P0377 – Timing Reference High Resolution Signal “B” Too Few Pulses

P0377 is one of those ignition-related trouble codes that can be confusing when you first see it on a scan tool. It doesn’t sound as familiar as misfire or cam sensor codes, but it still points to a timing and signal problem your engine relies on to start and run correctly. In this guide, you’ll learn what P0377 means in plain language, what usually causes it, the symptoms you might notice, and how a mechanic would diagnose and fix it so you can decide what to do next.

What Does P0377 Mean?

P0377 is an OBD-II diagnostic trouble code that typically translates to “Timing Reference High Resolution ‘B’ Too Few Pulses” (wording varies slightly by manufacturer). In simple terms, your ECU is not seeing enough high-resolution timing pulses from a specific reference circuit or sensor channel, usually tied to a crankshaft or injection pump timing wheel.

The engine computer uses these high-resolution pulses to precisely control ignition timing and fuel injection. When the pulse count drops below what the ECU expects, it sets P0377 and may switch to a backup, low-resolution strategy. That can protect the engine, but it often leads to rough running and reduced performance.

Quick Reference

• Code: P0377
• Meaning: Timing reference high-resolution “B” – too few pulses
• Systems involved: Crankshaft / injection pump timing, ignition control, ECU
• Severity: Moderate – can affect drivability and starting
• Typical fixes: Wiring repair, sensor replacement, timing wheel inspection, ECU reflash or replacement (rare)

Real-World Example / Field Notes

In the shop, I most often see P0377 on vehicles that have had recent engine work or timing-related repairs. One example: a diesel truck came in with hard starting and poor throttle response. The owner had just replaced the injection pump. The scan tool showed P0377, and scope testing revealed a weak high-resolution signal from the pump timing sensor. The sensor wiring had been pinched during installation, causing intermittent low voltage and missing pulses. Repairing the harness and securing it away from the pump pulley cleared the code and restored normal operation.

Symptoms of P0377

• Hard starting – longer crank time or multiple attempts before the engine fires.
• Rough idle – unstable idle speed, shaking, or occasional stumble at stoplights.
• Loss of power – sluggish acceleration and reduced performance, especially under load.
• Poor fuel economy – increased fuel consumption due to less precise timing control.
• Engine hesitation – brief hesitation or flat spots when you press the throttle.
• Check engine light on – MIL illuminated, sometimes with other timing or misfire codes.
• Intermittent stalling – in more severe cases, the engine may stall at idle or low speed.

Common Causes of P0377

Most Common Causes

• Faulty high-resolution timing sensor – often a crankshaft position sensor, cam sensor, or injection pump timing sensor on diesels. Internal failure or contamination can reduce the pulse signal.
• Damaged or corroded wiring – broken conductors, poor splices, or corroded connectors in the timing sensor “B” circuit reduce voltage and cause missing pulses.
• Loose or damaged timing wheel / reluctor – bent, cracked, or misaligned teeth on the high-resolution wheel can cause the ECU to see fewer pulses than expected.
• Poor sensor mounting or gap – incorrect air gap between the sensor and reluctor ring after engine work or sensor replacement can weaken the signal.
• Aftermarket parts issues – low-quality replacement sensors or incorrect part numbers that don’t match the OEM signal characteristics.

Less Common Causes

• ECU internal fault – failed input circuitry inside the engine computer misinterpreting a good signal as low pulse count.
• Electrical noise / interference – routing sensor wiring too close to ignition coils, alternator, or high-current wiring can create signal distortion.
• Mechanical timing problems – severely stretched timing chain, slipped belt, or incorrectly installed injection pump altering the relationship between low- and high-resolution signals.
• Previous tuning or ECU flash – incorrect software calibration affecting how the ECU expects and counts pulses.

Diagnosis: Step-by-Step Guide

To properly diagnose P0377, you’ll want at least a decent scan tool that can read live data, a digital multimeter, and ideally a lab scope. A wiring diagram and service information for your specific vehicle are also very helpful.

1. Confirm the code and check for companions. Connect a scan tool, confirm P0377 is present, and look for related codes like crank/cam sensor or misfire DTCs. Multiple timing codes can point toward a mechanical issue.
2. Check freeze frame data. Note engine speed, load, and temperature when P0377 set. If it sets only during start-up or at specific RPM, that narrows down the problem area.
3. Perform a visual inspection. Inspect the wiring harness to the crank/cam or injection pump timing sensor. Look for chafing, oil-soaked connectors, broken clips, or loose plugs, especially near moving parts and hot exhaust components.
4. Verify sensor power and ground. With a multimeter, check that the sensor receives proper reference voltage (often 5V) and has a solid ground. A bad ground can cause low signal amplitude and missing pulses.
5. Check sensor signal with live data. On the scan tool, monitor engine speed and any available “high-resolution” or “sync” PID while cranking and running. Unstable or dropping RPM readings can indicate a weak signal.
6. Scope the signal if possible. Using a lab scope, back-probe the signal wire and compare the waveform to service specs. You’re looking for clean, evenly spaced pulses. Missing or distorted pulses confirm the issue.
7. Inspect the reluctor / timing wheel. If accessible, remove covers and inspect the high-resolution wheel for bent, missing, or dirty teeth. On some diesel pumps, this may require more disassembly.
8. Check mechanical timing. If other timing codes are present or the engine runs very poorly, verify cam/crank or pump timing marks. A slipped belt or chain can cause mismatch between low- and high-resolution signals.
9. Substitute a known-good sensor if needed. If the wiring and wheel look good but the signal is weak or erratic, replacing the suspect sensor with OEM-quality is a logical next step.
10. Clear codes and road test. After repairs, clear the codes, perform a cold start, and drive under similar conditions to the freeze frame data. Re-scan to confirm P0377 does not return.

Pro tip: Use Mode $06 data (if your scan tool supports it) to check misfire and crank/cam correlation counters. Sometimes you’ll see borderline counts that point to a marginal sensor or wiring issue before the code becomes permanent.

Possible Fixes & Repair Costs

Fixing P0377 usually involves restoring a clean, consistent timing reference signal to the ECU. The exact repair depends on what you find during diagnosis.

• Repair or replace damaged wiring/connectors – cleaning corrosion, repairing broken wires, or replacing a connector pigtail.
• Replace faulty timing sensor – crankshaft, camshaft, or injection pump high-resolution sensor with an OEM or high-quality aftermarket part.
• Adjust or remount sensor – correcting the air gap or securing a loose sensor bracket.
• Replace or repair timing wheel/reluctor – fixing bent or missing teeth, or replacing a damaged wheel.
• Correct mechanical timing – re-timing the belt/chain or injection pump if it was installed incorrectly.
• ECU repair or replacement – rare, but possible if all external components test good and the input circuit is faulty.

Typical repair costs for P0377 range from about $150–$350 for simple wiring repairs or a basic sensor replacement, up to $600–$1,200+ if timing components or an injection pump sensor buried in the engine require significant labor. Costs vary with vehicle make, engine layout, part quality, and shop labor rates in your area.

Can I Still Drive With P0377?

In many cases, you can still drive with P0377 for a short period, but the engine may not run at its best. The ECU often falls back to a less precise timing strategy, which can mean reduced power, rough idle, and higher fuel consumption. If you notice hard starting, stalling, or worsening drivability, you should avoid long trips and heavy towing until it’s fixed. Continued driving with a compromised timing signal can also stress other components over time.

What Happens If You Ignore P0377?

If you ignore P0377, the engine may run for a while, but the inaccurate timing can lead to chronic hard starts, misfires, and potential damage to catalytic converters or diesel aftertreatment systems due to unburned fuel. In severe cases, a failing sensor or loose reluctor can progress to a no-start condition, leaving you stranded. Addressing the issue early usually keeps the repair simpler and cheaper.

Last updated: March 1, 2026

Vehicles Commonly Affected by P0377

P0377 shows up most often on vehicles that use high-resolution crank or injection pump timing systems, especially certain diesel and performance-oriented gasoline engines. You’ll commonly see it on GM trucks and SUVs (Chevrolet, GMC), some Ford and Dodge/Ram diesel pickups, and various European diesels from Volkswagen, Audi, and BMW. It can also appear on Japanese brands like Toyota, Nissan, and Isuzu that use detailed crank/cam timing strategies. While any OBD-II vehicle can set this code, it’s more frequent on engines with complex timing and injection control.