P2636: Fuel Pump B Low Flow/Performance

P2636 is the flow check for fuel pump B, in practice mostly the transfer pump of trucks with two tanks. GM's 2014 table for the 6.6 L LML compares how fast the levels in both tanks fall while the pump runs; if they do not move as expected for 140 s, a Type B fault is stored. The 2020 diesel table weighs a commanded transfer of 5 litres against the level change in the main tank (limit 2.5 litres, 1,000 s), or flags a rear tank still holding over 141 litres after 1,150 s of pumping; both give Type C. Emission recall 17067 for 2015-2016 Silverado/Sierra HD LML with tanks N2N notes that the MIL comes on, the fuel gauge drops to empty and transfer is blocked; dealers re-plumb the lines and reflash the ECM. PIT5208A attributes a false P2636 to refuelling with the engine or ignition on.

Symptoms

• Fuel gauge shows empty although the second tank holds fuel
• Engine may stall when the main tank runs dry
• Malfunction indicator lamp on

Most likely causes

• Weak transfer pump not moving enough fuel between tanks
• Restricted transfer line or design-related flow limit (GM recall 17067)
• Refuelling with engine or ignition on confuses the level-based check
• Faulty fuel level sender in one of the tanks

What to check, in order

Start with the simplest. You can tick off each step.

1. Check the pump's fuse, relay, and connector
2. With the connector unplugged, check whether the signal wire is making contact with ground.
3. Replace the filter if it is due for maintenance
4. Compare commanded and actual pressure in live data
5. Measure the pressure with a suitable gauge
6. Check the sensor and regulator before replacing the pump

Good to know

• Repair difficulty: medium
• It affects emissions.

What each character of P2636 means

• P: Powertrain: engine and transmission
• 2: Generic code: same on every make
• 6: Control modules and outputs
• 36: Specific fault in that system

Frequently asked questions

Can I keep driving with code P2636?

Avoid long trips and hard acceleration. If you smell fuel or the car stalls, do not continue.

What causes code P2636?

The most common causes are: weak transfer pump not moving enough fuel between tanks; restricted transfer line or design-related flow limit (GM recall 17067); refuelling with engine or ignition on confuses the level-based check.

Related codes

• P2635Fuel Pump A Low Flow/Performance
• P2632Fuel Pump B Control Circuit/Open
• P2633Fuel Pump B Control Circuit Low
• P2634Fuel Pump B Control Circuit High
• P0461Fuel Level Sensor A Circuit Range/Performance

Sources

• gsi.ext.gm.com: 14OBDG10 Engine Diagnostics
• gsi.ext.gm.com: 20OBDG04 Diagnostics
• static.nhtsa.gov: MC 10143408 9999
• static.nhtsa.gov: MC 10112023 9999

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.