P0378 Code: Meaning, Symptoms, Causes & Fix

Overview

Symptoms

• engine noise or unstable timing
• loss of power at mid/high RPM
• hard start when cold

Common causes

• Connector pin fit issue affecting timing reference high resolution signal b intermittent/erratic pulses
• Temperature/vibration related circuit dropout
• Connector pin damage/corrosion or harness stress near related components

How to diagnose P0378

1. Confirm P0378 and capture freeze-frame data with a professional scan tool.
2. Inspect wiring, connectors, and grounds related to timing reference high resolution signal b intermittent/erratic pulses for obvious defects.
3. Reproduce the fault with harness movement and heat/vibration checks.
4. Check for related DTCs to identify the root-cause chain before replacing parts.
5. After repair, clear codes and confirm readiness monitors complete without recurrence.

Severity of code P0378

Moderate: This code is moderate severity: the vehicle may still operate, but fault progression can increase repair cost over time.

Can you drive with P0378?

Most vehicles can be driven short-term with P0378, but postpone heavy load/highway driving until repaired.

If the warning lamp is flashing or drivability/safety is affected, avoid driving and diagnose immediately.

How to fix it (step-by-step)

1. Repair obvious wiring/connector faults first to prevent repeat parts replacement.
2. Address the root fault mode (intermittent) in the timing reference high resolution signal b intermittent/erratic pulses circuit/system.
3. Apply the required relearn/adaptation procedure if specified by service information.
4. Road-test and verify P0378 remains cleared under the conditions that originally set the code.

Fix options

• Wiggle-test harness during live monitoring.
• Retest after thermal/vibration simulation.
• Clear DTCs and verify with live data and completed drive cycle.

Typical repair cost

$300-$1400

Common mistakes when diagnosing P0378

• Replacing parts without verifying voltage, ground, and signal integrity first.
• Ignoring related stored/pending codes that indicate upstream faults.
• Skipping post-repair verification drive cycle and readiness checks.

P0378 in common vehicles

P0378 appears across makes such as Mazda, Subaru, Lexus, Jeep. Failure patterns differ by platform: wiring routes, software calibration, and component supplier revisions can change root cause.

Frequently asked questions

What causes P0378?

P0378 is commonly set by intermittent faults affecting timing reference high resolution signal b intermittent/erratic pulses.

How much does it cost to fix P0378?

For P0378, repair costs are typically $300-$1400 depending on exact root cause, labor rates, and part quality.

Can I clear P0378 without repair?

You can clear P0378 temporarily, but the code usually returns until the underlying fault is corrected.

Helpful tools

• Estimate repair cost
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• Calculate fuel cost