OBD-II trouble code
P0378: Timing Reference High Resolution Signal 'B' Intermittent/Erratic Pulses
An unstable second timing channel. Because 'B' is usually generated by electronics rather than by a magnet, the first thing to test is not the signal — it is the power feeding whatever makes it.
Quick facts
- System
- Powertrain
- Category
- Sensors / Timing
- Severity
- High severity
- Drivable
- No — stop driving until repaired
- Repair cost range
- $120 – $1,600
- DIY difficulty
- Advanced DIY
What does P0378 mean?
P0378 describes a second high-resolution timing channel that behaves for a while and then does not — pulses jittering, edges appearing and vanishing, a train that momentarily stops making sense before recovering. It shares that unpredictability with the equivalent fault on channel 'A', but it has one cause class that channel 'A' does not have at all, and that class is where the money usually is.
On a system where 'B' is a relayed copy, the signal is not produced by a magnet passing a toothed wheel. It is produced by electronics — a driver stage inside a module that reads the incoming reference and retransmits a conditioned version of it. Electronics need a stable supply and a solid ground to do that reliably, and the module doing it is very often bolted somewhere unhelpfully hot, on the engine or against the bulkhead. So a 'B' channel that goes erratic is disproportionately a power quality problem rather than a signal problem, and the correct first measurement is the sending module's supply voltage rather than the waveform it is producing.
The test that follows is specific to this code and is worth setting up properly. Watch the reference signal on one scope channel and the sending module's supply voltage on a second, then drive or load the vehicle until the fault appears. If the signal goes erratic at the exact moment the supply dips, the problem is upstream of the module entirely and no amount of work on the reference wiring will touch it. If the supply holds steady while the signal falls apart, the module's output stage or the run downstream of it is genuinely at fault.
Cranking voltage deserves its own mention, because this is the member of the family most likely to be a charging or battery fault wearing a sensor fault's clothes. System voltage sags hardest during cranking, and a module running on nine volts can emit a marginal, ragged pulse train that it would never produce at fourteen. An owner who reports an intermittent no-start that fixes itself on the second attempt, on a vehicle with a battery of unknown age, has handed you the most likely answer already. Load-testing the battery and measuring cranking voltage costs nothing and is more likely to end this job than any wiring work.
The trap to name is the expensive one. This code is intermittent, and on many vehicles it originates in a module — a combination that leads to modules being replaced on suspicion. Establish the supply and the ground before anyone orders one, because a new module on the same bad feed produces the same code.
Common causes
- Voltage drop or intermittent supply to the module that generates or relays the reference signal
- High-resistance or corroded ground at the sending or receiving module
- Weak or failing battery producing marginal system voltage during cranking
- Failing alternator with poor regulation or a shorted diode disturbing system voltage
- Heat-related failure inside a module mounted in a hot engine bay location
- Corroded or relaxed terminal at an inter-module connector losing contact under vibration
- Chafed reference wire making intermittent contact as the harness moves with engine torque
- Intermittent output from a genuine second pickup where one is fitted, from a cracked body or varying air gap
Symptoms
- Check engine light that appears and later clears itself
- Intermittent no-start that succeeds on the second or third attempt
- Momentary cut, stumble or lurch while driving that clears immediately
- Rough running that comes and goes on one cylinder bank on split-bank injection systems
- Correlation or plausibility codes appearing alongside this one
- Communication or module fault codes stored at the same timestamp
- Fault appearing more readily on cold mornings or after long periods parked
Diagnostic steps
- 1.Read the freeze frame before clearing anything, and note system voltage as well as coolant temperature and engine speed. On this code the voltage field is often the answer.
- 2.Load-test the battery and measure cranking voltage before doing anything else. A module running on marginal voltage produces a marginal signal, and this is the cheapest test available.
- 3.Test the charging system for regulation and AC ripple. A poorly regulated alternator disturbs every module on the vehicle, not just this one.
- 4.Identify from the wiring diagram which module generates or relays the reference. That module's power and ground are the primary suspects, not the reference wire.
- 5.Measure voltage drop on the sending module's supply and ground under load. A connection that reads fine at rest can collapse when the module is drawing current.
- 6.Scope the reference signal on one channel and the sending module's supply on a second, then reproduce the fault. Signal degrading exactly as supply dips means the fault is upstream of the module.
- 7.Check whether module communication or network codes share a timestamp with this one. A module losing its feed usually complains in more than one way at once.
- 8.Wiggle-test the inter-module connectors and the harness with the scope armed, checking terminal tension with a matching pin rather than by eye.
- 9.Only after supply, ground and wiring are proven should the module itself be considered. A replacement fitted to an unhealthy feed will reproduce the same code.
Repair cost
$120 – $1,600
A surprising share of these end at the battery or the charging system. A battery is $150 to $350 fitted and an alternator $350 to $800, and either can be the whole repair. Cleaning and repairing a corroded module ground or supply connection is $150 to $450. Connector and terminal work on the inter-module run is $150 to $500, with most of the cost being the time spent reproducing the fault. Diagnosis is heavier than average because the fault has to be caught in the act, commonly $150 to $350. A module replaced for a heat-related internal failure is $600 to $1,600 with programming — a real outcome, but the last one to reach for, not the first.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with control module replacement and programming preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an advanced DIY job. It typically requires specialty tools, scan-tool access, lifting equipment, or careful sequencing to avoid causing new failures. Plan for extended downtime and have a backup vehicle. Most owners are better served by a shop that has done this repair before.