OBD-II scanner. Then, perform a comprehensive drive cycle to ensure the fault does not return and that all readiness monitors complete.
• Be wary of using low-quality aftermarket sensors, as they may not meet OEM specifications and could lead to recurrent issues or inaccurate readings.
• Always refer to the vehicle-specific factory service manual for precise wiring diagrams, component locations, sensor specifications (resistance values, voltage ranges), and testing procedures, as these can vary significantly between makes, models, and years.