Diagnostic article Generic OBD-II
P2635: Fuel-pump performance is judged by filtered pressure error
Learn why this returnless fuel-system monitor can flag pressure error in either direction and why its threshold changes with operating demand.
Read before applying this article
Documented application
GM 2009 09OBDG02 Fuel System Control Module section, electronic returnless fuel-system performance monitor on physical page 67. Typical error thresholds are not exact service specifications, and per-vehicle fitment remains unestablished.
The code alone does not identify a failed part or establish that a vehicle is safe to drive. Confirm the vehicle, reporting module and applicable service procedure.
P2635 is often labeled fuel pump A low flow or performance. The cited GM fuel-system control module evaluates degradation of an electronic returnless system using filtered fuel-rail pressure error. Its monitor includes both a low and a high error boundary, so the fault is broader than proof of insufficient pump volume.
The threshold depends on demand
The boundaries are functions of desired pressure and fuel flow. The source gives typical negative limits from −30 to −90 kPa and positive limits from 30 to 90 kPa, with exact values in a supporting worksheet. Choosing one of these typical numbers as a universal pressure target would misread the monitor.
The pressure-error filter has a 12.5-second time constant, and the controller evaluates the result on a 100-millisecond loop. A brief raw-pressure excursion and a sustained filtered error therefore do not represent the same diagnostic evidence.
Prerequisites change what can be concluded
Engine run time must reach 30 seconds, fuel level must not be low, and pump control must be enabled and normal. Pressure-sensor, pump-circuit, reference-voltage, driver-temperature and internal-controller faults can inhibit the assessment. Those conditions matter before assigning a hydraulic cause to the performance code.
A useful record pairs desired pressure, reported pressure, fuel demand and the qualified monitor state. The code alone does not identify the pump, pressure sensor or supply path as the failed component. Verification should show that the filtered comparison passed under the relevant demand; fuel-system testing requires the applicable professional procedure, not improvised pressurized-line checks.
Source and applicability
GM 2009 09OBDG02 Fuel System Control Module section, electronic returnless fuel-system performance monitor on physical page 67. Typical error thresholds are not exact service specifications, and per-vehicle fitment remains unestablished.
Article source documents
Source checks describe how the text was prepared; they do not certify a diagnosis or confirm compatibility with every vehicle using this code.
Original code reference
Dictionary definition
Fuel Pump A Low Flow/Performance
This definition retains its original catalog scope. The article above is limited to its explicitly documented application.
Reference scope
Generic OBD-II definition.
A manufacturer label does not establish the model, year, engine or module to which a diagnosis applies. Read the safety and diagnostic limits.
Definition source
- DTC Database: pinned community code definitions
Source checked · MIT