P0370 — Timing Reference High Resolution Signal A — 5.9 Cummins 24-Valve (VP44)

Diagnostic guide for P0370 on 5.9 Cummins 24-Valve VP44 (1998.5–2002).

Updated October 15, 2026

What it means

The ECM has detected a fault in the high-resolution timing reference signal. On the 5.9L 24-valve this is the VP44 pump’s speed/position (timing reference) signal.¹

Most likely causes

1. VP44 pump connector or wiring fault causing loss/interruption of the pump speed/position signal — Inspect the VP44 10-pin connector for corrosion, loose terminals, oil intrusion, and damaged wiring in the engine harness near the pump and firewall. Check continuity and isolation of the timing reference signal wires between the ECM and pump.¹
2. VP44 power/ground supply issue affecting the pump-mounted controller — Verify battery voltage and clean grounds at the VP44 power feed, ECM grounds, and chassis grounds. Poor supply/ground can corrupt the high-resolution timing reference signal.¹
3. Internal VP44 controller/encoder degradation — The pump-mounted controller provides the timing reference; heat and low lift-pump supply pressure are known stressors. If power/grounds and wiring test good, internal pump controller/encoder failure is likely.²

How to diagnose

1. Verify freeze-frame conditions and confirm P0370 is present with engine running. Check for accompanying VP44 codes such as P1688 or P0216.
2. Inspect VP44 connector and harness for corrosion, oil, bent pins, and chafing; repair as needed.
3. Check VP44 power and ground circuits for proper voltage and low resistance to ground; clean and tighten connections.
4. Monitor lift-pump supply pressure at idle and under load; low supply pressure stresses the VP44 and can contribute to controller faults.
5. If wiring and power are good and code persists, evaluate the VP44 pump-mounted controller/encoder as the source of the high-resolution timing reference fault.

Specs and test values

Common mistake to avoid

Replacing the VP44 without testing the lift pump first. A weak lift pump starves the VP44 of the fuel it uses for cooling, so a new pump installed on a bad supply side can fail again quickly. Put a gauge on the feed line and check pressure at key-on and under load before buying the pump. Source: Diesel Power Products — checking lift pump pressure on a 24-valve Cummins.

Parts and tools you may need

• Digital multimeter
• Wiring diagram for 1998.5–2002 Ram 2500/3500 5.9L 24-valve
• Terminal release tools
• Dielectric grease
• Fuel pressure test gauge (lift pump)

Tools you’ll need

• digital multimeter — Checks VP44 power, ground, and signal circuit continuity/voltage to isolate wiring faults causing loss of timing reference.
• OBD-II scan tool — Confirms P0370 presence with freeze-frame and reveals accompanying VP44 codes like P1688/P0216 for context.
• fuel pressure test gauge — Verifies lift-pump supply pressure to the VP44, as low pressure stresses the pump controller and can contribute to timing reference faults.

Rough cost and difficulty

Typical US independent-shop figures; labor runs about $120–$180 per hour (dealers charge more). Confirm the diagnosis before buying parts.

When to see a pro

If the pump-mounted controller is suspected, have a qualified diesel technician perform signal tests and confirm pump health before replacement. Do not attempt high-pressure fuel system service without proper training and safety equipment.

Check for recalls

Recalls are tied to your truck’s VIN, so check yours directly: enter the VIN at NHTSA’s recall lookup. Open safety recalls are repaired free at a dealer.

Sources