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DTC codes / P26xx · Computer and output circuits / P2633

P2633Fuel Pump 'B' Control Circuit Low

P2633 indicates that the powertrain control module (PCM) has detected an abnormally low voltage on the control circuit for fuel pump 'B', which is often a secondary or transfer pump in a dual-pump system. The PCM commands the pump on by supplying a ground or voltage, and when the feedback on that circuit stays below the expected threshold, it sets this code and may disable the pump to protect the circuit.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaComputer and output circuits

Symptoms you may notice

  • Illumination of the check engine light, often with no other noticeable symptoms
  • Engine cranks but fails to start or starts only after long cranking
  • Engine stalls shortly after starting or during deceleration
  • Reduced engine power or a limp-home mode under load
  • Rough running or misfire under heavy acceleration due to fuel starvation
  • Audible absence of the fuel pump prime when the key is turned to the on position

Common causes, most likely first

  1. Open or high-resistance wire in the fuel pump 'B' control circuit, often due to corrosion or a broken conductor
  2. Failed fuel pump 'B' relay or a faulty relay socket connection
  3. Defective fuel pump 'B' itself, such as a shorted or open internal winding
  4. Poor ground connection at the pump or chassis ground point
  5. Damaged or corroded connector pins at the pump or PCM
  6. Faulty powertrain control module (PCM) driver circuit, though this is less common

How to diagnose and fix it

  1. Verify the code with a scan tool and check for any related fuel pump codes before clearing
  2. Inspect the fuel pump 'B' fuse and relay, swapping the relay with a known-good identical one to test
  3. Perform a voltage drop test on the control circuit from the PCM to the pump to locate excessive resistance
  4. Check the ground circuit for the pump, cleaning and tightening any suspect connections
  5. Disconnect the pump connector and measure resistance across the pump terminals to rule out an open or shorted pump
  6. If the circuit and relay test good, command the pump on with a scan tool and verify power and ground at the pump connector
  7. Replace the fuel pump 'B' if it fails the resistance or command test, and clear the code to confirm the repair

What the low circuit code hides

  • A low control circuit code often results from a relay that clicks but has burned contacts, so always load-test the relay rather than just listening for it.
  • On many vehicles, fuel pump 'B' is a transfer pump that only runs under certain conditions, so the code may set without any driveability symptom until the tank is low.
  • Corrosion inside a connector can create enough resistance to set a low circuit code while still allowing the pump to run intermittently, so wiggle-test the harness while monitoring live data.
  • A weak battery or charging system can cause the PCM to misread circuit voltage, so verify system voltage is stable before condemning the pump.
  • The PCM driver for fuel pump 'B' can be damaged by a shorted pump, so always check pump resistance before installing a replacement module.

Can I keep driving with P2633?

It is not safe to continue driving with this code because the fuel pump may stop without warning, causing a stall that could lead to a crash. Have the vehicle towed or drive it only to the nearest repair shop if it runs, and avoid high-speed or heavy-load driving.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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