P1397 — Glow Plug Circuit High Input (Bank #1)

What is the most common cause of P1397?

The most common causes are: corroded glow plug connectors, blown fuse in the glow plug circuit, damaged glow plug wiring or connector.

How do I diagnose P1397 myself?

Start with a scan: confirm P1397, 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.

What are the symptoms of P1397?

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

Is it safe to drive with P1397?

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 P1397 clear itself?

Sometimes P1397 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.

When your vehicle's engine control module detects a problem in the diesel glow plug system on bank one, it stores diagnostic trouble code P1397. This code specifically indicates a high input signal from the glow plug circuit, which means the module is receiving voltage readings outside the expected range. Diesel engines rely on these heating elements to warm the combustion chambers during startup, especially in colder temperatures. When the electrical circuit feeding the glow plugs registers abnormal voltage levels—either too high or showing an open condition—the control module interprets this as a fault and illuminates the check engine light. P1397 falls within the powertrain category of OBD2 codes, meaning it relates directly to engine and transmission operation. The control unit doesn't flag this fault based on a single anomalous reading; instead, it requires the problematic condition to appear repeatedly under specific operating circumstances before registering the code and triggering the warning lamp.

Understanding why the module sets P1397 requires knowing how the glow plug system operates. Each glow plug contains a heating element that draws electrical current when the ignition is switched on, raising cylinder temperatures so fuel can ignite properly during cold starts. The engine control module monitors the electrical resistance and voltage passing through this circuit. When resistance drops to zero—indicating a short—or climbs to infinity—indicating an open circuit—the module recognizes the plug cannot function. It stores freeze frame data alongside the code, capturing engine load, revolutions per minute, coolant temperature, and vehicle speed at the exact moment the fault occurred. This snapshot proves invaluable during diagnosis, as it lets you recreate the precise conditions under which the problem appeared.

Common Symptoms of P1397

• The check engine light remains illuminated after starting the engine
• The vehicle struggles to start, particularly during cold weather or freezing temperatures
• The engine runs roughly at idle until reaching normal operating temperature
• The glow plug warning indicator on the dashboard stays lit longer than usual or fails to turn off
• White or gray exhaust smoke persists after a cold start, indicating unburned fuel due to insufficient cylinder heating

What Causes P1397?

• Frayed, melted, or otherwise damaged wiring within the glow plug circuit
• A burnt-out or mechanically failed glow plug that no longer produces heat
• Corrosion buildup on the electrical connectors, creating poor conductivity or intermittent contact
• A defective glow plug relay or control module that fails to regulate power delivery properly
• Loose or improperly seated connectors that interrupt the electrical circuit
• Short circuits within the wiring harness causing abnormally high voltage readings

How to Diagnose P1397

1. Connect a compatible OBD2 scanner to the vehicle's diagnostic port, retrieve code P1397, and record the associated freeze frame data. This information shows engine conditions—load percentage, RPM, temperature, and speed—when the fault first triggered.

2. Check for any additional stored trouble codes that may point toward the root cause. Simultaneously, search manufacturer technical service bulletins for your specific make and model using your VIN; some P1397 cases have documented factory fixes or software recalibrations.

3. Perform a thorough visual inspection of the glow plug wiring harness and electrical connectors. Look for damaged insulation, wire chafing, corrosion deposits, bent pins, or loose connections that could interrupt current flow.

4. Using a digital multimeter, measure the resistance of each individual glow plug. A functioning plug typically shows resistance between 0.5 and 2 ohms depending on the application. An infinite or open reading confirms that specific plug has failed.

5. Probe the glow plug supply rail for proper voltage during the pre-glow sequence when you turn the ignition to the on position. Insufficient voltage indicates a problem upstream in the control circuit, while proper voltage with a stored code points toward individual plug failures.

6. After clearing the code from memory, complete a full drive cycle that reproduces the conditions recorded in the freeze frame. If P1397 returns, the underlying fault remains active and requires further investigation.

Repair Options and Costs

Addressing P1397 starts with the most frequent culprits before progressing to costlier components:

• Replace all glow plugs as a set: $250–$600 for parts and labor
• Replace a single failed glow plug: $80–$200 for parts and labor
• Replace the glow plug relay or control module: $150–$450 for parts and labor

Independent shop labor rates typically run $75–$120 per hour, and glow plug replacement on most diesel engines requires 1–3 hours depending on engine configuration and accessibility.

How Much Does Fixing P1397 Cost?

The total expense for repairing code P1397 varies based on which component actually failed. Most common scenarios involve either corroded connectors, a single burnt-out plug, or the glow plug control relay. Full-set replacements command the highest parts cost but remain straightforward repairs. Replacing one glow plug offers the lowest entry point but only applies if the remaining plugs test healthy. The glow plug relay represents an intermediate expense but eliminates an entire circuit from suspicion. Across all scenarios, expect to pay between $80 and $600 total. Proper electrical diagnosis before purchasing parts prevents unnecessary spending—never assume the priciest scenario applies without confirming it through testing.

Can You Drive With P1397?

Yes, you can operate the vehicle safely once the engine reaches normal operating temperature. The primary concern with P1397 centers on cold starts: if multiple glow plugs have failed, the engine may refuse to start in cold weather or require extended cranking that drains the battery. Once running, the engine performs normally even with this code active. However, waiting until winter arrives to address the problem risks being stranded when temperatures drop. Schedule the repair before cold weather arrives, particularly if you live in a region experiencing regular freezing temperatures. P1397 carries a short-term severity rating—you can continue driving for a limited period, but prompt attention prevents inconvenience and potential tow charges.

Related OBD2 Codes

• P1300: Multi-faults - Bank 1 - With Low Side Shorts
• P1301: Multi-faults - Bank 2 - With Low Side Shorts
• P1302: Injector High Sides Shorted Together
• P1303: IDM Failure
• P1304: Cylinder Head Overtemperature Protection Active
• P1305: Boost Calibration Fault