Easy one-page printout for the garage or repair shop
This summary uses the existing diagnostic information shown in full below.
This code means there's a problem with the spark plug or coil that helps cylinder 9 in your car's engine make a spark. It's like when a lighter doesn't make a flame because it's broken.
The P138D code indicates an issue with the ion sensing capability of the ignition coil on cylinder 9, which is responsible for detecting misfires or combustion quality.
The ion sensing ignition coil monitors the ionization current in the combustion chamber. If the coil on cylinder 9 fails to detect this current properly, it triggers the P138D code. This can be due to a faulty coil, wiring issues, or a problem with the ECM.
• Engine misfire
• Rough idling
• Decreased fuel efficiency
• Faulty ignition coil on cylinder 9
• Damaged or corroded wiring to the ignition coil
• Faulty Engine Control Module (ECM)
Ignition Coil: A component that transforms the battery's voltage to a much higher voltage to create a spark in the spark plugs.
ECM: Engine Control Module, the computer responsible for controlling engine functions.
• Check for any other related codes.
• Inspect the ignition coil and wiring for cylinder 9.
• Test the ignition coil's functionality.
• Replace the faulty ignition coil.
• Repair or replace damaged wiring.
• Reprogram or replace the ECM if necessary.
• OBD-II scanner
• Multimeter
• Socket set
$150 - $400
Moderate
• BMW 7 Series
• BMW X5
• BMW 5 Series
• Regularly inspect ignition coils and wiring.
• Use high-quality fuel to prevent deposits.
• Follow the vehicle's maintenance schedule.
Ignition System Issue
• Avoid driving the vehicle for extended periods with this code active.
• Seek professional diagnosis if unsure about the repair.
Check if the ignition system components are covered under the vehicle's powertrain warranty.
This code has been more prevalent in vehicles using advanced ion sensing ignition systems, particularly in high-performance models.
Moderate – It should be addressed soon to prevent further engine damage.
Driving with an active misfire can damage the catalytic converter and is not recommended.
• Engine at operating temperature
• Vehicle in idle or low-load condition
The ignition coil for cylinder 9 is located on the engine's cylinder head, typically near the rear of the engine in V-type engines.
Q: Can I drive with a P138D code?
A: It's not recommended to drive for long with this code, as it can lead to further engine damage.
Q: How do I know if my ignition coil is bad?
A: Symptoms of a bad ignition coil include engine misfires, rough idling, and poor fuel economy.
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Universal · Digital multimeter
A solid entry-level multimeter for voltage, continuity, resistance, and basic electrical troubleshooting.
Helpful for checking voltage drops, grounds, and basic circuit health when modules are not communicating.
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Universal · Wire connectors and repair kit
Useful when diagnosis points toward damaged wiring, corroded terminals, or connector repairs.
Useful if diagnosis points toward damaged connectors, wiring, or terminal repairs.
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Fitment Required · Vehicle-fitment part
Commonly relevant for misfire, ignition-efficiency, and some drivability-related repairs.
Misfire and ignition-related faults commonly involve spark plugs as part of the repair path.
Check the correct plug type, gap, and engine application for your vehicle.
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Fitment Required · Vehicle-fitment part
Relevant when the fault points toward ignition-coil performance or cylinder-specific ignition issues.
Some ignition faults involve coil performance or cylinder-specific coil problems.
Match the coil to your engine and confirm the failed component before ordering.
View options on AmazonThese parts may be related to this fault category, but a code by itself does not confirm exact fitment or guarantee that replacing a part will solve the problem.