What does it mean?
This diagnostic trouble code (DTC) is a generic powertrain code, meaning it applies to vehicles equipped with OBD-II (Jeep, Nissan, Dodge, Ram, BMW, Ford, GM, etc.). Although generic, specific repair steps may vary by make/model.
In the past, when I diagnosed a stored P0338 code, it was due to the powertrain control module (PCM) detecting a high input voltage signal from the crankshaft position sensor (CKP).
CKP sensors monitor engine speed (RPM) and crankshaft position. The crankshaft position is used by the PCM to calculate ignition spark timing. Camshafts rotate at half the speed of the crankshaft, so it is important that the PCM is able to distinguish between the engine’s intake and exhaust strokes (rotations). The CKP sensor and each camshaft position sensor (CMP) have one or more dedicated circuits to provide the PCM with an input signal, a 5-volt reference signal, and a ground.
CKP sensors are typically Hall effect electromagnetic types and are mounted externally on the engine. They are placed in very close proximity (usually only a few thousandths of an inch) to a circuit completing the engine ground. This engine ground is typically a reluctor ring (with precision-machined teeth) attached to either end of the crankshaft or integrated into the crankshaft itself. Systems with multiple CMP sensors may use a reluctor ring at one end of the crankshaft and another machined at the center of the crankshaft.
Typically, the CKP sensor is mounted so that the crankshaft’s reluctor ring passes within a few thousandths of an inch of its magnetic tip. As the crankshaft rotates, the raised areas of the reluctor ring complete an electromagnetic circuit with the sensor. The indentations (between the raised areas) briefly interrupt the circuit. The PCM perceives these rapid circuit completions and interruptions as a waveform pattern representing voltage fluctuations.
The PCM continuously compares the input signals from the crankshaft position and camshaft position sensor circuits. If the CKP input voltage is too high over a defined period of time, a P0338 code will be stored and a MIL may be illuminated.
Other crankshaft position sensor fault codes include P0335, P0336, P0337, and P0339.
Code Severity and Symptoms
When a P0338 is stored, a no-start condition will likely accompany it. For this reason, this code should be classified as severe. If the engine starts and runs, the risk of a no-start remains high in the near future.
Symptoms of this code may include:
Engine does not start
The tachometer (if applicable) does not register RPM when the engine is cranked
Hesitation during acceleration
Poor engine performance
Reduced fuel efficiency
Causes
Possible causes for this code being set are as follows:
Faulty CKP sensor
Open or shorted wiring to the CKP sensor
Corroded or fluid-soaked connector at the CKP sensor
PCM programming error or faulty PCM
Alternator overcharging and affecting PCM operation
Diagnostic and Repair Procedures
Access to a suitable diagnostic scanner, with a built-in digital volt/ohmmeter (DVOM) and an oscilloscope, will be necessary before diagnosing a P0338 code. As a reliable source for vehicle information, I will also need access to All Data DIY.
The most successful code diagnostics start with a visual inspection of all wiring harnesses and connectors related to the system. Petroleum-based fluids can compromise the protective insulation of wiring and lead to short circuits or open circuits (and a stored P0338), so circuits, electrical sensors, and/or connectors that have been contaminated by engine oil, coolant, or power steering fluid should be carefully inspected.
Connect the scanner to the vehicle’s diagnostic port to retrieve stored fault codes and freeze frame data. Note this information as it will help if the P0338 proves to be intermittent. Test drive the vehicle (if possible) to see if the code resets. If it does:
Locate a system wiring diagram from the vehicle information source and check the voltage at the CKP sensor. Typically, a five-volt reference is used for CKP sensor operation, but check the manufacturer’s specifications for the vehicle in question. A ground signal and one or more output circuits should also be present. Proceed to the next step if reference voltage and ground signals are detected at the CKP sensor connector.
Using the DVOM, disconnect the electrical connector of the CKP sensor in question and test it according to the manufacturer’s specifications. Replace the CKP sensor if the resistance levels do not meet the manufacturer’s specifications. If the CKP sensor resistance values match the manufacturer’s specifications, proceed to the next step.
After reconnecting the CKP sensor in question, attach the oscilloscope’s positive test lead to the signal output wire and connect the negative lead to the CKP sensor’s ground circuit. Power on the oscilloscope and select the operating voltage setting. Start the engine and observe the waveform pattern on the oscilloscope. Focus on unexpected spikes or glitches. If spikes or glitches are detected, carefully wiggle the wiring harness and connector of the CKP sensor in question to determine if the issue is a loose connection or a faulty sensor. If there is a broken or worn reluctor ring, or if the magnetic tip of the CKP sensor has excessive metal debris attached, it can cause missing voltage blocks in the waveform pattern. Proceed to the next step if no issues are found in the waveform pattern.
Locate the PCM connector, insert the oscilloscope test leads into the CKP sensor input signal and ground circuits, and observe the waveform pattern. Suspect an open or shorted circuit between the CKP sensor connector and the PCM connector if the waveform pattern near the PCM connector is different from what was seen when the test leads were connected near the CKP sensor. If this is the case, disconnect all associated controllers and test the individual circuits with the DVOM. Shorted or open circuits will need to be repaired or replaced. You may have a faulty PCM or a PCM programming error if the waveform pattern is identical to what was seen when the test leads were connected near the CKP sensor.
Additional Diagnostic Notes:
Some manufacturers recommend replacing CKP and CMP sensors as a set
Use technical service bulletins to aid the diagnostic process