P0628 - Fuel Pump Relay Control Circuit Low Voltage

Introduction:

The diagnostic trouble code (DTC) P0628 indicates a problem within the fuel pump relay control circuit, specifically a low voltage condition. This circuit is crucial for powering the fuel pump, which supplies the engine with the necessary fuel for combustion. A malfunction in this circuit can lead to various drivability issues, ranging from a no-start condition to poor engine performance.

Table: P0628 - Fuel Pump Relay Control Circuit Low Voltage

Category	Description	Possible Causes
Definition	Fuel Pump Relay Control Circuit Low Voltage	The Powertrain Control Module (PCM) has detected a voltage lower than expected in the fuel pump relay control circuit. This means the PCM isn't seeing the voltage it expects when it tries to activate the fuel pump relay.
Symptoms	No start condition Extended cranking Engine stalling Poor engine performance (hesitation, lack of power) Fuel pump not running Check engine light illuminated	The severity of the symptoms can vary. A completely non-functional fuel pump will result in a no-start. Intermittent low voltage can cause stalling or performance issues.
Possible Causes	Faulty fuel pump relay Short to ground in the fuel pump relay control circuit Open circuit in the fuel pump relay control circuit (wiring, connectors) Corroded or damaged connectors Faulty Powertrain Control Module (PCM) Low battery voltage * Faulty fuel pump wiring harness	PCM failure is rare, but it's important to rule out other possibilities first. A short to ground is a common culprit. Rodent damage to wiring is also a frequent cause.
Diagnostic Steps	1. Visual inspection of wiring and connectors. 2. Check fuel pump relay functionality (swap with a known good relay). 3. Measure voltage at the fuel pump relay control circuit. 4. Check for shorts to ground in the control circuit. 5. Check for open circuits in the control circuit. 6. Test fuel pump relay control circuit resistance. 7. Examine fuel pump wiring harness for damage. 8. Check battery voltage. 9. PCM testing (as a last resort).	Use a multimeter to perform voltage and resistance checks. Consult the vehicle's wiring diagram for specific pin locations and expected values.
Fuel Pump Relay Location	Varies depending on vehicle make and model. Typically found in the under-hood fuse box or under the dashboard. Consult the vehicle's repair manual for the exact location.	Understanding the location is critical for performing tests and replacing the relay.
Related Codes	P0627 (Fuel Pump Relay Control Circuit High Voltage), P0230 (Fuel Pump Primary Circuit Malfunction), P0231 (Fuel Pump Secondary Circuit Low), P0629 (Fuel Pump Relay Control Circuit High), P062A (Fuel Pump "A" Control Circuit Range/Performance)	These codes often indicate related fuel pump or fuel system issues. Troubleshooting should consider the context of any other codes present.
Tools Needed	Multimeter, wiring diagrams, test light, fuel pressure gauge (optional), relay tester (optional), OBD-II scanner	A multimeter is essential for electrical testing. Wiring diagrams are crucial for tracing circuits.
Severity	High. A malfunctioning fuel pump relay can lead to a complete engine shutdown and potential safety hazards.	Immediate attention is recommended to prevent potential safety risks and further damage.
Common Causes by Vehicle Type	Older vehicles: Corroded connectors and wiring due to age. Vehicles in harsh environments: Damage from salt, water, or extreme temperatures. * Vehicles with aftermarket accessories: Improperly installed accessories can interfere with the fuel pump circuit.	Consider the vehicle's age and operating environment when diagnosing the problem.
Wiring Diagram Importance	Essential for identifying the correct wires, connectors, and components within the fuel pump relay control circuit. Provides valuable information for troubleshooting and testing.	Without a wiring diagram, accurate diagnosis and repair are significantly more difficult.
PCM Involvement	The PCM controls the fuel pump relay by providing a ground signal to the relay coil. A faulty PCM can incorrectly signal the relay, causing the P0628 code.	PCM failure is rare, but it should be considered after ruling out other possibilities.
Fuel Pump Operation	The fuel pump is responsible for delivering fuel from the fuel tank to the engine. A faulty fuel pump relay prevents the fuel pump from operating correctly.	Understanding fuel pump operation is essential for diagnosing fuel system problems.
Relay Testing Techniques	Visual inspection: Check for physical damage or corrosion. Continuity test: Verify continuity between the relay terminals. Voltage test: Verify voltage at the relay terminals. Relay swapping: Swap with a known good relay to see if the problem is resolved.	Relay testing is a critical step in diagnosing the P0628 code.
Connector Inspection	Check for corrosion, loose connections, and damaged wiring at the fuel pump relay connector and other related connectors.	Clean and repair any damaged connectors to ensure proper electrical contact.
Ground Circuit Testing	Verify that the ground circuit for the fuel pump relay is properly grounded. A poor ground connection can cause low voltage issues.	Clean and tighten any ground connections to ensure a good ground path.
Voltage Drop Testing	Perform voltage drop testing on the fuel pump relay control circuit to identify areas of excessive resistance.	Voltage drop testing can help pinpoint the location of wiring or connector problems.
Fuel Pump Relay Types	Typically a standard automotive relay with four or five terminals. Some vehicles may use a solid-state relay.	Understanding the type of relay used in the vehicle is important for proper testing and replacement.
Impact of Low Battery Voltage	Low battery voltage can cause the PCM to misinterpret the voltage in the fuel pump relay control circuit, triggering the P0628 code.	Ensure that the battery is properly charged and in good condition before troubleshooting the fuel pump relay circuit.

Detailed Explanations:

Definition: The P0628 code signifies that the vehicle's Powertrain Control Module (PCM) has detected a voltage level in the fuel pump relay control circuit that is lower than the expected threshold. The PCM monitors this circuit to ensure that the fuel pump relay is being properly activated and deactivated.

Symptoms: The most noticeable symptom is a no-start condition, as the fuel pump won't receive power to deliver fuel to the engine. Other symptoms include extended cranking, where the engine struggles to start, engine stalling, particularly under load, and poor engine performance characterized by hesitation or a lack of power. The check engine light will also be illuminated.

Possible Causes: A faulty fuel pump relay itself is a common culprit. A short to ground in the control circuit can draw excessive current, leading to low voltage. An open circuit, such as a broken wire or corroded connector, can interrupt the flow of voltage to the relay. A faulty PCM, although less common, can also cause the issue. Low battery voltage can also be a contributing factor. Finally, a faulty fuel pump wiring harness may contain damaged wires.

Diagnostic Steps: Begin with a visual inspection of all relevant wiring and connectors for damage or corrosion. Check the fuel pump relay by swapping it with a known good relay or using a relay tester. Measure the voltage at the fuel pump relay control circuit using a multimeter. Check for shorts to ground by measuring the resistance between the circuit and ground. Check for open circuits by performing continuity tests on the wiring. Examine the fuel pump wiring harness for any signs of damage. Finally, check battery voltage to ensure it is within the proper range. PCM testing should be performed last, only after all other possibilities have been ruled out.

Fuel Pump Relay Location: The location of the fuel pump relay varies considerably depending on the vehicle's make and model. It is often found in the under-hood fuse box or under the dashboard. Refer to the vehicle's repair manual or a reliable online resource for the specific location.

Related Codes: Several other codes can be related to fuel pump or fuel system issues. These include P0627 (Fuel Pump Relay Control Circuit High Voltage), P0230 (Fuel Pump Primary Circuit Malfunction), P0231 (Fuel Pump Secondary Circuit Low), P0629 (Fuel Pump Relay Control Circuit High), and P062A (Fuel Pump "A" Control Circuit Range/Performance). Consider these codes during diagnosis.

Tools Needed: A multimeter is essential for measuring voltage, resistance, and continuity. Wiring diagrams are crucial for identifying the correct wires and components. A test light can be used to quickly check for voltage. A fuel pressure gauge (optional) can help verify fuel pressure. A relay tester (optional) can simplify relay testing. An OBD-II scanner is needed to read and clear diagnostic trouble codes.

Severity: The severity of the P0628 code is considered high because a malfunctioning fuel pump can lead to a complete engine shutdown, potentially creating a safety hazard. Immediate attention is recommended.

Common Causes by Vehicle Type: Older vehicles are more prone to corroded connectors and wiring due to age and environmental exposure. Vehicles in harsh environments may experience damage from salt, water, or extreme temperatures. Vehicles with aftermarket accessories may have improperly installed accessories that interfere with the fuel pump circuit.

Wiring Diagram Importance: A wiring diagram is essential for accurately diagnosing the P0628 code. It provides valuable information about the circuit's layout, including the location of wires, connectors, and components.

PCM Involvement: The PCM plays a crucial role in controlling the fuel pump relay by providing a ground signal to the relay coil. A faulty PCM can incorrectly signal the relay, leading to the P0628 code.

Fuel Pump Operation: The fuel pump's primary function is to deliver fuel from the fuel tank to the engine. A properly functioning fuel pump relay is essential for ensuring the fuel pump operates correctly.

Relay Testing Techniques: Several techniques can be used to test the fuel pump relay. These include a visual inspection for physical damage, a continuity test to verify continuity between the terminals, a voltage test to verify voltage at the terminals, and relay swapping with a known good relay.

Connector Inspection: Thoroughly inspect the fuel pump relay connector and other related connectors for corrosion, loose connections, and damaged wiring. Clean and repair any damaged connectors to ensure proper electrical contact.

Ground Circuit Testing: Verify that the ground circuit for the fuel pump relay is properly grounded. A poor ground connection can cause low voltage issues.

Voltage Drop Testing: Perform voltage drop testing on the fuel pump relay control circuit to identify areas of excessive resistance. This can help pinpoint the location of wiring or connector problems.

Fuel Pump Relay Types: Fuel pump relays are typically standard automotive relays with four or five terminals. Some vehicles may use a solid-state relay.

Impact of Low Battery Voltage: Low battery voltage can cause the PCM to misinterpret the voltage in the fuel pump relay control circuit, triggering the P0628 code.

Frequently Asked Questions:

What does the P0628 code mean? The P0628 code indicates a low voltage condition in the fuel pump relay control circuit, meaning the PCM isn't seeing the voltage it expects when activating the fuel pump relay.
What are the symptoms of a P0628 code? Symptoms include a no-start condition, extended cranking, engine stalling, and poor engine performance, all stemming from the fuel pump not receiving adequate power.
Can a bad fuel pump cause a P0628 code? While a bad fuel pump itself doesn't directly cause the P0628 code, the increased load it places on the relay circuit can exacerbate an existing low voltage issue.
Is it safe to drive with a P0628 code? It is generally not safe to drive with a P0628 code, as it can lead to engine stalling and a complete loss of power, which can be dangerous.
How do I fix a P0628 code? Fixing a P0628 code involves diagnosing the cause of the low voltage (relay, wiring, PCM) and repairing or replacing the faulty component. Start with the simplest fixes (relay replacement, wiring inspection) before moving on to more complex issues.

Conclusion:

The P0628 code signals a critical issue within the fuel pump relay control circuit, potentially leading to significant drivability problems. Thorough diagnosis, starting with simple checks and progressing to more complex testing, is essential to identify and rectify the root cause. Addressing this issue promptly is crucial to ensure safe and reliable vehicle operation.