When the engine computer counts more timing pulses than it should see between two reference marks, it knows the signal is wrong. A P0371 code means timing reference high resolution signal A showed too many pulses.
This code is uncommon and appears mostly on certain GM engines. This guide explains what the signal is, which engines use it and how to track down the fault.
P0371 Code Breakdown
Every OBD-II code follows the same five-character pattern set by the SAE J2012 standard. Each character narrows down which system has the fault. This table shows what each part of the code tells you.
| Character | Meaning | What It Tells You |
|---|---|---|
| P | Powertrain | The fault is in the engine or transmission systems. |
| 0 | Generic (SAE) code | The definition is the same on every OBD-II vehicle. |
| 3 | Ignition system or misfire | The fault involves the ignition system, misfire detection or engine position sensors. |
| 71 | Specific fault | Timing Reference High Resolution Signal A Too Many Pulses |
What the P0371 Code Means
The SAE J2012 standard defines the P0371 code as Timing Reference High Resolution Signal A Too Many Pulses. A high resolution timing signal gives the powertrain control module (PCM) many pulses per revolution, compared with a low resolution reference that gives only a few. Well-known examples are the Optispark distributor on GM’s LT1 and LT4 V8 engines from the 1990s, which uses an optical sensor and a slotted disc, and the optical sensor inside the injection pump on GM’s 6.5L diesel.
The PCM compares the high resolution pulses with a low resolution reference signal. It knows exactly how many fine pulses should arrive between two reference pulses. If the count is off, or the pulses stop, it sets a timing reference code.
Optical sensors are sensitive to dirt and moisture. On LT1 Optispark units, coolant or water leaking from the water pump above them is a common cause of failure. On the 6.5L diesel, the optical sensor sits inside the pump and can be affected by fuel contamination and wear.
Too many pulses means the PCM counted more high resolution pulses between reference marks than the design allows. That usually points to electrical noise on the signal wire, a damaged or dirty optical disc creating false edges, or a failing sensor.
Symptoms of a Timing Reference Signal Fault
The engine may switch to a backup mode when this signal fails. Common signs of a P0371 code include:
- Check engine light on
- Rough idle or stumbling
- Hard starting or no start
- Stalling, especially in wet weather
- Loss of power
- Spark knock or poor fuel economy
On LT1 engines, problems that start after a rainstorm, a car wash or a coolant leak often point to moisture inside the Optispark. On the 6.5L diesel, these codes often show up with hard starting and stalling.
Common Causes of the P0371 Code
These are the usual causes:
- Electrical noise. Spark plug wires or a bad ground add false pulses.
- Moisture in the sensor. Water creates false readings on optical sensors.
- Damaged shielding. The signal wire picks up interference.
- Failing optical sensor. Its output becomes unstable.
- Wrong replacement part. A sensor with a different pulse count.
Tools You Will Need
You can do most of this testing at home with basic tools:
- OBD-II scan tool with live data
- Digital multimeter
- Lab scope or graphing scan tool
- Wiring diagram for your engine
- Flashlight to check for leaks above the sensor
Step-By-Step Diagnosis of the P0371 Code
Check where the signal comes from on your engine first. On LT1 engines, it is the Optispark behind the water pump. On the 6.5L diesel, it is inside the injection pump.
Check Live Data
Watch engine speed and any timing reference or sync status values in live data while cranking and idling. A stumble or rpm dropout at the same time the code sets points to a signal problem.
Inspect the Connector and Wiring
Find the timing reference sensor and check that the connector is fully seated and locked. Look for corrosion, bent pins and wiring that rubs on brackets or hot parts.
Test Reference, Ground and Signal
Back-probe the connector with the key on. The reference should read close to 5 volts, the ground close to 0 volts, and the signal should match the value your service manual lists. Any wire that fails tells you where to look next.
Check for Leaks and Moisture
On engines with a front-mounted optical distributor, look for coolant or water leaks above it, such as a weeping water pump. Moisture inside the cap or sensor housing is a common cause. Check that any vent hoses on the unit are connected and clear.
Repairs That Clear the P0371 Code
Match the repair to what your tests found. Then clear the code and drive under the same conditions that set it.
- Repair the connector or wiring, and route it away from plug wires and coils.
- Replace the optical distributor or sensor if it fails tests or shows moisture damage.
- Repair coolant or water leaks above the sensor before installing a new one.
- Clean and tighten engine and PCM grounds.
- Replace the injection pump optical sensor on diesels, often as part of a pump repair.
On LT1 and LT4 engines, many owners replace the water pump and the Optispark at the same time, since a leaking pump is a common reason the Optispark fails. Use a quality replacement unit with proper venting.
Common Mistakes to Avoid
These mistakes waste money or leave the real fault in place:
- Ignoring plug wire routing near the sensor wiring
- Replacing the PCM before checking for noise
- Installing a new Optispark without fixing a leaking water pump
- Buying a low-quality replacement optical unit
Typical Repair Costs
The cost to fix a P0371 code depends on the cause. This table shows typical ranges for parts and labor.
| Repair | DIY Cost | Typical Shop Cost |
|---|---|---|
| Wiring or connector repair | $10 to $40 | $100 to $300 |
| Optispark distributor | $200 to $500 | $500 to $1,000 |
| Water pump | $50 to $200 | $300 to $700 |
| Diesel pump optical sensor | $100 to $300 | $400 to $1,500 |
On LT1 engines, labor is a big part of the cost because the Optispark sits behind the water pump and harmonic balancer. On the 6.5L diesel, the optical sensor is inside the pump, so the pump often has to come off or be rebuilt.
Is It Safe to Drive with a P0371 Code?
It depends on how the engine runs. Many engines switch to a backup mode using the low resolution signal, so they run but with less power and rougher operation.
If the engine stalls, misfires heavily or will not restart, avoid driving it. A stall in traffic can be dangerous.
Related Codes
These codes often show up with this one or point to the same system:
| Code | Meaning |
|---|---|
| P0370 | Timing Reference High Resolution Signal A Malfunction |
| 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 Pulses |
| P0335 | Crankshaft Position Sensor A Circuit Malfunction |
| P0216 | Injection Timing Control Circuit Malfunction |
Restoring Timing Reference Signal A
A P0371 code means the PCM found a problem with timing reference high resolution signal A. Moisture-damaged optical sensors, wiring faults and electrical noise cause most cases.
- Find out where the signal comes from on your engine.
- Check for coolant or water leaks above the sensor.
- Inspect the connector and wiring.
- Check power, ground and signal.
- Replace the sensor if it fails tests.
Frequently Asked Questions
Can Spark Plug Wires Cause the P0371 Code?
Yes. Worn or poorly routed plug wires can leak electrical noise into the signal wiring, which adds false pulses. On Optispark engines, use quality wires and keep them routed as the factory intended.
What Is an Optispark?
Optispark is GM’s optical distributor used on LT1 and LT4 V8 engines in the 1990s. It sits behind the water pump at the front of the engine and uses a slotted disc and light sensors to produce both a high and a low resolution timing signal.
Is This Code Common?
Not on most modern engines. It is mostly seen on 1990s GM LT1 and LT4 V8s and on GM’s 6.5L diesel. On most newer vehicles, crank and cam sensor codes cover similar faults.
Can I Clean an Optical Sensor?
Sometimes, if the problem is light dust or moisture. Dry it carefully and use only cleaners safe for electronics and plastic. If the disc is scratched or the sensor is damaged, cleaning will not help and the unit needs replacement.