Comprehensive diagnostic guide for OBD-II code P0779

What This Code Means

• P0779 is a Powertrain/Transmission-related code indicating an intermittent fault in the pressure control solenoid B (often referred to as Shift Solenoid B in many transmissions). In plain terms: the B solenoid's electrical circuit or hydraulic control is failing intermittently, which can affect transmission hydraulic pressure and shift quality.
• Source reference: Open Source GitHub definition notes (Intermittent fault of the pressure control solenoid B), aligning with the P0779 identification as a pressure solenoid B issue. This sits within the broader OBD-II Powertrain/ transmission code framework described by Wikipedia's OBD-II sections.

Context: where this fits in the OBD-II system

• P0779 is categorized as a Powertrain code (transmission/Hydraulic control) under the OBD-II framework. This aligns with the general description of how modern vehicles monitor transmission controls and flag problems when a solenoid, valve, or related circuit behaves unexpectedly.

Symptoms

• Intermittent or harsh/shuddering transmission shifts, especially during upshifts or downshifts
• Delayed or incomplete gear engagement, or occasional failure to shift beyond certain gears
• Transmission may surge during shifts or exhibit rough torque delivery
• Check Engine Light or Transmission/TCM indicator lamps may illuminate intermittently
• DTC P0779 may not be the only code present; look for related transmission solenoid codes (e.g., other P077x family codes) or codes indicating hydraulic pressure or valve body concerns
• Note: Because the fault is intermittent, symptoms can appear sporadically, sometimes correlating with temperature, fluid condition, or road/load conditions. (General transmission solenoid behavior; parsing typical owner reports)

Preliminary data to collect (before teardown)

• Freeze frame data from the scan tool at the time of symptoms (drive cycles, engine/vehicle speed, transmission gear, load, fluid temperature)
• Any related codes (e.g., P0770-P0778 for other shift solenoids, P2710-P2712 for transmission control circuit issues, etc.)
• Transmission fluid level and condition (level checked hot/at proper operating temperature; smell and color)
• Vehicle make/model/year and transmission type, since solenoid configurations vary by application (Valve body design and solenoid layout differ by transmission family)
• Bi-directional control readiness: whether the scan tool can command solenoid B on/off (where supported)
• Safety note: when working around the transmission area, ensure the transmission is properly supported and the vehicle cannot move. Follow all shop safety protocols for fluid handling and electrical work.

Diagnostic Approach

1) Verify the DTC and collect data

• Confirm P0779 is present with a current or pending status and review freeze frame data for relevant conditions (speed, load, fluid temperature). Check for related transmission codes that might point to a broader hydraulic issue.

2) Inspect the external wiring and connectors

• Inspect the wiring harness and connector to solenoid B for signs of abrasion, corrosion, pushed-in pins, bent terminals, or poor grounds.
• Disconnect/Reconnect the solenoid harness; reseat any connectors with corrosion and ensure proper seating.
• Look for trapped moisture or contaminants in the connector. Clean or replace as needed.

3) Test solenoid B coil resistance and electrical behavior

• With the appropriate service information/specs, measure the solenoid B coil resistance. Compare against the manufacturer's specification.
• If resistance is out of spec, replace the solenoid B assembly.
• If resistance is within spec but the circuit shows intermittent voltage or current in operation, proceed to circuit testing.
• If capable, use a bi-directional scan tool to command solenoid B on and off to verify response and verify that the TCM/ECU recognizes the commanded state.

4) Inspect and test power/ground supply to the solenoid circuit

• Check the power supply (VB+ or equivalent) and ground for the solenoid B circuit. Look for voltage drop, poor grounding, or parasitic loads that could cause intermittent operation.
• Verify battery condition and charging system; a weak battery can cause intermittent solenoid operation or ECU misreading.

5) Examine transmission fluid level and condition

• Check transmission fluid level when hot and at specified temperature; ensure correct type/spec.
• Assess fluid condition (color, odor, sludging). Dirty or burnt fluid can impact hydraulic pressure and valve operation, potentially mimicking or exacerbating solenoid issues.
• If fluid is degraded or level low, address first or in parallel with solenoid tests.

6) Hydraulic/pressure path assessment (if available)

• If a pressure test tool is available, measure line pressure or hydraulic pressure in relevant ranges to see if the system pressure is within spec when solenoid B is commanded.
• Evaluate the valve body and hydraulic passages for contamination or sticking. In some cases, internal valve body wear or debris can cause intermittency that correlates with solenoid operation.

7) Look for related codes and re-test

• Clear codes after repairs and road-test the vehicle to verify that P0779 does not reappear and that shifting behavior is normalized.
• If P0779 persists, re-check the solenoid B circuit for intermittent faults, and also review related solenoids and the overall transmission control logic. Consider updated PCM/TCM software if indicated by OEM service information. (General post-repair verification steps)

Decision tree: likely causes and corresponding fixes

• Likely: Intermittent or faulty Shift Solenoid B or its wiring
  • Repair/replace solenoid B; fix wiring/connection; reseat harness; re-test
• Possible: Poor electrical connection or grounding in the B solenoid circuit
  • Repair wiring harness, clean grounds, secure connectors; re-test
• Possible: Transmission fluid level/condition contributing to hydraulic pressure issues
  • Correct fluid level, replace if contaminated, perform fluid change per manufacturer spec
• Possible: Valve body or hydraulic contamination or wear
  • Valve body service or transmission servicing may be required; consider professional valve body inspection or transmission rebuild if needed
• Possible: PCM/TCM control issue or software calibration
  • Check for OEM software updates or re-flash as specified; re-test
• Possible: Other related solenoids or circuits affecting hydraulic sequencing
  • Inspect adjacent solenoids and circuits (A solenoid, etc.) for shared harness issues

Probable Causes

• Faulty/intermittent Shift Solenoid B (most likely): 40-50%
• Wiring harness/connectors or poor electrical connection to B solenoid: 25-30%
• Transmission fluid level/condition (low or degraded): 10-15%
• Valve body hydraulic issues or contamination: 5-10%
• PCM/TCM software/calibration issue: 5%
• Other electrical issues (battery, grounds, parasitic loads): 5%

Safety Considerations

• Use proper PPE and lockout/tag-out procedures when working around the vehicle's electrical system and transmission ports.
• When performing any hydraulic pressure checks or valve body inspections, use appropriate jigs, adapters, and safety equipment. Transmission fluid is hot and can cause burns; handle with care.
• If you must remove the valve body or solenoids, follow OEM service procedures for torque specs, gaskets, and reassembly to avoid leaks or misalignment.
• Be mindful that OEMs may classify P0779 with slightly different definitions across brands; rely on OEM service information for exact solenoid B specs and replacement procedures in addition to the general diagnostic steps outlined here. (General safety and procedure reminders; OEM variations)

What you'll need (tools and equipment)

• OBD-II scan tool with ability to read live data and, if possible, bidirectional control for solenoid circuits
• Multimeter (for resistance checks, voltage drop checks, and continuity testing)
• Optional: oscilloscope for more advanced solenoid waveforms
• Manufacturer service information for solenoid B resistance spec and voltage expectations
• Transmission fluid and a supply of the correct ATF for the vehicle (per manufacturer spec)
• Vehicle jack stands or hoist, wheel chocks, and safe workplace setup

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.

---