P0379 Code: Timing Reference High-Resolution Signal “B” No Pulses

The P0379 code means the powertrain control module detected no pulses from timing-reference high-resolution Signal “B.”

This is a complete signal-loss condition. Signal “B” is not merely weak, missing a few pulses or dropping out occasionally—the PCM sees no usable pulse activity while the engine is turning.

Common causes include a disconnected sensor, missing sensor power or ground, an open signal circuit, a failed optical or position sensor, a blocked optical path, stationary encoder hardware or a signal that disappears before reaching the PCM.

P0379 is commonly associated with electronically controlled diesel injection systems, but the exact Signal “B” source varies by manufacturer. Always consult the correct wiring diagram and service information before replacing parts.

P0379 Quick Reference

Item	Details
Code	P0379
Definition	Timing Reference High-Resolution Signal “B” No Pulses
System	Engine-position or injection-timing reference
Primary fault	Zero Signal “B” pulses detected
Common causes	Failed sensor, missing power or ground, open circuit or stationary encoder
Common symptoms	Crank-no-start, stalling, extended cranking or reduced power
Severity	Serious
Safe to drive?	Generally not recommended
Best diagnostic tools	Scan tool, multimeter and oscilloscope

For additional definitions, visit the complete P-series OBD-II code list or browse the Pro Street OBD-II Doctor.

What Does the P0379 Code Mean?

P0379 is stored when the PCM expects high-resolution timing-reference Signal “B” pulses but receives none.

The PCM uses timing-reference signals to determine precise rotational position. Depending on the vehicle, Signal “B” may help identify crankshaft position, injection-pump position, ignition timing or fuel-injection timing.

Possible Signal “B” sources include:

An optical sensor inside a diesel injection pump
A secondary timing-reference sensor
A crankshaft position sensor
A distributor-mounted optical pickup
A Hall-effect position sensor
A magnetic position sensor
A slotted encoder disc
A toothed reluctor wheel
A pump-mounted electronic control module

The “B” designation identifies a particular signal circuit. It does not automatically mean Bank 2, Sensor 2, Camshaft Sensor “B” or Ignition Coil “B.”

How High-Resolution Signal “B” Works

Many systems associated with P0379 use an optical sensor and rotating encoder disc.

The disc contains precisely arranged slots. As the disc rotates, these slots allow or block light traveling to an optical sensor. Each light transition creates an electrical pulse.

Other systems use a Hall-effect or magnetic sensor reading a toothed reluctor wheel. Each tooth generates a voltage transition corresponding with rotational position.

The PCM may use Signal “B” to calculate or verify:

Engine position
Injection-pump position
Engine speed
Fuel-injection timing
Injection synchronization
Ignition timing
Correlation with Signal “A”
Combustion-event timing

A normal oscilloscope waveform displays a repeating pulse train while the engine cranks or runs. A P0379 waveform remains inactive or flat.

What Does “No Pulses” Actually Mean?

P0379 means no usable Signal “B” transitions reach the PCM.

That can happen in several ways:

The sensor produces no output.
The sensor has no power or ground.
The encoder or reluctor does not rotate.
The optical path is completely blocked.
The signal wire is open.
The signal is shorted continuously high or low.
The sensor generates pulses, but the wiring prevents them from reaching the PCM.
The PCM receives the waveform but cannot process it.

The code describes what the PCM sees, not which component failed. A flat waveform is evidence—not permission to order the most expensive part in the system.

Common P0379 Symptoms

Common symptoms include:

Illuminated Check Engine Light
Engine cranks but will not start
Extended cranking
Engine starts and immediately stalls
Sudden engine stalling
Difficult hot or cold starting
Rough engine operation
Reduced power
Poor throttle response
Limp mode
Incorrect fuel-injection timing
Increased diesel exhaust smoke
No engine-speed reading from the affected circuit
Additional synchronization or position-sensor codes

Some vehicles may continue running by relying on another position input. Others will not start without Signal “B.”

Common Causes of P0379

Possible causes include:

Disconnected timing-reference Sensor “B”
Failed optical sensor
Failed Hall-effect sensor
Failed magnetic position sensor
Missing sensor supply voltage
Failed sensor ground
Open signal wire
Signal circuit shorted to ground
Signal circuit shorted to voltage
Broken wire beneath intact insulation
Bent or backed-out connector terminal
Corroded connector
Water, oil or fuel contamination
Completely blocked optical path
Failed optical light source
Stationary encoder disc
Broken injection-pump drive
Damaged reluctor wheel
Excessive sensor air gap
Incorrectly installed sensor
Failed injection-pump electronics
Low cranking voltage
PCM connector damage
PCM software problem
Internal PCM failure

How Serious Is P0379?

P0379 is a serious trouble code.

Without a usable high-resolution Signal “B,” the PCM may not be able to determine engine or injection-pump position accurately. Possible consequences include:

Complete no-start
Sudden stalling
Incorrect injection timing
Reduced engine power
Limp mode
Failure to restart after stalling

A vehicle that stalls or loses power should not be driven. If the engine still runs normally, it may be using a backup signal, but that does not make the fault harmless.

P0379 Compared With Related Codes

Code	Meaning	Diagnostic focus
P0374	Signal “A” No Pulses	Complete loss of Signal “A”
P0375	Signal “B” Malfunction	General Signal “B” fault
P0376	Signal “B” Too Many Pulses	Excessive Signal “B” pulse count
P0377	Signal “B” Too Few Pulses	Some pulses present, but too few
P0378	Signal “B” Intermittent/Erratic	Signal drops out or becomes unstable
P0379	Signal “B” No Pulses	Continuous absence of Signal “B” pulses

P0379 should remain focused on a completely inactive Signal “B” waveform.

P0379 Versus P0377

P0377 means Signal “B” is present, but the PCM counts fewer pulses than expected.

P0379 means the PCM detects no pulses.

On an oscilloscope:

P0377 displays a waveform with missing pulses or gaps.
P0379 displays a consistently flat or inactive waveform.

P0379 Versus P0378

P0378 identifies a Signal “B” waveform that works, fails and returns unpredictably.

P0379 identifies complete signal loss. No pulses are detected during the monitoring period.

A vehicle that occasionally stalls and restarts may fit P0378. A vehicle with a consistently dead Signal “B” circuit is more likely to set P0379.

P0379 Versus P0374

P0374 and P0379 describe equivalent failures on different circuits:

P0374 applies to Signal “A.”
P0379 applies to Signal “B.”

The two signals may use separate sensor channels, encoder tracks, wiring and PCM terminals.

P0379 Versus P0322

P0322 identifies no signal from the ignition or distributor engine-speed input circuit.

P0379 specifically identifies no pulses from high-resolution timing-reference Signal “B.” The two codes should not target the same primary keywords.

P0379 Versus P0335

P0335 identifies a crankshaft position Sensor “A” circuit malfunction.

P0379 may involve a crankshaft-derived signal on some vehicles, but other applications use an optical injection-pump sensor, distributor or separate timing-reference component.

How to Diagnose the P0379 Code

1. Scan All Modules

Connect a capable scan tool and record:

Stored codes
Pending codes
Freeze-frame data
Engine RPM during cranking
Battery voltage
Crankshaft and camshaft synchronization
Injection timing
Signal “A” and Signal “B” data, when available
Communication or power-supply codes

Do not clear the codes before saving this information.

Related low-voltage, crankshaft, camshaft or injection-pump codes may identify a shared cause.

2. Verify the Engine and Signal Hardware Are Turning

Before testing the electrical circuit, confirm that the engine and relevant timing components rotate.

Inspect as applicable:

Crankshaft rotation
Camshaft rotation
Distributor-shaft rotation
Injection-pump drive
Encoder-disc rotation
Reluctor-wheel rotation
Timing belt or chain operation

A stationary encoder cannot produce pulses. Sometimes the sensor is simply reporting that the mechanical component it watches has decided not to participate.

If the engine cranks unusually fast or lacks a normal compression rhythm, inspect mechanical timing before continuing.

3. Identify the Signal “B” Source

Consult the correct wiring diagram and determine:

Which component creates Signal “B”
Sensor location
Sensor type
Power terminal
Ground terminal
Signal terminal
PCM input terminal
Expected voltage
Correct waveform
Required sensor air gap
Whether another module processes the signal

Do not assume “B” means Bank 2 or Camshaft Sensor “B.”

4. Inspect the Connector

Disconnect the Signal “B” connector and inspect:

Bent pins
Backed-out terminals
Spread female terminals
Corrosion
Moisture
Oil or fuel contamination
Damaged seals
Broken locking tabs
Heat damage
Weak terminal tension

Reconnect the connector and confirm that it locks securely.

A connector can look attached while one terminal remains just far enough away to accomplish absolutely nothing.

5. Inspect the Wiring Harness

Trace the circuit from the sensor to the PCM or associated module.

Look for:

Open wiring
Chafed insulation
Pinched wires
Harness contact with hot components
Broken wires near connectors
Damage near engine mounts
Poor previous repairs
Corroded splices
Missing shielding
Rodent damage

Gently tug each wire near the connector. Internal conductors may break while the outer insulation remains intact.

6. Check Battery and Cranking Voltage

Test:

Battery state of charge
Battery load capacity
Cranking voltage
Starter current draw
Positive-cable voltage drop
Ground-cable voltage drop
Engine grounds
PCM power and grounds

Low cranking voltage can prevent the sensor, injection-pump electronics or PCM from operating correctly.

Correct battery, cable or starter problems before condemning the timing-reference system.

7. Verify Sensor Power

Back-probe the sensor according to the manufacturer’s procedure.

Confirm:

Correct supply voltage
Stable reference voltage
Voltage remains present during cranking
No short to ground
No excessive circuit resistance

If power is missing, trace the circuit through its fuse, relay, splice or module supply.

8. Verify Sensor Ground

Check:

Ground continuity
Ground-side voltage drop
Terminal tension
Shared sensor-ground operation
Ground stability during cranking

A static continuity test is not enough. A circuit may show continuity while still developing excessive voltage drop under load.

9. Check the Signal Circuit

With the circuit powered down and the appropriate modules disconnected, test the Signal “B” wire for:

Continuity
Excessive resistance
Short to ground
Short to voltage
Short to another circuit
Poor terminal contact

Where allowed, use a loaded-circuit test rather than relying only on an ohmmeter.

10. Examine Signal “B” With an Oscilloscope

Observe the waveform during cranking and, if possible, while the engine runs.

A normal waveform should have:

Repeating pulses
Predictable voltage amplitude
Clean switching edges
Consistent spacing
Frequency that increases with engine speed

A P0379 waveform may show:

A completely flat line
No switching activity
No frequency
Signal stuck at supply voltage
Signal stuck near ground
No change during cranking

A digital multimeter may display an average voltage without proving that pulses exist. An oscilloscope shows whether the circuit is actually switching.

11. Compare the Sensor and PCM Waveforms

Test Signal “B” at the sensor and PCM.

Possible results include:

No pulses at the sensor: Check sensor power, ground, air gap, encoder movement and sensor condition.
Pulses at the sensor but none at the PCM: Repair the signal wire, connector or terminal.
Pulses at the PCM terminal but P0379 remains: Check terminal contact, PCM power, software and internal PCM operation.

This comparison quickly separates a sensor-side failure from a harness or PCM-side failure.

12. Compare Signals “A” and “B”

Use a dual-channel oscilloscope when possible.

If Signal “A” operates normally while Signal “B” remains flat, shared mechanical rotation and PCM power may be less likely causes.

Focus on:

Signal “B” sensor channel
Separate encoder track
Signal “B” wiring
Sensor output stage
PCM Signal “B” input

If both signals are absent, inspect shared power, ground, mechanical drive and module operation.

13. Inspect the Optical System

On optical systems, inspect:

Sensor lens
Light source
Encoder disc
Encoder slots
Optical path
Fuel or oil contamination
Internal alignment
Encoder rotation

A fully blocked optical path can prevent every expected pulse.

14. Inspect the Reluctor and Air Gap

On magnetic or Hall-effect systems, check:

Reluctor-wheel condition
Missing teeth
Loose reluctor
Metal debris
Sensor-tip contamination
Sensor mounting
Specified air gap
Reluctor runout
Shaft movement

An excessive air gap may reduce the waveform below the sensor or PCM recognition threshold.

15. Inspect Injection-Pump Operation

On affected diesel engines, inspect:

Injection-pump drive
Internal optical sensor
Encoder rotation
Pump connector
Internal wiring
Pump-mounted electronics
Pump shaft and bearing condition

Confirm the external circuit before replacing the complete injection pump. The pump is expensive enough to deserve a trial before sentencing.

16. Evaluate the PCM Last

Consider PCM failure only after verifying:

Sensor power and ground
Mechanical rotation
Correct sensor installation
Encoder or reluctor condition
Signal output at the sensor
Signal integrity at the PCM connector
PCM power and ground
Connector terminal tension
Applicable software updates

PCM replacement may also require programming and relearn procedures.

Common P0379 Diagnostic Mistakes

Replacing the Sensor Without Checking Power and Ground

A new sensor will produce exactly zero pulses if it receives exactly zero power.

Ignoring Mechanical Rotation

A sensor cannot read an encoder, reluctor or distributor shaft that is not turning.

Using Only a Multimeter

Average voltage does not prove that the signal switches. Confirm the pulse train with an oscilloscope.

Treating P0379 Like P0377

P0377 has some pulses. P0379 has none. That changes the most likely causes and diagnostic priorities.

Confusing P0379 With P0322

P0322 covers an ignition or distributor engine-speed input. P0379 covers high-resolution timing-reference Signal “B.”

Replacing the Injection Pump Too Early

External connector, power, ground and wiring failures can imitate an internal injection-pump problem.

Testing Only at the Sensor

A valid sensor waveform can disappear before it reaches the PCM. Test both ends of the circuit.

Common P0379 Repairs

Possible repairs include:

Reconnecting the Signal “B” sensor
Cleaning corroded terminals
Replacing damaged connector pins
Repairing an open signal wire
Repairing a shorted signal circuit
Restoring sensor power
Repairing the sensor ground
Replacing timing-reference Sensor “B”
Cleaning the optical sensor
Cleaning or replacing the encoder disc
Correcting the sensor air gap
Replacing a damaged reluctor wheel
Repairing the mechanical sensor drive
Repairing injection-pump electronics
Replacing a weak battery
Repairing battery cables or grounds
Updating PCM software
Replacing and programming a confirmed defective PCM
Performing a required timing or crankshaft relearn

If the repair requires a crankshaft-position variation relearn, consult the P0315 crankshaft position variation guide.

Estimated P0379 Repair Costs

Repair	Estimated cost
Diagnostic testing	$100–$300
Connector or wiring repair	$100–$500
Battery or ground repair	$100–$500
Timing-reference sensor replacement	$180–$700
Encoder or reluctor repair	$350–$1,300
Injection-pump electronics repair	$500–$1,800
Injection-pump replacement	$1,000–$4,000+
PCM software update	$100–$300
PCM replacement and programming	$700–$2,000+

Actual costs depend on vehicle design, component accessibility, programming requirements and local labor rates.

Can You Drive With P0379?

Driving with P0379 is generally not recommended.

If the vehicle still runs, the PCM may be relying on another position signal or a backup strategy. The engine can still stall or fail to restart.

Do not drive if the vehicle:

Stalls
Loses power
Runs roughly
Enters limp mode
Becomes difficult to restart
Shows unstable synchronization
Produces abnormal combustion noise

If the engine will not start, avoid repeatedly cranking it until the battery is depleted. Adding a low-voltage code collection does not improve the original diagnosis.

How to Verify the Repair

After completing the repair:

Fully charge the battery.
Clear stored and pending codes.
Monitor cranking voltage.
Observe Signal “B” with an oscilloscope.
Confirm pulse activity at the sensor.
Confirm the same waveform reaches the PCM.
Compare Signals “A” and “B.”
Verify synchronization data.
Allow the engine to reach operating temperature.
Road-test the vehicle when safe.
Rescan for pending codes.
Complete any required relearn procedure.

The repair is confirmed when Signal “B” produces a stable pulse train throughout cranking, warm-up and changing engine speeds.

P0379 Frequently Asked Questions

What does the P0379 code mean?

P0379 means the PCM detected no pulses from timing-reference high-resolution Signal “B.”

What commonly causes P0379?

Common causes include a disconnected or failed sensor, missing power or ground, an open signal circuit, a blocked optical path or stationary encoder hardware.

Can P0379 prevent an engine from starting?

Yes. If Signal “B” is required for synchronization, the PCM may disable or delay fuel injection or ignition.

Can a weak battery cause P0379?

Yes. Low cranking voltage can prevent the sensor or control electronics from operating correctly.

Is Signal “B” the same as Bank 2?

No. Signal “B” identifies a timing-reference circuit and does not universally mean Bank 2.

Is P0379 a crankshaft position sensor code?

Not necessarily. Signal “B” may originate from a crankshaft sensor, optical injection-pump sensor, distributor or another timing-reference component.

What is the difference between P0377 and P0379?

P0377 means some Signal “B” pulses are detected, but too few. P0379 means no pulses are detected.

What is the difference between P0378 and P0379?

P0378 means Signal “B” becomes intermittent or erratic. P0379 means the signal remains completely absent.

Do you need an oscilloscope to diagnose P0379?

An oscilloscope is strongly recommended because it confirms whether the circuit produces actual pulses rather than an unchanging average voltage.

Final Thoughts

P0379 identifies a complete loss of high-resolution timing-reference Signal “B” pulses.

Begin by verifying that the engine and encoder are turning. Then test battery voltage, connector condition, sensor power, ground and signal continuity. Finally, compare the waveform at the sensor and PCM.

A consistently flat waveform narrows the problem considerably. It does not identify the failed part by itself, but it gives the diagnosis somewhere useful to start—which is already better than ordering an injection pump because the internet looked confident.

For more diagnostic coverage, visit the Pro Street OBD-II Doctor and the complete P-series OBD-II code index.