What P0378 means
P means powertrain (engine and transmission). The 0 marks a generic code defined by SAE, so the same number means the same thing across makes. The third character, 3, places it in the ignition system or misfire subsystem; the last two digits identify the specific fault.
Some engines use a high-resolution timing signal (many pulses per revolution) alongside a low-resolution reference for precise ignition and injection timing. The engine computer counts pulses between reference events; too many, too few or erratic pulses set these codes.
The letter "B" distinguishes between two or more of the same component on one vehicle. Which physical part is "B" is set by the manufacturer, so confirm it in the service information before replacing anything.
What “Intermittent” means here
"Intermittent" (or "Erratic") means the high-resolution timing reference signal signal dropped out or jumped in a way a working sensor could not, but not continuously. Intermittent faults are almost always connections: a loose or corroded terminal, a chafed harness, or a wire that breaks contact with vibration or heat. Watching live data while gently flexing the harness is the classic way to catch one.
What you may notice
Symptoms depend on the vehicle and on how the module responds to the fault. Any of these can accompany P0378:
- Hard starting
- Misfire or rough running
- Stalling
Where to start diagnosing P0378
- Inspect the sensor connector and harness, particularly for interference from nearby ignition wiring.
- Check the sensor and its target wheel for damage.
- Confirm the signal with a scope where possible.
Clear the code only after recording freeze frame data: it shows the engine speed, load and temperature at the moment the fault set, which is often the best clue to reproducing it.
Live data to watch
These standard OBD2 values relate to P0378. Support varies by vehicle; a value your car does not report will simply be missing.
| Value | What to look for |
|---|---|
| Engine RPM (PID 0x0C) | Look for dropouts. |
Related codes
Codes from the same high-resolution timing reference signal family. If several are stored together, the shared part — wiring, a common reference voltage, or the component itself — is the place to start.
| Code | Definition |
|---|---|
| P0371 | Timing Reference High Resolution Signal "A" Too Many Pulses |
| P0372 | Timing Reference High Resolution Signal "A" Too Few Pulses |
| P0373 | Timing Reference High Resolution Signal "A" Intermittent/Erratic Pulses |
| P0374 | Timing Reference High Resolution Signal "A" No Pulse |
| P0375 | Timing Reference High Resolution Signal "B" |
| P0376 | Timing Reference High Resolution Signal "B" Too Many Pulses |
| P0377 | Timing Reference High Resolution Signal "B" Too Few Pulses |
| P0379 | Timing Reference High Resolution Signal "B" No Pulse |
Further reading
Frequently Asked Questions
What does P0378 mean?
P0378 is the generic SAE J2012 code for "Timing Reference High Resolution Signal "B" Intermittent/Erratic Pulses". In plain terms, it means the engine computer has detected a "intermittent" fault on the high-resolution timing reference signal.
Can I keep driving with P0378?
Can cause stalling. Have it diagnosed before relying on the vehicle.
What should I check first for P0378?
Inspect the sensor connector and harness, particularly for interference from nearby ignition wiring.
Definition source: SAE J2012 / ISO 15031-6 generic DTC definitions. This is the generic meaning that applies across makes; a manufacturer can define a code more narrowly or add its own sub-codes, so confirm against the service information for your vehicle before replacing parts. Written by the OBDAssistant engineering team — see our editorial policy.