The P1399 code is a manufacturer-specific powertrain code. Because it is manufacturer-specific, its exact definition depends entirely on the vehicle application, ranging from random engine misfires to glow plug circuit faults.
This code identifies a detected system or circuit condition, not necessarily a failed component. Proper diagnosis requires verifying the exact factory definition and testing the relevant ignition, fuel, or electrical circuits.
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Key Takeaways
- The P1399 code is manufacturer-specific, indicating a random misfire on Honda/Acura applications or a glow plug circuit fault on Ford diesel applications.
- Possible causes for misfire applications include EGR system faults, ignition component failures, or valve clearance issues.
- Possible causes for glow plug applications include a failed glow plug relay, blown fuses, or wiring faults.
- Symptoms vary by application and may include rough idle, hard cold starts, or an illuminated Check Engine Light.
- Diagnosis requires confirming the exact factory definition and testing the specific system components and circuits.
P1399 Code Description and Implications
P1399 is a manufacturer-specific code. Its exact definition depends entirely on the vehicle application.
Some Honda and Acura applications: The code indicates a “Random Misfire,” meaning the Powertrain Control Module (PCM) has detected misfires occurring randomly across multiple cylinders without isolating a single specific cylinder.
Certain Ford diesel applications: The code typically indicates a “Wait to Start Lamp Circuit” or “Glow Plug Control Circuit” fault, where the PCM detects an electrical issue preventing the glow plug system from operating correctly.
Because definitions vary, verifying the exact factory service information is mandatory before beginning diagnosis.
Common Causes of the P1399 Code
Several conditions can trigger this manufacturer-specific malfunction code. Identifying the exact cause requires targeted testing.
- For Honda/Acura misfire applications: A clogged or faulty EGR valve, incorrect valve clearance, or failing ignition coils and spark plugs.
- For Ford diesel applications: A failed glow plug relay, blown high-amperage fuses, or broken wiring in the glow plug control circuit.
- Low fuel pressure or restricted fuel injectors causing a lean condition across all cylinders.
- Vacuum leaks introducing unmetered air into the intake manifold.
- A faulty PCM or internal module driver issue (rare, usually only after circuit verification).
Symptoms Associated With P1399
Symptoms vary significantly depending on which system the code monitors on your specific vehicle.
- Illuminated Check Engine Light, which may flash if the misfire is severe enough to damage the catalytic converter.
- On misfire applications: Rough idle, engine hesitation, loss of power, and increased exhaust emissions.
- On glow plug applications: Extended cranking times, hard starting in cold weather, and the Wait to Start light failing to illuminate or staying on continuously.
Diagnosing the P1399 Trouble Code
Diagnosis must begin by confirming the exact factory definition for your specific make and model.
- Verify the exact manufacturer definition and locate the wiring diagram or diagnostic flowchart for the specific system.
- For misfire applications: Inspect the EGR system for carbon buildup, check valve clearances, and test ignition coils and spark plugs.
- For glow plug applications: Check the relevant high-amperage fuses and test the glow plug relay for proper switching and power delivery.
- Use a digital multimeter to test circuit continuity, checking for opens, shorts to ground, or shorts to voltage.
- Use a scan tool to monitor live data, such as misfire counters or glow plug duty cycles, to pinpoint the fault.
Repairing the P1399 Issue
Repairs depend entirely on the confirmed root cause of the system malfunction.
- If an EGR valve is clogged or failing, clean the passages or replace the valve using a correct component.
- If valve clearances are out of specification, perform a valve adjustment according to the manufacturer procedure.
- If the glow plug relay or fuses fail testing, replace them with components meeting the correct vehicle specification.
- Repair or replace any damaged wiring harness sections and secure the routing away from heat sources.
- If testing confirms the PCM driver is faulty and all external circuits are intact, follow the manufacturer procedure for module replacement.
Cost Considerations for P1399 Repairs
Cost depends on diagnostic time, component accessibility, required parts, programming requirements, and labor.
Replacing a glow plug relay or cleaning an EGR valve is generally less expensive than performing a full engine valve adjustment or replacing multiple ignition coils.
Prevention Strategies for Future Occurrences
While electrical and mechanical failures are not always preventable, some practices can reduce the risk of system issues.
- Adhering to recommended maintenance intervals for spark plugs and engine oil can help prevent misfires and carbon buildup.
- Inspecting exposed wiring and high-amperage fuse links can help identify developing damage from heat exposure or chafing.
- Using high-quality fuel and keeping the fuel system clean helps maintain proper injector operation across all cylinders.
Troubleshooting Tips and Best Practices
Always consult the exact manufacturer service information for your vehicle’s diagnostic flowchart and expected live data values.
- Do not assume ignition components are faulty simply because a misfire code is present; EGR system faults and valve clearance issues must be ruled out first on many Honda applications.
- Check for related codes, such as specific cylinder misfire codes or EGR flow codes, which may help narrow down the root cause.
- Verify technical service bulletins for your specific make and model, as some manufacturers issue updated EGR valves or revised PCM software calibrations for recurring misfire faults.
More OBD-II Codes
| P1397 | P1396 | P1395 | P1394 |
| P1393 | P1392 | P1400 | P1401 |
| P1403 | P1404 | P1405 | P1406 |
| P0300 | P0301 | P0302 | P0303 |
| P0304 | P0305 | P0306 | P0313 |
| P1389 | P1390 | P1391 | P1398 |
Frequently Asked Questions
Is the P1399 code generic or manufacturer-specific?
No. P1399 is a manufacturer-specific code. Its exact definition varies by vehicle make, indicating a random misfire or a glow plug circuit fault.
Can I drive my car with a P1399 code?
It depends on the application and symptoms. If the Check Engine Light is flashing due to a severe misfire, driving the vehicle can damage the catalytic converter and should be avoided.
Will a bad EGR valve cause a P1399 code?
Yes. On Honda and Acura applications, a clogged or malfunctioning EGR valve is a known cause of random misfires, which can trigger this specific code.
Can a faulty glow plug relay trigger P1399?
Yes. On Ford diesel applications, a failed glow plug relay will prevent the system from heating the cylinders, causing the PCM to detect a circuit fault and trigger the code.
Will a P1399 code cause an emissions test failure?
Yes. Any code that illuminates the Check Engine Light or indicates an active misfire will cause an automatic failure during an OBD-II emissions inspection.
What Should You Do About the P1399 Code?
Confirm the exact factory definition for your specific vehicle application. Follow the applicable manufacturer diagnostic procedure to test the relevant ignition, fuel, or electrical circuits.
Repair or replace the confirmed failed component or fix the identified mechanical issue, then retest the system to ensure proper operation.
