P1378 — FICM Low Supply Voltage

Does P1378 mean the fuel injectors are bad?

Not necessarily. P1378 specifically identifies low voltage at the fuel injection control module, so the battery, charging system, power feeds, grounds, and wiring should be tested before blaming injectors or replacing the module.

Can I drive with P1378 stored?

Only if the engine runs normally and does not stall. If P1378 is active and the engine hesitates, shuts down, or will not restart, continued driving could worsen damage to the FICM or related control circuitry.

Why does P1378 appear only when the engine is cranking?

Cranking places the heaviest demand on the battery. A weak battery, damaged connection, poor ground, or high-resistance supply path can allow voltage to fall below the FICM operating threshold during that moment.

How is P1378 repaired and confirmed?

Repair the source of low module voltage, then clear the code with an OBD scanner and verify it remains absent during cranking and running. If all upstream tests pass, the FICM power supply and internal module may require further diagnosis.

When your vehicle's check engine light comes on and a scan tool pulls code P1378, it means the powertrain control module has detected a problem in one of the sensor or actuator circuits that it monitors continuously. This isn't a generic emissions code—P1378 points to a specific electrical fault that the ECU verified across multiple drive cycles before lighting up the dashboard. The control module won't set this code on a single glitch; it waits for patterns that confirm an actual failure rather than a momentary sensor spike.

Alongside P1378, the ECU saves a freeze frame snapshot that captures engine rpm, load percentage, coolant temperature, and vehicle speed at the exact moment the fault occurred. That snapshot is invaluable during diagnosis because it tells you what conditions triggered the code. Whether the failure happened during cold startup, heavy acceleration, or steady cruising—knowing that context helps you reproduce the issue and verify your fix. Checking that data first keeps you from chasing phantom problems that occurred under completely different operating conditions.

The root cause behind P1378 typically stems from electrical degradation somewhere in the affected circuit. More often than not, the culprit is environmental: moisture intrusion causing connector corrosion, vibration loosening a harness plug, or road debris chafing through wire insulation. Less frequently, the fault lies within the control module itself or a severely corroded ground path. Understanding these common failure modes points your diagnostic effort in the right direction from the start.

Symptoms of P1378

The Check Engine Light illuminates on the dashboard as the primary indicator. In many vehicles, this may be the only noticeable symptom initially.

Fuel economy tends to suffer when the system referenced by P1378 can't maintain proper operation. The engine may compensate through closed-loop adjustments, burning slightly more fuel to keep emissions in check.

Depending on which component is affected, the vehicle may enter limp mode—a reduced-power state designed to protect the drivetrain. This typically happens with more critical sensor failures.

The affected system itself may function erratically or stop working entirely. This could manifest as intermittent operation that comes and goes, making the fault harder to pin down during casual driving.

In severe cases, related systems may exhibit degraded performance until the root cause is addressed.

Common Causes

Corroded, loose, or damaged electrical connectors rank as the most frequent source of P1378 codes. Female terminals oxidize over time, increasing resistance and disrupting signal quality.

Open or shorted wiring within the affected circuit creates the same detection pattern. Broken strands inside insulation may still carry power intermittently until the wire fails completely.

A blown fuse protecting the circuit will obviously prevent normal operation. Checking fuse condition takes seconds and should never be skipped.

The control module driver responsible for that circuit can fail internally, though this represents a less common scenario compared to external wiring issues.

Poor ground connection at the component or control module creates signal anomalies that the ECU interprets as a fault. Ground paths deserve investigation when no obvious wire damage exists.

How to Diagnose P1378

Start by connecting an OBD2 scanner and retrieving all stored codes, not just P1378. Related codes often point toward the true root cause—for instance, multiple sensor faults may indicate a shared power or ground problem rather than individual component failures.

Before disassembling anything, search for technical service bulletins specific to your vehicle make, model, and year. Many P0xxx codes have documented factory fixes, software updates, or known issues that bypass hours of unnecessary troubleshooting. The NHTSA website and manufacturer portals let you search by VIN.

Perform a thorough visual inspection of the wiring harness and connectors tied to the affected system. Look for damaged insulation, broken wires hidden inside the loom, corrosion buildup on terminals, and any signs of prior repairs that may have been done incorrectly.

Use a multimeter to verify reference voltage, ground integrity, and signal continuity at the component connector. Compare your readings against manufacturer specifications when available. Unexpected voltage drops or open circuits indicate exactly where the problem lies.

While monitoring live data on your scan tool, perform a wiggle test on the harness—gently flex the wiring along its length and watch for signal dropout. Intermittent faults often reveal themselves through momentary interruptions that don't show up with the vehicle sitting still.

Clear the code after your inspection, then complete a drive cycle that reproduces the conditions recorded in the freeze frame data. If P1378 returns, the fault is confirmed and active. If it stays cleared, the issue may have been a temporary connection problem.

Repair Options and Costs

Wiring or connector repair — $80–$250. This covers terminal replacement, harness splicing, or connector reconstruction. It's usually the right starting point since wiring issues cause most P1378 codes.

Sensor or actuator replacement — $150–$500. If testing confirms the component itself has failed, replacement restores proper circuit operation. Always verify before swapping parts.

Control module repair — $300–$1,000. Required only when internal circuitry has failed. Diagnosis should rule out all external causes first, since control modules rarely fail without external evidence like water damage or voltage spikes.

Total Repair Cost

Depending on what actually failed, expect to spend between $80 and $1,000 to resolve P1378. Labor rates vary by region and shop, but independent shops typically charge less than dealerships for the same work. Getting an accurate diagnosis before purchasing parts prevents wasting money on components that don't address the actual problem.

Can You Drive With P1378?

Most vehicles remain drivable with this code stored, though you may notice reduced performance, poorer fuel economy, or an inoperative auxiliary system. Drive safely and prioritize repair within a reasonable timeframe—continuing to operate with sensor failures can cascade into more serious problems. If the vehicle exhibits stalling, hard starting, or warnings affecting brakes and steering, treat it as urgent and address it immediately.

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