OBD-II trouble code
P0377: Timing Reference High Resolution Signal 'B' Too Few Pulses
Pulses arriving short on the second channel while the first is perfect. That combination rules out the reference wheel entirely and turns this into a voltage problem, not a timing one.
Quick facts
- System
- Powertrain
- Category
- Sensors / Timing
- Severity
- High severity
- Drivable
- No — stop driving until repaired
- Repair cost range
- $150 – $1,600
- DIY difficulty
- Shop recommended
What does P0377 mean?
P0377 reports a shortfall in the pulse count on the second high-resolution timing channel. The interesting case — and the common one — is P0377 stored on its own, with the 'A' channel reporting nothing wrong. That pairing carries a conclusion you can bank before touching a tool.
A missing tooth on a reference wheel starves every channel that reads that wheel. If 'A' is counting correctly, the teeth are all present and the crankshaft is turning as expected. So a shortfall confined to 'B' cannot be mechanical. Whatever is happening is happening between the point where the two paths diverge and the point where the count is taken, and that is a much smaller search area than the engine.
What lives in that area is mostly a question of signal amplitude. On systems where 'B' is a conditioned copy passed from one module to another, the pulse train is a squared-up digital waveform and the receiving module counts it by comparing each pulse against a threshold voltage. Pulses that clear the threshold are counted; pulses that do not are simply invisible. Anything that shaves amplitude off the train therefore removes pulses without altering their timing in the slightest — a corroded splice, a green terminal, a partially broken strand inside insulation that looks perfect, a repair crimp that has gone high-resistance. The waveform leaves the sending module in good order and arrives at the receiver too small to count.
That is why the measurement that matters here is taken at the receiving end, not the sending end. Scoping the signal where it is generated tells you what was sent, which is rarely the question. Scoping it at the receiving module's input pin tells you what was actually available to be counted, and the difference between the two is the fault.
There is a second mechanism worth understanding because it produces the same symptom without any fault in the signal wire at all. The receiving module measures pulse height against its own ground. If that ground has gone high-resistance, the module's reference floats upward, and every pulse in the train appears shorter than it really is. Enough of them fall under the threshold and the count comes up short. Testing the module's ground with a voltage drop measurement takes minutes and prevents a wiring repair that would never have helped.
Where 'B' really is a separate pickup, the ordinary air gap and sensor-strength causes apply — but so does a cost point worth raising early. The second sensor is almost always the awkward one, tucked against the bulkhead or behind the far bank, and the same part can carry several times the labour of its twin.
Common causes
- Corroded splice, terminal or crimp reducing pulse amplitude below the receiving module's counting threshold
- High-resistance ground at the receiving module raising its reference and making every pulse read short
- Partially broken conductor inside intact insulation, common where a harness bends near a mount
- Water ingress into an inter-module connector leaving resistance behind after it dries
- Weak or failing output driver in the sending module producing an undersized pulse train
- Drifted input threshold or degraded input circuit in the receiving module
- Excessive air gap or weakened magnet where 'B' is a genuine second pickup
- Poor previous harness repair, such as a twisted-and-taped joint that has oxidised
Symptoms
- Check engine light on with the engine running acceptably in most conditions
- Rough running or a weak, hesitant cylinder bank on split-bank injection systems
- Extended cranking, especially when the battery is cold and system voltage is lower
- Loss of power under sustained load as the receiving controller falls back on a protected strategy
- Correlation or plausibility codes stored alongside this one
- Fault that worsens in wet weather where water ingress into a connector is responsible
Diagnostic steps
- 1.Check whether an 'A' channel code is also stored. If it is not, the reference wheel and crankshaft are proven good and every mechanical cause can be set aside immediately.
- 2.Confirm from the wiring diagram where the two paths diverge. Everything before that point is already exonerated by a clean 'A' channel; the search area is everything after it.
- 3.Scope the signal at the receiving module's input pin, not at the sending module's output. What matters is the amplitude that was available to be counted, and that is only visible at the receiving end.
- 4.Measure pulse height against specification. A train of the right shape and the wrong height is the signature of resistance in the run, and it will not show up as a timing error.
- 5.Compare the same waveform at the sending module's output pin. Full amplitude leaving and reduced amplitude arriving isolates the loss to the wiring between them.
- 6.Test the receiving module's ground with a voltage drop measurement under load. A floating ground makes healthy pulses read short and produces this code with no wiring fault present.
- 7.Open and inspect every connector and splice on the run, looking for green corrosion, water staining and previous repairs. A twisted-and-taped joint from an old repair is a frequent finding.
- 8.Flex the harness at bends and near mounting points while watching pulse amplitude on the scope. A partly broken conductor inside good insulation shows up as amplitude that changes with position.
- 9.Where 'B' is a real second sensor, check its air gap, seating and part number before assuming a wiring fault.
Repair cost
$150 – $1,600
Cleaning and repairing a corroded splice or terminal, or restoring a module ground, is typically $150 to $450 and is the most common outcome when 'A' is clean. Tracing a broken conductor inside intact insulation costs more in labour than in parts, commonly $200 to $600. Where 'B' is a genuine second sensor, budget $40 to $200 in parts but allow for the harder access on the far bank, so $250 to $700 fitted rather than the $150 to $450 the 'A' side would cost. A failed sending driver or receiving input stage means a module at $600 to $1,600 with programming, and that should be the last conclusion rather than the first.
Estimate your repair
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Open the Repair Cost Estimator with wiring harness repair preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
Leave this one to a qualified shop. It typically involves emissions-critical components, refrigerant handling, or other work that requires manufacturer-grade tooling, training, or certification. DIY attempts often produce a more expensive problem than the original code.