P0378 – Timing Reference High Resolution Signal “B” Intermittent/Erratic Pulses

P0378 is one of those codes that sounds scary but is usually very fixable once you understand what’s going on. It points to a problem with a high‑resolution timing signal that your engine computer relies on for precise fuel and ignition control. When this signal glitches, you may notice hard starting, rough running, or even stalling. In this guide, you’ll learn exactly what P0378 means, the most common causes, symptoms to watch for, how a mechanic would diagnose it, and what repairs and costs you can realistically expect.

What Does P0378 Mean?

P0378 is typically defined as “High Resolution B Signal – No Pulses” or “Timing Reference High Resolution B – Intermittent/No Signal,” depending on the manufacturer. In plain English, your engine control unit (ECU) is losing or not seeing a clean timing signal from one of its high‑resolution reference channels, often tied to a crankshaft or injection pump sensor.

This timing signal helps the ECU determine exactly when to fire the injectors and ignition. If the signal is missing, noisy, or out of range, the ECU logs P0378 and may switch to a backup strategy, which can hurt performance and drivability.

Quick Reference

• Code: P0378
• Meaning: High‑resolution timing “B” signal missing or intermittent
• Main systems: Crankshaft/injection pump timing, ignition, fuel control
• Common causes: Sensor failure, damaged wiring, poor connections, rare ECU fault
• Risk level: Moderate – may cause stalling or hard starting

Real-World Example / Field Notes

In the shop, I most often see P0378 on diesel pickups and some older gasoline engines that use a high‑resolution timing disk or injection pump sensor. A typical case: a truck comes in with intermittent stalling at idle and a rough cold start. No obvious misfire codes, just P0378 stored. Wiggling the harness at the timing sensor makes the engine stumble. Pull back the loom and you’ll usually find chafed or oil‑soaked wires. Repair the wiring, clean the connector, clear the code, and the truck is back to normal. The sensor itself is bad less often than the wiring, but both happen.

Symptoms of P0378

• Check engine light on: P0378 will usually set the MIL, sometimes with no other codes.
• Hard starting: The engine may crank longer than normal, especially when cold.
• Rough idle: Unstable or lumpy idle due to inaccurate timing control.
• Stalling: Engine may stall at stoplights or when coming off throttle.
• Loss of power: Noticeable lack of acceleration or sluggish response.
• Poor fuel economy: Incorrect timing can cause extra fuel consumption.
• Intermittent hesitation: Brief cutouts or surging while cruising or accelerating.
• Hard hot restart: Engine may be harder to restart once warmed up.

Common Causes of P0378

Most Common Causes

• Faulty high‑resolution timing sensor: The sensor on the crankshaft, camshaft, or injection pump (depending on design) can fail internally or go out of spec, causing a weak or missing signal.
• Damaged or chafed wiring: Harnesses routed near the exhaust, engine mounts, or sharp brackets can rub through, shorting or opening the timing signal circuit.
• Corroded or loose connector: Oil, coolant, road salt, or moisture inside the sensor connector can create high resistance and intermittent signal loss.
• Improper air gap or sensor alignment: On some setups, the sensor must sit a specific distance from the tone wheel or timing disk. If it’s too far or misaligned (after engine work, for example), the signal becomes unreliable.
• Aftermarket modifications: Poorly routed wiring, piggyback tuners, or non‑OEM sensors can introduce noise or weak signals that trigger P0378.

Less Common Causes

• Damaged tone wheel or timing disk: Missing teeth, bent disks, or cracks can cause the ECU to see gaps in the high‑resolution signal.
• Internal ECU fault: Rare, but a failing engine control unit can misread or lose the B‑channel signal.
• Low system voltage or poor grounds: Weak battery, bad engine ground, or corroded ground straps can distort sensor signals.
• Previous incorrect repairs: Spliced wiring with poor solder joints, wrong gauge wire, or twisted‑together connections can create intermittent issues.
• Water intrusion: Flood damage or pressure‑washing the engine bay can push water into connectors or harnesses.

Diagnosis: Step-by-Step Guide

To properly track down P0378, you’ll want a good scan tool (preferably with live data and Mode $06 access), a digital multimeter, basic hand tools, and if available, an oscilloscope for viewing sensor waveforms. A wiring diagram for your specific vehicle is also extremely helpful.

1. Confirm the code and check for others. Connect a scan tool, read all stored and pending codes, and note freeze‑frame data. If you see related timing or crank/cam sensor codes, diagnose them together.
2. Visual inspection of the sensor and harness. Locate the high‑resolution timing sensor (often on the crankshaft, camshaft, or injection pump). Look for oil saturation, broken clips, melted insulation, or rubbed‑through wires.
3. Check the connector. Unplug the sensor and inspect for corrosion, bent pins, moisture, or loose terminals. Clean with electrical contact cleaner and ensure a tight fit when reconnecting.
4. Wiggle test. With the engine idling and scan tool monitoring live data (RPM, timing, and any “high‑res” signal PIDs), gently move the harness and connector. If the engine stumbles or data drops out, you’ve likely found a wiring or connector issue.
5. Measure sensor power and ground. Using a multimeter, verify that the sensor receives proper reference voltage (often 5V) and has a solid ground. A missing supply or bad ground can mimic a failed sensor.
6. Check signal output. If you have an oscilloscope, back‑probe the signal wire and watch for a clean, consistent waveform as the engine cranks or runs. A weak, flat, or noisy signal indicates sensor or tone wheel problems.
7. Inspect the tone wheel or disk. Where accessible, look for broken teeth, cracks, or excessive rust. Rotate the engine by hand if needed and check for uniform spacing and condition.
8. Review Mode $06 data (if available). Some vehicles store counters for timing signal errors. High counts on the high‑resolution B channel support a sensor or wiring fault even if the code is intermittent.
9. Check grounds and battery voltage. Load‑test the battery and inspect engine and chassis grounds. Correct any corrosion or loose connections that could affect sensor performance.
10. Substitute a known‑good sensor. If all wiring, power, ground, and mechanical checks pass but the signal remains erratic, replacing the sensor with a quality OEM or equivalent unit is the next logical step.

Pro tip: On intermittent P0378 issues, heat and vibration matter. Run the engine until hot, then repeat wiggle tests and live‑data monitoring. Many “no problem found” vehicles only act up when everything is at operating temperature.

Possible Fixes & Repair Costs

Most P0378 repairs are straightforward once you pinpoint the fault. Common fixes include repairing or replacing damaged wiring, cleaning and tightening connectors, replacing the high‑resolution timing sensor, or in rare cases, addressing a damaged tone wheel or ECU issue. At a typical shop, you might spend around $100–$200 for diagnosis, $150–$350 for a sensor replacement, $100–$300 for wiring repairs, and $600+ if a tone wheel or ECU needs replacement. Costs vary with vehicle make, parts quality (OEM vs aftermarket), labor rates, and how difficult access is.

Can I Still Drive With P0378?

In many cases, you can still drive with P0378 present, but it’s not something you should ignore for long. If the ECU is able to fall back on a lower‑resolution timing signal, the engine may run reasonably well with only mild symptoms. However, if the signal drops out completely, you can experience stalling, no‑start conditions, or sudden loss of power. For safety and to prevent getting stranded, limit driving and schedule diagnosis as soon as possible, especially if the engine runs rough or stalls.

What Happens If You Ignore P0378?

Ignoring P0378 can lead to more frequent stalling, hard starting, and potential damage from backfires or unburned fuel entering the exhaust. Over time, that can shorten the life of your catalytic converter or diesel aftertreatment components and may mask other timing‑related issues. Fixing the root cause early is usually much cheaper than dealing with secondary damage later.

Last updated: February 13, 2026

Vehicles Commonly Affected by P0378

P0378 shows up most often on diesel pickups and SUVs that use high‑resolution timing or injection pump sensors, including many Chevrolet/GMC Duramax, Ford Power Stroke, and Dodge/Ram Cummins applications. It can also appear on certain Volkswagen/Audi TDIs, older GM gasoline engines with optical distributor or crank sensors, and some Japanese models that use detailed crank/cam timing wheels. While the exact hardware differs, the underlying issue is the same: the ECU isn’t happy with the high‑resolution B‑channel timing signal.