OBD-II Diagnostic Trouble Code
P0629 — Fuel Pump A Control Circuit High
P0629 means the ECM saw the fuel pump A control circuit voltage higher than expected. That indicates a short to battery voltage or an open in a circuit the ECM pulls low. The ECM may not be able to turn the pump on or off.
Severity 4/5
Truck-relevant
Stop within 50 mi
Severity
Stop within 50 mi
Fuel Pump A Control Circuit High
Code
P0629
Category
Powertrain
Subsystem
Fuel
Truck-relevant
Yes
What this means for truckers
P0629 (Fuel Pump A Control Circuit High) — fuel pump A circuit open.
Before you replace parts — try this first
P0629 on the Fuel system is most often relay contacts stuck closed or a failed driver module — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
- Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
- Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
- Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
The fuel or lift pump may run all the time even with the key off and drain the batteries, or may not run at all, causing hard start or no-start. The check-engine light comes on.
Common causes
- CommonRelay contacts stuck closed or a failed driver module ($60–$500)
- CommonOpen circuit in the relay coil or control wire ($60–$350)
- OccasionalControl wire shorted to a battery or ignition feed ($120–$450)
- RareFailed ECM pump driver ($1200–$3500)
How to fix
Check whether the pump runs with the key off and replace a stuck relay
When to use: First step
With the relay or driver removed, check the control circuit for voltage with the key off
When to use: To find a short to power
Check the circuit for opens and check the relay coil resistance
When to use: When the pump doesn't run
Repair the wiring or replace the driver module
When to use: Based on test results
Affects most
No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.
Related
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P0670 — Glow Plug Control Module Circuit
- → P0671 — Cylinder 1 Glow Plug Circuit
- → P0672 — Cylinder 2 Glow Plug Circuit
- → P0673 — Cylinder 3 Glow Plug Circuit
- → P0674 — Cylinder 4 Glow Plug Circuit
Frequently Asked Questions
Can I drive with code P0629?
Drive with caution. P0629 is severity 4 — stop within ~50 miles. Most commercial diesel codes at this severity (turbo, fuel rail, transmission solenoid) can damage adjacent expensive components if left unrepaired. Get to a service location before highway-speed loads.
How much does it cost to fix P0629 on a commercial truck?
Total repair cost ranges $60–$3500 depending on the root cause. The cheapest fix is usually the first one listed below — Relay contacts stuck closed or a failed driver module — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will P0629 fail a DOT inspection?
P0629 itself doesn't automatically fail an inspection, but if the truck is in reduced-power mode (limp mode) at the time of inspection, it WILL be flagged. Most inspectors will note the check engine light but pass the truck if it's running and performance is normal. Fix it before a Level 1 inspection regardless.
What's the difference between P0629 and related codes?
P0629 is closely related to: P0670 (Glow Plug Control Module Circuit), P0671 (Cylinder 1 Glow Plug Circuit), P0672 (Cylinder 2 Glow Plug Circuit), P0673 (Cylinder 3 Glow Plug Circuit). These codes share the same subsystem (fuel) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only P0629 is set without the companions, the issue is more isolated.
How do I prevent P0629 from coming back after a repair?
For P0629 (Fuel Pump A Control Circuit High), prevention centers on the fuel system. Replace fuel filters at the OEM interval — don't stretch them. Use only diesel from high-volume fuel stops (avoid bottom-of-tank fuel at low-traffic stations). Add a water separator if you don't have one. After repair, prime the fuel system per the engine manual.