Decoding EOBD/OBDII Error P1391: Crankshaft Position Sensor Issues and No-Start Problems

Experiencing a frustrating “cranks but won’t start” situation with your vehicle? If you’ve encountered the error code P1391 on your OBD-II scanner, the Crankshaft Position Sensor (CPS), sometimes also referred to as the Crankshaft Position Sensor (CKP), is a prime suspect. This article, tailored for car owners and DIY mechanics, will delve into the intricacies of the P1391 error code, focusing on how to diagnose and address issues related to your vehicle’s CPS.

Understanding Error Code P1391 and the Crankshaft Position Sensor

The EOBD/OBDII error code P1391 typically indicates a problem within the Crankshaft Position Sensor (CPS) circuit. This sensor plays a crucial role in your engine’s operation. It monitors the crankshaft’s position and speed, providing essential data to the engine control unit (ECU) or powertrain control module (PCM). This information is critical for:

• Ignition Timing: The ECU uses CPS data to determine when to fire the spark plugs.
• Fuel Injection Timing: Accurate crankshaft position is needed for precise fuel delivery.
• Engine Speed (RPM) Measurement: The CPS signal helps the ECU calculate engine RPM.

When the CPS malfunctions or the circuit encounters issues, the ECU may not receive the necessary signals, leading to a range of problems, most notably a no-start condition or intermittent engine stalling.

Symptoms Indicating a Faulty Crankshaft Position Sensor (CPS) and P1391 Error

A failing CPS can manifest in various symptoms. Recognizing these can help you pinpoint the P1391 error’s root cause:

• Engine Cranks But Won’t Start: This is the most common symptom. The starter motor engages, but the engine fails to ignite and run.
• Intermittent Stalling: The engine might start and run normally for a while, then suddenly stall without warning.
• No Spark at Spark Plugs: If the CPS isn’t sending signals, the ECU might not trigger the ignition system, resulting in no spark.
• Fuel Pump Issues: In some cases, a faulty CPS can prevent the fuel pump from priming correctly (usually a 2-3 second whirring sound when you turn the key to “ON”).
• Gauge Malfunctions: You might observe erratic behavior in the fuel gauge or voltage gauge.
• “No BUS” Error on Odometer: After turning the ignition on, the odometer might display “No BUS,” indicating a communication problem within the vehicle’s network, potentially linked to CPS failure.
• Check Engine Light (CEL) and Error Code P1391: While not always guaranteed, a failing CPS may trigger the CEL and store the P1391 code. However, sometimes the CPS failure can be so complete that the OBD-II scanner struggles to communicate with the ECU initially.

Diagnosing the P1391 Error and Potential CPS Issues

Before assuming the CPS is faulty, it’s wise to perform basic troubleshooting steps to rule out other potential issues:

1. Check Basic Start and Charge Systems:

   • Battery Connections: Ensure battery terminals are clean, tight, and free of corrosion.
   • Wiring and Grounds: Inspect all wiring and ground connections to the battery, starter, alternator, engine block, and chassis. Poor grounds or low voltage can cause various electrical issues, mimicking CPS problems.

2. OBD-II Scanner Communication Test:

   • Initial Scan Attempt: Try connecting your OBD-II scanner to read codes. If it fails to connect or cannot read codes, suspect a CPS issue.
   • CPS Disconnect Test: Disconnect the CPS connector, then try connecting the OBD-II scanner again. If the scanner can now communicate with the ECU, this strongly suggests a faulty CPS.

3. Voltage and Gauge Check (CPS Unplugged):

   • Unplug the CPS connector.
   • Turn the ignition key to the “ON” position (without starting the engine).
   • Observe the fuel gauge and voltage gauge. If they now display correctly, or if a “No BUS” error disappears, this is another indicator of a failing CPS.

4. Relay and Fuse Check:

   • Swap Relays: Exchange the fuel pump relay and the ASD (Automatic Shut Down) relay with similar relays in your vehicle’s power distribution center (PDC). This eliminates faulty relays as a potential cause.
   • Check ASD Relay Fuse: Inspect the fuse for the ASD relay to ensure it’s not blown.

5. Fuel Pump Priming Check:

   • Turn the ignition key to the “ON” position.
   • Listen for the fuel pump to run for a few seconds (a brief whirring sound). If you don’t hear it, the CPS could be preventing fuel pump activation.

6. Neutral Safety Switch (NSS) Check (for Automatic Transmissions):

   • Try starting the vehicle in “Neutral” as well as “Park.”
   • Wiggle the shift lever while attempting to start.
   • Check if the reverse lights illuminate when the shifter is in “Reverse.” NSS issues can sometimes mimic no-start problems.

7. Oxygen (O2) Sensor Wiring Inspection:

   • Inspect the wiring and connectors of the O2 sensors on the exhaust pipes. Short circuits from melted or damaged O2 sensor wiring can blow fuses and disrupt ECU communication.

Testing the Crankshaft Position Sensor (CPS)

To definitively confirm a faulty CPS, you can perform resistance and AC voltage tests. The procedures vary slightly depending on your engine type and year.

CPS Connector Location:

The CPS is typically located on the transmission bell housing, near the engine’s flywheel or flexplate. The connector is usually found near the rear of the intake manifold or on the passenger side of the engine bay, depending on the vehicle model.

Testing Procedure for 1991-2001 4.0L H.O. Engines (Resistance Test):

1. Locate and disconnect the CPS connector from the main wiring harness.
2. Set your multimeter to the 1K to 10K Ohms scale.
3. Place the multimeter probes across terminals B and C of the CPS connector (refer to the image above for terminal identification).
4. A healthy CPS should show open circuit (infinite resistance). If you read low resistance, the sensor is likely faulty and needs replacement.

Testing Procedure for 1987-1990 4.0L Engines (Resistance and AC Voltage Test):

Test #1: Resistance Test

1. Set your multimeter to read resistance in the 0-500 Ohms range.
2. Disconnect the CPS connector.
3. Measure the resistance across terminals A and B of the CPS connector.
4. The resistance should be between 125-275 Ohms. If it’s outside this range, replace the CPS.

Test #2: AC Voltage Output Test (Requires a Helper)

1. Set your multimeter to read AC voltage in the 0-5 AC Volts range (or the closest AC voltage scale your meter has).
2. Connect the multimeter probes across terminals A and B of the CPS connector.
3. Have a helper crank the engine (do not start it).
4. The meter should read 0.5 – 0.8 VAC (Volts AC) while cranking. If it’s below 0.5 VAC, the CPS is likely faulty.

Note: For 2000 and 2001 models, the CPS location on the bell housing remains the same, but the connector might be on the passenger side. Follow the wire from the sensor to locate the connector.

Conclusion

If you are facing a “cranks but won’t start” issue accompanied by the EOBD/OBDII error code P1391, a faulty Crankshaft Position Sensor (CPS) is a highly probable cause. By systematically following the diagnostic steps and testing procedures outlined in this guide, you can confidently determine if the CPS is the culprit. Replacing a faulty CPS is often a straightforward repair that can restore your vehicle’s reliable starting and running performance. Remember to always consult your vehicle’s repair manual for specific instructions and safety precautions.