P2635 on 2016-2017 Chevrolet Volt: Fuel Pump Low Flow, Stalling Causes and Fixes

What's Unique About the 2016-2017 Chevrolet Volt

The 2016-2017 Volt is a second-generation plug-in hybrid where the gas engine acts as a range extender. While the fuel system can sit dormant for long periods, the P2635 code on this vehicle is most famous for a specific design flaw. GM has issued multiple Technical Service Bulletins (TSBs) confirming that high ambient temperatures (above 100°F) can cause vapor to form in the fuel line. The original fuel pump struggles to overcome this vapor lock, leading to fuel starvation, stalling, and the P2635 code. The official fix involves an updated fuel pump module specifically designed to correct this heat-related issue.

Symptoms You May Notice

• Engine stalling, particularly in hot weather (above 100°F / 37.7°C)
• Engine hesitation or surging during acceleration
• Reduced engine power or 'Propulsion Power Reduced' message
• Check Engine Light illuminated
🎬 Watch: Understanding P2635 causes and fixes
• Engine is hard to restart after stalling, especially when hot

Most Likely Causes

1. Failing In-Tank Fuel Pump Module (Heat-Related Failure) 🔴 High Probability → Shop Fuel Pump As per GM Technical Service Bulletins (TSBs) #PIP5507B and #17-NA-335, the original pump design is susceptible to vapor lock in ambient temperatures over 100°F (37.7°C), causing fuel flow to drop. This is a widely documented pattern failure for this vehicle.
   How to confirm: A technician can monitor desired vs. actual fuel pressure using a scan tool. A significant drop in actual pressure that coincides with hesitation or stalling, especially when the weather is hot, confirms the issue. The presence of TSBs for this exact condition is strong evidence.
   Typical fix: Replace the entire in-tank fuel pump module assembly with the updated GM-specified part number. TSB 17-NA-335 explicitly states this is the correction.
   Est. part cost: $250-$500

Rare But Worth Checking

• Faulty Fuel Pump Control Module (FPCM): → Shop Fuel Pump While the pump itself is the most common culprit for this specific issue on the Volt, the FPCM is responsible for powering the pump. A module failure can mimic a pump failure. It's less likely but worth considering if a new pump doesn't resolve the code.
• Wiring or Connector Issue: A damaged wire or loose connector at the FPCM or the fuel pump module can interrupt power or communication, causing a low flow condition. TSB 16-NA-225 mentions checking for loose connectors inside the tank.

Diagnosis Steps

1. Connect a professional OBD-II scan tool to confirm P2635 is present and check for other codes, especially P018B.
2. Check for relevant TSBs (PIP5507B, 16-NA-225, 17-NA-335) based on the vehicle's VIN.
3. Using the scan tool's live data function, monitor 'Desired Fuel Pressure' vs. 'Actual Fuel Pressure'.
4. Attempt to replicate the failure conditions, if safe to do so (often occurs in hot weather during city driving or after an extended idle).
5. Observe if the actual fuel pressure drops significantly below the desired pressure just before or during a hesitation or stall event. This is the key diagnostic indicator.
6. If the pressure drop is confirmed, the diagnosis points to a failing fuel pump module, as outlined in TSB 17-NA-335.
7. If pressure readings are stable but the code persists, inspect the wiring and connectors for the Fuel Pump Control Module (FPCM) and the fuel pump module for any signs of damage or corrosion.

Parts You'll Likely Need

• Fuel Pump Module Assembly (OEM #12673525 (supersedes 12666611)) — This is the complete assembly in the fuel tank. The original part has a design flaw causing heat-related failure. GM has released updated versions to correct the problem. TSB 17-NA-335 advises consulting the GM Electronic Parts Catalog (EPC) with the vehicle's VIN to get the correct, most current part number, but 12673525 is the widely accepted updated part.
  Trusted brands: ACDelco, GM Genuine Parts
  OEM price range: $300-$500
  Aftermarket price range: $250-$400
• Fuel Pump Control Module (OEM #23382564) — This module controls the fuel pump. While it is not the most common cause for this specific P2635 issue on the Volt, it can fail and cause similar symptoms. It is a potential, but less likely, replacement part.
  Trusted brands: ACDelco, GM Genuine Parts
  OEM price range: $70-$120
  Aftermarket price range: $50-$90

Related Codes That Often Appear With This One

• P018B — This code for 'Fuel Pressure Sensor 'B' Circuit Range/Performance' is frequently set alongside P2635. The TSBs from GM for the hot-weather stalling issue list both codes together as primary indicators of the failing fuel pump.
• P069E — This code, 'Fuel Pump Control Module Requested MIL Illumination', may be stored. It signifies that the FPCM has detected a fault (like the low flow condition of P2635) and has requested the ECM to turn on the Check Engine Light.
🎬 See how to diagnose P069E and P2635 codes

Technical Service Bulletins (TSBs) & Recalls

• PIP5507B: Preliminary information noting that P2635 and P018B may set in ambient temperatures of 100°F or higher, causing hesitation or stalling. It notes that excessively hot and sustained ambient temperatures over 100F can trigger these conditions.
• 16-NA-225: Details the procedure for replacing the fuel pump module and advises checking for loose in-tank connectors if the DTC P2635 and/or P018B are set.
• 17-NA-335: Supersedes previous bulletins, confirms the cause is vapor buildup in the fuel feed line, and directs technicians to replace the fuel pump module with an updated version.

Platform-Specific Known Issues

• A well-documented pattern failure where high ambient temperatures (100°F+) cause the original-equipment fuel pump to fail, resulting in engine stalling. This is acknowledged by GM in multiple service bulletins.
• Owners on forums like gm-volt.com describe the failure as happening in summer heat, often after the car has been sitting for a short period (heat soak) and then restarted, leading to a stall within minutes of driving.

Mechanic-Grade Diagnostic Values

• Low-Pressure Fuel System Pressure (Desired vs. Actual) — expected: Actual pressure should be within 45 kPa (6.5 PSI) of desired pressure at a 300 kPa (43.5 PSI) request, or within 60 kPa (8.7 PSI) at a 400 kPa (58 PSI) request.. Failure: A deviation greater than the specified values, especially a significant drop in actual pressure under load or in hot weather, points to a failing pump.
• FPCM Power Supply Voltage — expected: Should be close to battery voltage (12.0-12.6V) at the module connector with the ignition on.. Failure: Voltage below 11.5V indicates a problem in the power supply wiring or fuse block before the module.
• FPCM Ground Circuit Voltage Drop — expected: Less than 0.2V (200mV).. Failure: A reading higher than 0.2V indicates high resistance in the ground circuit, which can cause the module to operate incorrectly.
• FPCM Fuel Pump Command Signal (Dark Green/White wire) — expected: A pulsing 12V signal for 2-3 seconds at key-on, and constantly while cranking or running.. Failure: No voltage on this wire when the ECM should be requesting fuel points to a wiring break between the ECM and FPCM, or a faulty ECM.

Scan Tool Commands That Help

• GDS2 (GM Dealer Tool): Fuel Pump Trim Reset — This procedure must be performed after replacing the fuel pump module or FPCM to clear learned values and allow the system to adapt to the new components.
• GDS2 (GM Dealer Tool): Fuel Pump Enable/Disable — Allows a technician to manually command the fuel pump on and off to verify its operation and check fuel pressure without starting the engine.
• GDS2 (GM Dealer Tool): Module Programming (FPCM) — If the Fuel Pump Control Module is replaced, it must be programmed with the correct software for the vehicle's VIN. An unprogrammed module will result in a no-start condition.

Wiring & Ground Locations

• Fuel Pump Control Module (FPCM) — Located in the rear of the vehicle, typically under trim panels near the fuel tank area. On similar GM platforms, it's often on the frame rail near the spare tire.. This module directly controls the fuel pump. A failure of the module or its wiring can cause low fuel flow, mimicking a bad pump.
• FPCM Ground — The FPCM ground wire (typically black) attaches to a chassis ground point near the module's location in the rear of the vehicle.. A loose or corroded ground is a common failure point that can cause intermittent or complete loss of fuel pump operation, triggering P2635.
• Fuel Pump Module Connector — On top of the fuel tank, connecting the vehicle harness to the in-tank fuel pump module assembly. The harness part number is GM 23142154.. Corrosion or damage to this connector or its pins can cause a voltage drop to the pump motor, resulting in low flow.
• ECM to FPCM Command Wire (Dark Green/White) — Runs from the Engine Control Module (ECM) to Pin 13 of the FPCM connector. At the ECM, it is often in Pin 50 of one of the main connectors.. A break or short in this wire will prevent the FPCM from receiving the 'on' signal from the ECM, leading to a no-fuel-flow condition.

Real Owner Repair Stories

• gm-volt.com forum user (2017 Chevrolet Volt) — Check Engine Light with codes P2635 and P018B, engine stalling in hot weather (100°F+), especially after a short stop and restart (heat soak).
  ❌ Tried (didn't work) Initial dealer diagnosis was inconclusive as the problem would not replicate in their shop.
  ✅ What actually fixed it The owner referenced TSB 17-NA-335. The dealer then agreed to replace the in-tank fuel pump module assembly with the updated part. The problem was resolved and did not return.

OEM Part Supersession History

• 12666611 → 12673525 — The original fuel pump module design was susceptible to vapor lock in high temperatures. The updated part (12673525) and its ACDelco equivalent (M10239) are designed to mitigate this issue.
  Heads up: Using an old-stock original part (12666611) or a used pump from a salvage yard will likely lead to a repeat failure.

Model Year Variations Within This Range

• 2016 vs. 2017: The 2016 model year had a limited rollout, while the 2017 was available nationwide. The 2017 model also introduced optional Adaptive Cruise Control and complied with new Tier 3 emissions standards. However, the core 1.5L powertrain and fuel system related to the P2635 fault are identical between the two years.

Diagnostic Flowchart

Other Known Issues on This Vehicle

Issues unrelated to this code that are worth knowing about as an owner of this generation:

• Battery Energy Control Module (BECM) Failure 🔴 High — Common failure. Can cause no-start, inability to charge, and propulsion power loss messages. Often sets U26xx codes. (Ref: Special Coverage Program N232432680 extends warranty to 15 years/150,000 miles.)
• 'Shift to Park' Message Error 🟠 Medium — Very common. A faulty microswitch in the shifter assembly prevents the car from detecting it's in Park, which can drain the 12V battery. (Ref: TSB 19-NA-206 (and others) address this. Repair is often covered under the Voltec warranty.)
• EGR Valve and Cooler Clogging 🟠 Medium — Common, especially on vehicles with infrequent gas engine use. Carbon buildup clogs the EGR valve and/or cooler, leading to rough running, reduced power, and check engine lights (e.g., P0401). (Ref: No recall, but a well-known issue. Parts availability can be a problem, leading many owners to perform periodic cleaning.)

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: For this specific repair, a used part is NEVER a smart choice. The primary cause of P2635 is a known design defect in the original fuel pump. A used pump from a donor vehicle will either have the same defect or be close to failure.

What to inspect on the donor part:

• Not applicable. Do not purchase a used fuel pump for this repair.

OEM-only on this vehicle (don't cheap out):

• Fuel Pump Module Assembly

Aftermarket brands forum-validated for this vehicle:

• ACDelco (This is the GM Original Equipment brand)

Brands owners have reported issues with on this vehicle:

• Unknown or unbranded 'white box' parts. Given the critical safety nature of this part (preventing engine stalls), using a non-reputable brand carries significant risk.

Real Owner Stories

Aggregated from forums and TSBs cited above. Mileages and costs reflect what owners reported in those sources.

Related OBD-II Codes

• P018B — Often sets alongside P2635 in ambient temperatures of 100°F or higher due to fuel pump performance issues as noted in TSB PIP5507B.
• U26XX — Codes associated with Battery Energy Control Module (BECM) failure, which is another common cause of 'Propulsion Power Reduced' messages in this vehicle.
• P0401 — Related to EGR valve and cooler clogging, another common known issue for the 2016-2017 Volt that causes reduced power and check engine lights.

Frequently Asked Questions