P1398 — Glow Plug Circuit Low Input (Bank #2)

What is the most common cause of P1398?

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

Will P1398 clear itself?

Sometimes P1398 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.

Is it safe to drive with P1398?

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 much does it cost to fix P1398?

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

How do I diagnose P1398 myself?

Start with a scan: confirm P1398, 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.

Diesel engines rely on glow plugs to warm the combustion chambers before starting, especially in cold weather. Without adequate heat, fuel doesn't ignite properly and the engine struggles to run. Code P1398 indicates the powertrain control module has detected abnormally low electrical activity in the glow plug circuit for bank two. This isn't a one-time sensor glitch — the module requires the fault to repeat under specific conditions before it stores the code and turns on the check engine light.

When P1398 sets, the control unit captures a freeze frame snapshot showing engine speed, load, temperature, and vehicle speed at the moment of failure. That data is invaluable during diagnosis because it tells you exactly which driving scenario triggered the fault. Reproducing those conditions during testing helps confirm whether the problem is still present. The underlying issue is always electrical — either a failed component, a broken connection, or excessive resistance blocking current flow to the heating elements.

Symptoms of P1398

• Check Engine Light remains illuminated after startup
• Visible white or gray smoke from the exhaust pipe during cold starts
• Engine cranks longer than normal or fails to start in cold temperatures
• Rough, uneven idle that smooths out once the engine reaches operating temperature
• Glow plug warning light stays on in the instrument cluster

What causes P1398?

• One or more failed glow plugs in the bank two cylinder bank
• Blown fuse protecting the glow plug control circuit
• Corrosion or moisture intrusion at the glow plug electrical connectors
• Faulty glow plug relay or integrated control module
• Damaged wiring harness — chafed insulation, broken wires, or loose terminal connections

How to diagnose P1398

1. Connect an OBD2 scan tool and verify P1398 is stored. Record the freeze frame data, which shows engine conditions when the fault occurred — engine load, rpm, coolant temperature, and vehicle speed.
2. Look for any accompanying diagnostic trouble codes. Related codes often point toward the actual root cause, such as circuit shorts or open grounds. Check manufacturer technical service bulletins for your specific make and model — factory-issued updates may address known issues.
3. Perform a visual inspection of the glow plug wiring and connectors for bank two. Look for damaged insulation, corroded terminals, disconnected plugs, or evidence of heat damage.
4. Test each glow plug's resistance with a multimeter. A healthy plug shows a low resistance reading, typically between 0.5 and 2 ohms depending on design. An open circuit indicates a failed element.
5. Check for battery voltage at the glow plug rail during the pre-glow phase using a digital multimeter. No voltage means the control circuit or relay is not activating properly.
6. Clear the code and perform a complete drive cycle that includes the conditions captured in the freeze frame. If P1398 returns, the fault is confirmed and active.

Repair options for P1398

Start with the most common and least expensive possibilities before replacing major components:

• Replace a single faulty glow plug — parts and labor: $80–$200
• Replace the glow plug relay or control module — parts and labor: $150–$450
• Replace the complete set of glow plugs — parts and labor: $250–$600

Total repair cost for P1398

Your actual cost depends entirely on which part is faulty. For a single glow plug replacement, expect to pay between $80 and $200 including labor. If testing reveals the relay or control module is the culprit, the bill typically falls between $150 and $450. Replacing all glow plugs in the bank — which some technicians recommend when multiple plugs show signs of wear — generally costs $250 to $600. In most cases, the problem comes down to one or two bad plugs, putting the total on the lower end of that range. Always verify the exact failure before committing to a full-set replacement.

Can you drive with P1398?

After the engine warms up, you can continue driving without immediate concern. The real danger surfaces during cold starts — if several glow plugs are dead, the engine may not start at all on a frosty morning. Addressing P1398 before cold weather arrives is strongly recommended. The issue carries a short-term severity rating: it won't leave you stranded once the engine is running, but it will create serious problems when you need to restart in low temperatures.

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