Comprehensive diagnostic guide for P0258 -- Fuel Pump B Control Circuit Low

What This Code Means

• The official Open Source code definition lists P0258 as which translates to the Fuel Pump B Control Circuit Low (i.e., the B circuit signal to the fuel pump is reading as low). This reflects a problem in the control circuit that powers or signals the pump B. Source: GitHub OBD2 code definitions (Portuguese description of P0258)
• General context on OBD-II codes and how powertrain diagnostics work is described in Wikipedia's OBD-II sections: Diagnostic Trouble Codes and Powertrain Codes. These sections describe that modern systems monitor various parameters and generate codes when issues are detected, and that powertrain codes relate to engine/performance and emissions-related controls. Source: Wikipedia - OBD-II: Diagnostic Trouble Codes; OBD-II: Powertrain Codes

Diagnostic Approach

1) Confirm and document the DTC and data

• Use a scan tool to confirm P0258 is current (not historical) and note any freeze-frame data (engine rpm, load, fuel trim, battery voltage, temperature, etc.).
• If available, review live data for the Fuel Pump B Control Circuit voltage when the PCM is attempting to energize the pump (and compare to expected value, typically close to battery voltage or a defined control-signal level depending on design).
• Cross-check for related codes (P0257, P0259, P0260-P0263, etc.) that may indicate a broader fuel system or injector/control issue.

2) Visual inspection and basic electrical checks

• Inspect relevant fuses and relays for the fuel pump B circuit; replace any blown fuse and test circuit continuity after replacement.
• Inspect the fuel pump relay (coil resistance, contacts) and wiring harness for chafing, pin damage, or corrosion at the PCM connector, pump connector, and ground points.
• Check the battery condition and charging system voltage; a weak supply can masquerade as a control-circuit fault.
• Check for damaged, frayed, or exposed wiring in the B circuit run; ensure connectors are clean, dry, and properly seated.

3) Verify power, ground, and control signals at the B circuit

• With the ignition ON, measure the supply voltage at the pump B circuit connector (as close to the pump as practical). Confirm presence of battery voltage on the feed side (or proper signal rail per design) and verify the ground side is solid.
• With the engine cranking, observe the B control signal from the PCM to the pump/relay (or the pump itself, depending on vehicle design). Document whether the signal is within spec (e.g., 12V or a commanded duty/level) or remains abnormally low/zero.
• If the circuit is commanded but the pump does not energize, this points toward a wiring/connector issue, a faulty relay/driver, or a pump itself.

4) Check for good grounds and wiring integrity

• Confirm a solid ground reference for the pump and PCM ground path. A weak or intermittent ground can cause a low signal and pump underperformance.
• Perform continuity checks for the B circuit between the PCM/relay input and the pump connector; look for high resistance, shorts to power, or opens.

5) Pump operation verification and fuel system prerun checks

• If possible, perform a controlled pump test: energize the B circuit via a known-good relay and observe pump operation (listen for pump noise, measure fuel pressure rise with a gauge).
• Connect a fuel pressure gauge to the rail and compare the readings with the OEM spec under engine cranking and running conditions. A successful energizing signal with low fuel pressure indicates pump or injector/filter issues; a low signal with normal pressure still points to an electrical/control issue.
• Consider testing with a scope to view the PWM or duty cycle of the pump drive if the vehicle uses a variable control strategy.

6) Evaluate pump, filter, and mechanical fuel-system health

• If the pump B circuit is energizing correctly and voltage is solid but fuel pressure remains low, suspect a failing pump (wear, reduced flow), a clogged/blocked fuel filter, or a restriction along the feed line.
• If two-pump systems exist or two circuits are present, confirm whether the "B" circuit is the primary or secondary pump path and test accordingly.

7) PCM/software and vehicle-specific considerations

• If electrical tests are clean and the pump responds correctly when directly energized but still reports P0258 in the scan tool, a PCM control fault or software calibration issue could be involved. In such cases, consult the manufacturer service bulletin or perform OEM-grade diagnostics, which may include software reflash or module replacement.

8) Rule-in/rule-out decision-making checklist

• Rule-in: The B circuit shows low signal while commanded or exhibits abnormally low voltage when the pump is supposed to energize; pump does not run or fails to deliver expected pressure; all wiring and fuses test good.
• Rule-out: The B circuit voltage is correct and pump runs when alternative feed path is provided or directly energized; the problem resolves when the PCM is bypassed or a known-good harness is used.

Repair or replacement considerations (based on findings)

• Repair/replace wiring harness or connectors that show damage, corrosion, or high resistance.
• Replace faulty fuel pump B relay or fix PCM-driven driver circuits if diagnosed as the source.
• Replace blown fuses; repair power supply lines to the pump as required.
• Replace a weak or failing fuel pump (or the B circuit's secondary pump, if applicable) and verify system pressure after installation.
• Replace or re-flash the PCM/ECM if a PCM control fault is identified by OEM diagnostics or service bulletins.
• Replace the fuel filter if flow restriction is found to be the root cause of low fuel pressure readings.

Safety Considerations

• The fuel system operates under pressure; depressurize the fuel system before disconnecting any lines and follow shop protocol to avoid fuel spray injuries.

• Work in a well-ventilated area away from open flames, sparks, or heat sources; use proper PPE and containment for fuel spills.

• Relieve system pressure only after disconnecting the battery and following the vehicle manufacturer's procedure.

• Disconnect the battery when performing electrical tests or connector repairs to avoid short circuits or shock.

• The general concept of OBD-II DTCs and powertrain codes (diagnostic approach, monitoring by the PCM, etc.) is described in Wikipedia's OBD-II sections: Diagnostic Trouble Codes and Powertrain Codes. These pages outline that modern systems continuously monitor and generate codes when issues arise, underscoring the diagnostic flow used here.

• The precise meaning for P0258 is provided by the Open Source GitHub definition: (Fuel Pump B Control Circuit Low), which anchors this guide to the exact DTC interpretation for P0258 in practice. Source: GitHub OBD2 code definitions

• This guide follows the general diagnostic framework described in the OBD-II reference sections: diagnosing by verifying electrical circuits (fuses, relays, wiring), measuring fuel pressure, and assessing PCM control signals. While the Wikipedia sources don't give vehicle-specific P0258 details, they provide the necessary context for how DTCs are used and diagnosed in powertrain systems.

Quick-reference troubleshooting summary

• If P0258 is confirmed current with data showing a low B-control signal: focus on the B circuit wiring, relay/driver, and pump condition; verify voltage when energized; check grounds.
• If voltage to the B circuit is correct and pump energizes but pressure remains low: suspect pump wear, fuel-filter restriction, or line obstruction.
• If a fault is found in fuse/relay or wiring corrosion: repair/replace wiring or components; re-test.
• If all electrical tests pass but the DTC persists: consult OEM service information for PCM/software issues or module replacement.

Diagnostic Tests

• Scan tool: verify P0258, review freeze-frame data, capture live data for B-control circuit, pump voltage, and fuel pressure.

• Electrical: inspect fuses/relays, test the B-circuit feed and ground, verify connectors for corrosion/damage.

• Functional: energize pump directly via a known-good relay to confirm pump operation and measure fuel pressure.

• Mechanical: check for fuel-filter clogging or line restrictions.

• PCM/software: check for service bulletins, possible reflash, or module replacement if electrical tests are inconclusive.

• GitHub OBD2 CODE DEFINITIONS : P0258 description - primary code meaning in practice

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

---