P1399: Glow Plug Circuit High Side High Input

Definition

The OBD2 code P1399 indicates a malfunction in the glow plug circuit, specifically that there is a high input signal detected, which may suggest an issue with the glow plug relay, wiring, or the glow plugs themselves

Common causes

Faulty glow plug
Open or shorted wiring in the glow plug circuit
Faulty glow plug relay
Poor ground connection
Malfunctioning engine control module (ECM)
Corroded or damaged connectors
Incorrectly installed or incompatible glow plugs

Common misdiagnoses

Faulty glow plugs
Defective glow plug relay
Wiring issues or shorts in the glow plug circuit
Faulty engine control module (ECM)
Incorrectly installed or damaged glow plug harness
Battery or charging system problems
Fuel system issues
Injector problems

Troubleshooting steps

1. Check the Code

Use an OBD-II scanner to confirm the P1399 code and check for any additional codes that may provide more context about the issue
2. Inspect the Glow Plugs
3. Visually inspect the glow plugs for any signs of damage or wear
4. Test each glow plug for continuity using a multimeter. A good glow plug should show low resistance (typically between 0.5 to 2 ohms)
5. Check the Glow Plug Relay
6. Inspect the glow plug relay for proper operation. A faulty relay can cause high input signals
7. Test the relay by applying power to it and checking if it activates the glow plugs
8. Examine Wiring and Connectors
9. Inspect the wiring harness and connectors associated with the glow plugs and relay for any signs of damage, corrosion, or loose connections
10. Repair or replace any damaged wiring or connectors
11. Test the Glow Plug Control Module
12. If your vehicle is equipped with a glow plug control module, check its operation. This module controls the timing and duration of the glow plug operation
13. Use a multimeter to test the module's output signals
14. Check for Short Circuits
15. Look for any short circuits in the glow plug circuit that could cause a high input signal. This can be done by checking the resistance between the glow plug circuit and ground
16. Inspect the Battery and Charging System
17. Ensure that the battery is in good condition and fully charged, as low voltage can affect the operation of the glow plugs
18. Check the alternator and charging system to ensure they are functioning properly
19. Clear the Codes and Test Drive
20. After performing the necessary repairs, clear the trouble codes using the OBD-II scanner
21. Take the vehicle for a test drive to see if the code reappears
22. Consult a Professional
23. If the issue persists after performing these steps, it may be necessary to consult a professional mechanic or technician for further diagnosis. By following these troubleshooting steps, you should be able to identify and resolve the issue related to the P1399 code

Definition

Common causes

Common misdiagnoses

Troubleshooting steps

1. Check the Code

2. Inspect the Glow Plugs

3. Visually inspect the glow plugs for any signs of damage or wear

4. Test each glow plug for continuity using a multimeter. A good glow plug should show low resistance (typically between 0.5 to 2 ohms)

5. Check the Glow Plug Relay

6. Inspect the glow plug relay for proper operation. A faulty relay can cause high input signals

7. Test the relay by applying power to it and checking if it activates the glow plugs

8. Examine Wiring and Connectors

9. Inspect the wiring harness and connectors associated with the glow plugs and relay for any signs of damage, corrosion, or loose connections

10. Repair or replace any damaged wiring or connectors

11. Test the Glow Plug Control Module

12. If your vehicle is equipped with a glow plug control module, check its operation. This module controls the timing and duration of the glow plug operation

13. Use a multimeter to test the module's output signals

14. Check for Short Circuits

15. Look for any short circuits in the glow plug circuit that could cause a high input signal. This can be done by checking the resistance between the glow plug circuit and ground

16. Inspect the Battery and Charging System

17. Ensure that the battery is in good condition and fully charged, as low voltage can affect the operation of the glow plugs

18. Check the alternator and charging system to ensure they are functioning properly

19. Clear the Codes and Test Drive

20. After performing the necessary repairs, clear the trouble codes using the OBD-II scanner

21. Take the vehicle for a test drive to see if the code reappears

22. Consult a Professional

23. If the issue persists after performing these steps, it may be necessary to consult a professional mechanic or technician for further diagnosis. By following these troubleshooting steps, you should be able to identify and resolve the issue related to the P1399 code