What does it mean?
This diagnostic trouble code (DTC) is a generic powertrain code, meaning it applies to OBD-II equipped vehicles (Jeep, Nissan, Dodge, Ram, BMW, Ford, GM, etc.). Although generic, specific repair steps may vary by make/model.
In the past, when I’ve diagnosed a stored P0388 code, it was due to the powertrain control module (PCM) detecting a high input voltage signal from the secondary crankshaft position sensor (CKP). In a system using multiple CKP sensors, the B sensor typically refers to a secondary CKP sensor.
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’s important for the PCM 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 providing 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’re placed in very close proximity (usually only a few thousandths of an inch) to an engine ground-completing circuit. 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 crankshaft center.
Typically, the CKP sensor is mounted so 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 raised areas) briefly interrupt the circuit. The PCM perceives these rapid circuit completions and interruptions as a waveform pattern representing voltage fluctuations.
The PCM constantly compares input signals from the crankshaft position and camshaft position sensor circuits. If the CKP input voltage is too high over a defined time period, a P0388 code will be stored and a MIL may be illuminated.
Other B crankshaft position sensor fault codes include P0385, P0386, P0387, and P0389.
Code severity and symptoms
When a P0388 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 no-start remains high in the near future.
Symptoms of this code may include:
Engine will not start
Tachometer (if equipped) does not register RPM when engine is cranked
Hesitation during acceleration
Poor engine performance
Reduced fuel economy
Causes
Possible causes for setting this code include:
Faulty CKP sensor
Open or shorted wiring to CKP sensor
Corroded or fluid-soaked connector at CKP sensor
PCM programming error or faulty PCM
Alternator overcharging affecting PCM operation
Diagnostic and repair procedures
Access to a proper diagnostic scanner with integrated digital volt/ohmmeter (DVOM) and oscilloscope will be necessary before diagnosing a P0388 code. As a reliable vehicle information source, I’ll also need access to All Data DIY.
Most successful code diagnostics begin with visual inspection of all wiring harnesses and connectors related to the system. Petroleum-based fluids can compromise wiring’s protective insulation and cause short or open circuits (and a stored P0388), therefore, 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 trouble codes and freeze frame data. Note this information as it will help if P0388 proves 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 voltage at the CKP sensor. Typically, a five-volt reference is used for CKP sensor operation, but check manufacturer specifications for the vehicle in question. A ground signal and one or more output circuits should also be present. Proceed to next step if reference voltage and ground signals are detected at CKP sensor connector.
Using the DVOM, disconnect the electrical connector from the CKP sensor in question and test it according to manufacturer specifications. Replace the CKP sensor if resistance levels don’t meet manufacturer specifications. If CKP sensor resistance values match manufacturer specifications, proceed to 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 ground circuit. Power the oscilloscope and select appropriate voltage setting. Start the engine and observe the waveform pattern on the oscilloscope. Focus on unexpected spikes or dips. If spikes or dips are detected, carefully wiggle the wiring harness and connector of the CKP sensor in question to determine if the problem is a loose connection or faulty sensor. If there’s a broken or worn reluctor ring, or if the CKP sensor’s magnetic tip has excessive metal debris attached, it may cause missing voltage blocks in the waveform pattern. Proceed to 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 PCM connector if the waveform pattern near the PCM connector differs from what was seen when test leads were connected near the CKP sensor. If so, disconnect all associated controllers and test individual circuits with the DVOM. Shorted or open circuits will need repair or replacement. You may have a faulty PCM or PCM programming error if the waveform pattern is identical to what was seen when 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 facilitate the diagnostic process