P1399 — Glow Plug Circuit High Input (Bank #2)
For Ford vehicles
glow plug circuit high side – signal too high
powertrain (engine/transmission)
Is it safe to drive?
Short-term driving is OK Safe to drive once the engine is started and warmed up. The main risk is cold-weather starting: with several dead plugs the engine may not start on a cold morning. Fix before winter.
How to diagnose with an OBD scanner
- Read the code and freeze frame Connect an OBD2 scanner, confirm P1399 is stored, and save the freeze frame data — it shows engine conditions when the fault was detected.
- Check for companion codes and TSBs Related codes often reveal the real root cause. Also check manufacturer technical service bulletins (TSB) for your make and model — many P0xxx issues have factory fixes or software updates. Search by VIN on the manufacturer site or nhtsa.gov.
- Inspect wiring and connectors Visually inspect the wiring harness and connectors related to the diesel glow plug system for damage, corrosion, chafing or poor contact.
- Subsystem check Measure resistance of each glow plug — an open plug reads infinite resistance.
- Subsystem check Check voltage at the glow plug rail during the pre-glow phase.
- Verify the repair Clear the code, complete a full drive cycle including the conditions from the freeze frame, and confirm P1399 does not return.
Approximate repair cost
Repair cost depends on which component actually failed. Typical scenarios for P1399 (parts and labor, USD, independent shop rates): Replace a full set of glow plugs: $250–$600; Replace the glow plug relay/module: $150–$450; Replace one glow plug: $80–$200. Overall, fixing P1399 usually costs between $80 and $600. An accurate diagnosis before buying parts saves money — the most expensive scenario is rarely the first one to check.
Common questions
Will P1399 clear itself?
Sometimes P1399 clears after a few drive cycles if the fault was intermittent — for example a loose connector or a tank of bad fuel. The underlying cause usually remains, so the code tends to return. Clear it once; if it comes back, diagnose rather than erase it repeatedly.
What is the most common cause of P1399?
The most common causes are: faulty glow plug relay or control module, damaged glow plug wiring or connector, burned-out glow plug.
What are the symptoms of P1399?
Drivers most often notice: hard starting in cold weather, rough idle until warmed up, Check Engine Light on, glow plug warning light staying on.
What does OBD2 code P1399 mean?
Glow Plug Circuit High Input (Bank #2) (P1399): a powertrain (engine/transmission) fault affecting the diesel glow plug system. The car is usually drivable while you plan the repair. Most often caused by faulty glow plug relay or control module, damaged glow plug wiring or connector, burned-out glow plug.
How much does it cost to fix P1399?
Replace a full set of glow plugs — $250–$600; Replace the glow plug relay/module — $150–$450; Replace one glow plug — $80–$200. Exact cost depends on the root cause, your vehicle and local labor rates — a proper diagnosis is the cheapest first step.
When your diesel engine's control unit detects that something is wrong with the glow plug system on the second bank of cylinders, it stores trouble code P1399. The diagnostic message reads "Glow Plug Circuit High Input (Bank #2)" — a designation that points directly at the heating elements responsible for getting diesel combustion started in cold conditions. Bank number two simply refers to the second set of cylinders in engines with a V-configuration or multiple cylinder banks.
The glow plug system works by drawing electrical current through resistive heating elements mounted in each cylinder's combustion chamber. When you turn the key, the engine control module energizes these plugs for several seconds, raising internal temperatures so that the air-fuel mixture ignites properly during startup. Diesel engines do not compress air alone like gasoline engines do — they rely on the heat generated by compression plus the additional warmth from glow plugs to initiate combustion, especially when ambient temperatures drop below freezing. Without adequate pre-heating, fuel cannot ignite reliably and the engine stumbles or refuses to start altogether.
The ECU does not set P1399 after a single anomalous reading. It monitors the glow plug circuit during each pre-heat cycle and compares actual electrical behavior against expected parameters. If the fault condition persists across multiple monitored events under specific operating conditions — such as engine temperature below a threshold or battery voltage within a certain range — the module logs the code and illuminates the malfunction indicator lamp. The associated freeze frame data captures engine load, revolutions per minute, coolant temperature, and vehicle speed at the exact moment the fault was detected, which proves invaluable when attempting to reproduce and troubleshoot the problem.
Symptoms of P1399
The most noticeable sign of P1399 is difficulty starting the engine when temperatures are low. Cylinders with malfunctioning glow plugs fail to reach the temperature needed for reliable combustion, so the starter motor spins the engine longer than usual before fuel ignites.
Rough idling after a cold start frequently accompanies P1399. The engine may shake or stumble until normal operating temperature is reached, because cylinders that did not fire properly during warm-up continue running inconsistently until everything stabilizes.
The Check Engine Light activates once the ECU has confirmed the fault condition and logged the code. This warning typically remains illuminated regardless of whether the engine eventually starts or runs normally afterward.
The dedicated glow plug indicator lamp on the instrument cluster may stay illuminated longer than normal or fail to extinguish after the normal pre-heat cycle completes. This differs from the Check Engine Light in that it specifically relates to the glow system rather than indicating a broader emissions-related issue.
White or gray smoke may billow from the exhaust pipe during cold starts. Unburned fuel passes through the combustion chamber without igniting properly when glow plugs are not functioning, creating visible emissions that dissipate once the engine warms up and normal combustion resumes.
Common Causes of P1399
A blown fuse in the glow plug circuit ranks among the most frequent culprits. The circuit receives dedicated fusing to protect wiring from overload conditions. A failed fuse interrupts current flow entirely, preventing all plugs on that bank from heating.
Damaged wiring or connectors within the glow plug harness represent another common failure point. Wire insulation can crack from age, heat cycling, or abrasion against engine components, exposing conductors that short to ground or break continuity entirely.
Corrosion at electrical connections compromises conductivity and increases resistance. Moisture intrusion, road salt, and thermal expansion/contraction cycles all contribute to connector degradation over time, creating intermittent or complete circuit failures.
A faulty glow plug relay or control module may fail to energize the circuit properly. The relay acts as an electrically-controlled switch that handles the high current demands of multiple glow plugs; when contacts wear or the coil fails, the system does not activate as commanded.
A burned-out individual glow plug occasionally triggers P1399 if the failure creates a specific electrical signature the control module interprets as excessive current draw or an open circuit condition.
How to Diagnose P1399
Begin by connecting a professional-grade OBD2 scanner to the vehicle diagnostic port. Confirm that P1399 is stored as an active or pending code, then record all available freeze frame data. This snapshot reveals engine load, rpm, coolant temperature, and vehicle speed when the fault first occurred — recreate these exact conditions during your road test to reliably trigger the failure.
Before beginning physical inspection, research any factory technical service bulletins or software updates specific to your vehicle make and model. Some glow plug system faults have known root causes documented by manufacturers with published repair procedures or module reprogramming that eliminates the problem without component replacement.
Perform a thorough visual inspection of the wiring harness routing near the cylinder head and intake manifold. Look for damaged insulation, rubbed-through wires, melted connectors, or any signs of overheating. Check that all ground connections are secure and free from corrosion.
Use a digital multimeter to measure resistance across each glow plug individually. A functioning plug typically reads between one and six ohms depending on design specifications. An open circuit reading indicates a failed heating element that requires replacement.
Verify voltage availability at the glow plug supply rail during a deliberate pre-heat cycle. Use a back-probe technique or appropriate test lead to access the connector without damaging seals. Compare readings against manufacturer specifications for battery voltage at the relay output.
After completing repairs or ruling out suspected components, clear all stored codes and perform a complete drive cycle that includes cold soak conditions matching those captured in the freeze frame data. Monitor the scanner for code return over multiple ignition cycles before considering the diagnosis complete.
Repair Options and Costs
Glow plug replacement varies significantly in cost depending on which approach your situation requires. Individual plug replacement runs approximately $80 to $200 for parts and associated labor on one cylinder. However, if multiple plugs are suspected or confirmed faulty, replacing the complete set typically costs between $250 and $600 when using quality aftermarket or OEM-equivalent components.
The glow plug relay or control module itself can fail and requires replacement at a cost of roughly $150 to $450 for the part alone. Labor charges apply on top of component pricing depending on relay location and accessibility within your specific vehicle architecture.
Total Cost Summary
Professional diagnosis typically adds $100 to $150 in labor charges at independent repair facilities before any actual repair work begins. The total investment to resolve P1399 ranges from approximately $80 for a single plug replacement up to $600 for a complete glow plug set plus professional installation. Most situations fall in the $200 to $400 range. Correct diagnosis before purchasing replacement parts prevents unnecessary expense — the most expensive repair scenario is rarely the actual problem.
Can You Drive with P1399?
The severity level for this code is short, meaning immediate danger is unlikely once the engine reaches normal operating temperature. After the engine warms up, cylinder temperatures are sufficient for reliable combustion even without glow plug assistance, so the vehicle remains drivable under most conditions. However, cold-weather starting becomes increasingly unreliable as additional plugs fail, potentially leaving you stranded on freezing mornings. Addressing P1399 before winter arrives is strongly recommended to avoid inconvenient no-start situations.