P0377Generic code

P0377 Code: Timing Reference High Resolution Signal B Too Few Pulses

The P0377 code means the computer receives fewer pulses than expected on high resolution timing reference signal B, which it uses to work out engine position precisely. A damaged wheel or disc, dirt or a weak signal is the usual reason. The engine usually keeps running on the low resolution signal alone, but less smoothly.

Severity
Medium
Can I keep driving?
Yes, but with caution
Estimated cost
$50 to $600
Difficulty
Medium
System
Ignition and Misfires
Applies to
All OBD2 vehicles

Updated Reviewed by Renzo Chacca

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What the P0377 code means

Some ignition systems, such as the GM Optispark or the optical distributors on Nissan and Mitsubishi engines, generate two signals. One is a low resolution signal used to synchronize the engine. The other is a high resolution signal, with hundreds of pulses per revolution, used to fine-tune ignition and injection timing.

The P0377 code means the computer receives fewer pulses than expected. That points to a damaged wheel or disc, dirt or a weak signal. The computer usually falls back to using only the low resolution signal, so the engine runs less smoothly.

On optical distributors, moisture or oil inside the distributor is a very common cause of these codes. The optical sensor works with a light emitting diode and a photodetector separated by a slotted disc. Any dirt, condensation or oil film on the disc blocks the light and makes pulses get lost.

On the GM Optispark, the distributor sits behind the water pump. When the pump leaks coolant, the drip lands on the distributor and puts it out of action. That is why, when you repair these codes, you should also check the water pump and the distributor vent. Some versions have a vent tube that gets clogged.

P0377 code symptoms

  • Hard starting, or the engine does not start.
  • Hesitation and misfires.
  • Rough idle.
  • Loss of power.
  • Check engine light on.

Likely causes of the P0377 code

  1. Moisture or oil inside the distributor on optical systems. Very frequent.
  2. Damaged optical sensor or slotted disc. Scratched, broken or with dirt caked on.
  3. Signal wires running close to spark plug wires, which induces noise and creates false pulses.
  4. Corroded connector. The resistance weakens the signal.
  5. Sensor power supply missing. Check the fuse and the 5 V or 12 V reference.
  6. Leaking water pump on GM Optispark systems.

How to diagnose the P0377 code step by step

  1. Inspect the inside of the distributor. Look for moisture, oil or a damaged disc.
  2. Check the sensor power supply. It is normally 5 V or 12 V.
  3. Measure the signal with an oscilloscope. There should be a regular, dense train of pulses. Missing pulses point to a damaged disc or dirt.
  4. Separate the signal wires from the high-voltage wires.
  5. Check the connector and the ground.

How to fix the P0377 code

  • Dry and clean the distributor, and fix the water or oil entry (seal).
  • Replace the distributor or the optical sensor.
  • Repair the wiring and route it away from the high voltage.
  • Clear the codes and verify the repair.

Technical reference data

  • High resolution signal: up to 360 pulses per revolution on optical systems.
  • Low resolution signal: one pulse per cylinder.
  • These are approximate values. Check the manufacturer data for your engine.

Brand-specific notes

This code is strongly associated with the GM LT1 V8 engines with Optispark and with the optical distributors that Nissan and Mitsubishi used in the 1990s.

P0377 on GM

Closely tied to the LT1 V8 engines (Camaro, Corvette, Caprice) with the Optispark distributor.

P0377 on Nissan

Seen with the optical distributors Nissan used in the 1990s.

P0377 on Mitsubishi

Seen with the optical distributors Mitsubishi used in the 1990s.

Frequently asked questions about P0377

Can I drive with a P0377 code?
With caution. The engine can misfire or stall, so get it checked before you drive far.
What is the Optispark?
An optical GM distributor mounted behind the water pump on the LT1 V8 engines. Leaks from the pump damage it often.
Can the distributor be repaired?
Sometimes a cleaning is enough. If the optical sensor is damaged, it gets replaced.
How much does it cost to fix P0377?
Roughly $50 to $600, depending on whether it needs a cleaning, a sensor or a whole distributor.
What does the P0377 code mean?
It is a generic OBD2 code that means the computer is seeing too few pulses in high resolution timing reference signal B, which points to a damaged wheel or disc, dirt or a weak signal.
Will a P0377 code clear itself?
A P0377 code can clear itself if the cause was a bit of moisture that dried out, but a damaged disc or a dirty distributor will bring it back. Find the source of the problem.
What is the most common cause of P0377?
Moisture, oil or dirt on the slotted disc of an optical distributor, which blocks the light and makes pulses go missing. A damaged disc is the next most likely cause.

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