P1404 — Glow Plug Circuit High Side

How do I diagnose P1404 myself?

Start with a scan: confirm P1404, save the freeze frame and note any companion codes. Then inspect the wiring and connectors related to the diesel glow plug system, and verify the suspect component against live data before buying parts.

Is it safe to drive with P1404?

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.

Will P1404 clear itself?

Sometimes P1404 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 are the symptoms of P1404?

Drivers most often notice: glow plug warning light staying on, hard starting in cold weather, rough idle until warmed up, Check Engine Light on.

How much does it cost to fix P1404?

Replace a full set of glow plugs — $250–$600; Replace one glow plug — $80–$200; Replace the glow plug relay/module — $150–$450. Exact cost depends on the root cause, your vehicle and local labor rates — a proper diagnosis is the cheapest first step.

When your diesel vehicle's computer detects a problem with the glow plug circuit's high-side voltage, it logs OBD2 code P1404. The term "high side" refers to the power supply side of the glow plug circuit — essentially the wire that carries voltage from the battery through the relay to the plugs themselves. This is a powertrain fault code that shows up in the engine management system when voltage readings stray outside expected parameters.

Diesel engines rely on glow plugs to preheat the combustion chambers before starting, particularly in cold weather. The engine control unit (ECU) monitors the glow plug circuit continuously and compares live electrical readings against factory-set thresholds. When those readings fall outside acceptable ranges, P1404 gets stored in the ECU's memory along with a freeze frame snapshot — a record of engine speed, load, coolant temperature and vehicle speed at the exact moment the fault was detected. That snapshot is valuable during diagnosis because it shows the conditions under which the problem occurred.

Once P1404 is confirmed, the check engine light typically stays illuminated until the issue is resolved. While the vehicle may still run once started, the underlying electrical fault can worsen over time, especially if cold-weather starting becomes increasingly difficult.

Symptoms of P1404

P1404 affects cold-start performance and can produce several noticeable changes in how your diesel runs and drives:

• Check Engine Light remains on after the engine starts
• Glow plug warning light stays illuminated on the dashboard
• Engine cranks longer than normal when starting cold
• Rough or uneven idle that smooths out once the engine warms up
• Excessive white smoke from the exhaust pipe during cold starts
• Reduced power or hesitation under load in cold conditions

Common Causes of P1404

The high-side glow plug circuit can fail in several ways. The most frequent culprits include:

• A burned-out or internally failed glow plug
• Corrosion or moisture damage at glow plug electrical connectors
• A faulty glow plug control relay or module
• Chafed, melted or broken wiring in the glow plug harness
• Poor electrical connections due to loose connectors or damaged pins
• Internal short in the glow plug circuit board within the ECU itself (rare)

How to Diagnose P1404

Follow these steps to isolate the root cause before buying any replacement parts:

1. Connect an OBD2 scanner and verify P1404 is stored. Save the freeze frame data — it documents the engine operating conditions when the fault was detected, which guides your inspection.
2. Pull all stored codes and note any related P0xxx entries. Check for manufacturer technical service bulletins (TSB) specific to your vehicle make, model and model year — some glow plug faults have published factory remedies or software updates.
3. Perform a thorough visual inspection of the glow plug wiring harness and connectors. Look for damaged insulation, evidence of heat damage, corrosion buildup or connectors that are not fully seated.
4. Measure the resistance of each glow plug individually using a multimeter. A functioning plug shows a low resistance value, typically between 0.5 and 2 ohms depending on the design. An open or failed plug displays infinite resistance.
5. Verify proper voltage at the glow plug rail during the pre-glow cycle using a digital multimeter or oscilloscope. No voltage or voltage significantly below specification indicates a problem upstream.
6. Clear the code and run a complete drive cycle that recreates the conditions from the freeze frame. If P1404 returns, the fault is confirmed and active.

Repair Options and Costs

Tackle the most likely causes first to avoid unnecessary spending. Replacement options for P1404 include:

• Replace a full set of glow plugs — $250–$600
• Replace one glow plug — $80–$200
• Replace the glow plug relay or control module — $150–$450

Total Repair Cost for P1404

The final bill depends entirely on which component triggered the fault. At independent shop rates in the United States, most drivers pay between $80 and $600 to resolve P1404. A single failed glow plug represents the least expensive fix, while replacing the entire set or the control module pushes costs toward the higher end. Always confirm the exact cause through proper diagnosis before purchasing parts — the worst-case scenario is rarely the first thing to check, and unnecessary parts purchases add up quickly.

Can You Drive with P1404?

P1404 presents a short-term driving safety concern. Once the engine is running and fully warmed up, the vehicle operates normally because glow plugs are only needed during startup. However, if multiple glow plugs have failed, you risk being unable to start the engine on a cold morning — especially in freezing temperatures. Schedule the repair before winter arrives or before any extended cold-weather trip to avoid being stranded.

Related OBD2 Codes

• P1400: Glow Plug Monitor Fault (Bank #1)
• P1401: Glow Plug Monitor Fault (Bank #2)
• P1402: System Voltage Out Of Self Test Range
• P1403: VVT Solenoid B Circuit High Input
• P1405: DPFE Circuit Low Input
• P1406: DPFE Circuit High Input