Comprehensive diagnostic guide for OBD-II code P0791

Important Notes

• The Wikipedia OBD-II references establish that OBD-II uses Diagnostic Trouble Codes (DTCs) to monitor vehicle systems, with a broad grouping into Powertrain Codes (which include most transmission codes). This guide aligns with that structure: P0791 is a powertrain/transmission-related code, and OEM definitions can vary by manufacturer.
• Open-source code definitions provided include an example in Portuguese for a rotation-sensor circuit, illustrating that generic code naming is widely varied and OEM-specific definitions differ. There is no universal, single P0791 definition in the supplied open-source entries. )
• No NHTSA complaint dataset is included , so probability estimates for causes are based on typical ASE-field experience and general troubleshooting logic rather than published complaint frequencies in this set.

What This Code Means

• P0791 is a powertrain/transmission code. OEM definitions for P0791 vary by manufacturer; common themes across brands include transmission control solenoid performance issues, valve-body related faults, or a detected incorrect gear ratio/shift condition. Always verify with OEM service information for the exact vehicle application.
• Given the generic nature of the sources, treat P0791 as a transmission-system fault that can be electrical (solenoids, wiring, TCM), hydraulic (valve body, pressure), or mechanical (clutch/gear wear) in origin. Expect potential related codes such as P0700 (transmission control system fault), P073x (gear ratio errors), P0740 (torque converter/clutch issues), etc., to appear in conjunction.

Symptoms

• Intermittent or harsh/late shifts, especially between gears 1-2 or 2-3.
• Slipping or surging when accelerating or under load.
• Transmission staying in a "default" or limp-mode-like state, with limited gears available.
• Check Engine/Transmission/Warning lights on, sometimes with a secondary code (e.g., P0700) indicating a generic transmission fault monitor.
• Delays or failure to engage in drive or reverse, accompanied by abnormal transmission fluid behavior (burnt smell, dark color, or metallic particles in fluid).

Diagnostic Approach

1) Confirm and document

• Read the exact P0791 definition from the vehicle's OEM Service Information System (SIS) or repair manual to confirm the manufacturer-specific meaning (shift solenoid A, valve-body condition, gear ratio mis-match, etc.).
• Retrieve freeze-frame data and any related DTCs (especially P0700 and other P07xx codes) to understand the conditions under which the fault occurred (engine load, RPM, road speed, etc.).
• Note any symptom onset pattern (e.g., only at operating temperature, only in cold starts, intermittent).

2) Baseline checks (safety-critical and low-cost)

• Transmission fluid: check level with engine at proper temperature and vehicle on level ground; inspect color, smell, and contamination (burnt odor, metal particles). Burnt or very dark fluid and/or metallic debris often points to hydraulic issues or wear.
• Fluid condition and maintenance history: has the system been serviced recently? Is the wrong type of fluid installed for the OEM specification?
• Visual inspection of electrical harnesses and connectors at the TCM/solenoids and the valve body: look for loose connectors, corrosion, chafed wires, or signs of water intrusion.
• Battery and charging system health: weak voltages can affect solenoid operation and TCM control.

3) Electrical and data verification

• Using a capable scan tool, confirm real-time transmission data:
  • Solenoid activation/duty cycles for shift solenoids A/B (if supported by the vehicle).
  • Transmission line pressure (if the tool and vehicle support live pressure data).
  • TCM power/ground status, sensor inputs (vehicle speed, engine RPM, throttle position).
• Check for any stored fault codes beyond P0791 that might pinpoint a more specific subsystem (solenoid, pressure sensor, gear ratio sensor, wiring fault, etc.).

4) Solenoids, valve body, and pressure checks

• If line pressure data indicates low or erratic pressure, investigate the hydraulic circuit:
  • Inspect the valve body for wear, clogging, or sticking valves; debris in the valve body is a common culprit for shift faults.
  • Test/measure resistance of shift solenoids (compare to OEM specs). A stuck or weak solenoid can cause incorrect gear engagement and trigger P0791-like conditions.
  • Check the pressure sensor(s) and wiring for fault conditions; a faulty sensor can give incorrect feedback to the TCM, causing a false gear-ratio/shift fault.
• If allowed by service procedure, perform a controlled solenoid test (reproduce a shift event and observe solenoid response) and record any abnormal duty cycles or failure to energize.

5) Fluid and hydraulic system repair approach

• If the fluid is contaminated or degraded, perform a proper fluid drain and fill with the manufacturer-specified fluid. In some cases, a full or dedicated valve-body cleaning is required instead of a fluid-only treatment.
• If the valve body is suspected to be damaged or clogged, cleaning or replacement of the valve body assembly or solenoid pack may be needed.

6) Mechanical assessment (if electrical/hydraulic checks don't resolve it)

• Transmission internal wear (clutches, bands, gears) can cause misbehavior that triggers P0791 in some vehicles. This typically requires a rebuild or replacement.
• Torque converter issues (slippage, overheating) can mimic gear-ratio faults or shift faults; verify torque converter operation if indicated by symptoms or other codes (e.g., P0740 for torque converter circuit issues).

7) Software and adaptation

• Some P0791 scenarios are caused or aggravated by outdated TCM software or learned/adaptive fault correction. If recommended by OEM, perform a TCM software update or reflash and re-learn/adapt the transmission control strategy.
• After any repair, clear codes, perform a drive cycle to re-learn adaptation, and verify that the fault does not return.

8) Verification and testing

• After repairs, perform a controlled road test to verify smooth shifting across gears under multiple loads and RPM ranges.
• Re-check for any stored codes. Confirm that no additional misconfiguration codes reappear and that the vehicle leaves limp mode (if it previously did) and operates within OEM specifications.

Probable Causes

Note: The exact likelihoods vary by vehicle, application, and maintenance history. These are general ASE-style probabilities to guide prioritization:

• Faulty shift solenoids or valve-body-related hydraulic faults: ~40-50%

• Transmission fluid condition or level problems (low, burnt, or wrong spec): ~20-25%

• Wiring, grounds, or sensor/solenoid electrical issues (including pressure sensor): ~15-20%

• Internal mechanical wear (clutch packs, gears, bands) or valve-body damage: ~5-10%

• TCM software/adaptation issues: ~5%

What to gather and bring to the repair shop

• Vehicle make, model, year, and VIN; OEM service information for the exact P0791 definition.
• Exact symptoms and driving conditions (temperature, speed, gear, load) when the fault occurs.
• Any related DTCs (P0700, P073x, P0740, etc.) and freeze-frame data.
• Maintenance history: transmission fluid service status, fluid type/spec, filter service if applicable.
• Recent repairs or modifications affecting the transmission or engine control system.

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.

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