Comprehensive diagnostic guide for OBD-II code P0991

Topic focus: Intermittent signal from the transmission fluid pressure sensor/switch E

Summary

• DTC P0991, corresponds to an intermittent signal in the transmission fluid pressure sensor/switch circuit (designated as sensor/switch E). In practice, this typically points to intermittent electrical issues in the sensor circuit or a sensor with a sporadic output, rather than a constant mechanical fault in the hydraulic system.
• Technically, P0991 sits within the OBD-II P0xxx (powertrain) category, per Wikipedia's overview of Diagnostic Trouble Codes and Powertrain Codes. This places P0991 in the generic powertrain fault domain rather than a manufacturer-specific code.
• The code definition from open-source repositories

Symptoms

• MIL (Check Engine Light) illuminated or intermittently on
• Erratic or hard/harsh shifting behavior, especially under load or at certain temperatures
• Transmission may default to a limp mode or exhibit shifting anomalies when commanded (e.g., kick-down, upshifts, downshifts)
• Transmission warning lamp or message on the instrument cluster
• Scanner data shows intermittent or inconsistent transmission fluid pressure sensor readings or a fluctuating sensor signal
  Note: Actual symptoms can vary by vehicle make/model and whether the fault is sensor signal integrity versus a hydraulic pressure issue.

Probable Causes

• Electrical/wiring issues around the transmission fluid pressure sensor (harness chafing, damaged insulation, loose/dirty connectors, corrosion)
• Intermittent sensor ground or supply voltage problems (poor ground path, flaky 5V reference)
• Faulty transmission fluid pressure sensor or switch itself (sensor intermittently failing or drifting)
• PCM/ECU input or internal processing fault causing inconsistent interpretation of a healthy signal
• Mechanical transmission hydraulic issue that intermittently affects pressure readings (less common as the sole cause of P0991, but possible if the sensor/readings are marginal)

Probabilities

• Wiring/connectors or harness faults: ~45-60%
• Sensor/sensor circuit faults (sensor itself, ground, or reference): ~25-30%
• PCM/ECU input processing fault or software-related issue: ~5-15%
• Transmission hydraulic issues that could produce intermittent pressure reading: ~5-10%
  Note: These are approximate, experience-based estimates. They reflect the "intermittent signal" nature of P0991 and the fact that electrical/intermittent wiring issues are the most common culprits in similar DTCs. If you find contrary evidence in a specific vehicle family, adjust accordingly.

Diagnostic Approach

1) Verify and document

• Confirm the DTC P0991 is current and note any freeze-frame data, monitor readiness, and recent drive history when the fault occurred.
• Check for related DTCs that may provide additional context (e.g., other transmission-related codes or sensor supply/ground concerns).
• Verify the vehicle make, model, year, and transmission type, since sensor locations and wiring harness routing can vary by model.

2) Visual inspection and basic integrity checks

• Inspect the transmission fluid level and condition (level within spec, clean fluid, no metallic sheen or burnt smell). While this does not directly prove the sensor, it guides you toward hydraulic concerns if fluid state is abnormal.
• Locate the transmission fluid pressure sensor/switch E wiring harness and connector. Look for:
  • Damaged insulation, exposed wires, or signs of chafing against moving parts
  • Corrosion, bent pins, bent connector receptacles, or signs of water intrusion
  • Loose or dirty electrical connectors; reseat and re-terminate as needed
• Inspect grounds and sensor supply circuits at the PCM/ECU side if accessible; look for corrosion or loose connections.

3) Baseline data and sensor signal check (live data)

• Using a capable scan tool with live data, observe the transmission fluid pressure sensor signal (and any referenced pressure readings) during various driving conditions.
• Check the following in real time:
  • Sensor signal vs. engine/transmission rpm and vehicle speed
  • The 5V reference (supply) voltage to the sensor and the sensor ground integrity
  • Any instances of signal dropouts, spikes, or erratic behavior correlating to vehicle conditions (temp, load, or vibration)
• If the sensor is accessible for back-probing, verify continuity to the PCM, proper voltage reference, and ground integrity.

4) Electrical testing (sensor circuit)

• With the ignition ON (engine off if recommended by vehicle service information), perform resistance checks on the sensor circuit wiring harness:
  • Verify continuity from the sensor to the PCM/ECU pins
  • Verify no short to power, short to ground, or cross-talk between adjacent wires
• Measure every expected pin voltage:
  • Confirm the 5V (or vehicle-specified reference) supply to the sensor
  • Confirm a solid sensor ground path
  • Confirm the sensor signal pin varies within expected range during sensor excitation
• Wiggle and move tests on the wiring harness near connectors and along harness routes to provoke intermittent contact, noting any changes in signal behavior.

5) Sensor functional test

• If accessible, test the sensor using manufacturer service procedures or by substituting with a known-good sensor (if permitted by the vehicle's service guidelines). Observe whether the intermittent signal issue resolves with a known-good sensor.
• If removal is required, ensure proper handling of the transmission fluid system and follow safety procedures to avoid leaks or exposure.

6) Correlate data and perform a controlled drive test

• After addressing obvious wiring or connector issues, perform a controlled road test or data-logging drive to attempt to reproduce the intermittent condition.
• Monitor sensor signal and transmission pressure readings under various operating conditions (cold start, idle, acceleration, steady cruising, and high load). Look for correlation between sensor signal interruptions and RPM, temperature, or road-induced vibrations.

7) Rule out related software/ECU issues

• Check for service bulletins (TSBs) for the vehicle that mention transmission pressure sensor, sensor circuits, or related powertrain control issues.
• If the vehicle's software/ECU has an available update or reflash, consider performing the update per manufacturer guidelines, ensuring you are using the correct calibration for the vehicle.

8) Consider hydraulic/transmission internal possibilities (if electrical faults are ruled out)

• If sensor wiring and sensor performance are verified and the signal intermittently remains abnormal, consider hydraulic-related issues or a transmission pressure sensor that intermittently reads incorrectly due to an internal fault.
• Consider checking the transmission fluid pressure relief valve, pump, and related hydraulic paths if you have the appropriate diagnostic tools and service procedures.

9) Code reset and verification

• After repair or suspected mitigation, clear the DTCs and recheck for reoccurrence during road testing.
• If P0991 returns, re-evaluate the wiring and sensor integrity and consider alternate causes (ECU input, etc.).

Safety Considerations

• When working around transmission fluid circuits, avoid exposure to hot surfaces and avoid fuel or exhaust cross-overs.
• Use appropriate PPE and contain transmission fluid to prevent skin contact or environmental contamination.
• If the vehicle requires jacking or lifting to access connectors, follow safe lifting procedures and ensure the vehicle is stable.

Common Repairs

• Post-repair drive test results showing no reappearance of P0991
• Any persistent intermittent readings or new codes that may appear
• Updated freeze-frame data indicating a stable sensor signal
• Documentation of parts replaced (sensor, harness, connectors) and service bulletins consulted

What to cite

• General code framework and category: OBD-II documentation and "Diagnostic Trouble Codes" and "Powertrain Codes" sections in the Wikipedia OBD-II article. This underpins the concept that powertrain-related DTCs (P0xxx) are the class this code would belong to.
• Definition alignment: The Open Source/MIT repository entry states which aligns with the suspected fault area for P0991. Use this to describe the nature of the fault (intermittent signal from the sensor).
• Contextual note: Emissions testing section can be consulted to understand how DTCs affect emissions readiness and the importance of clearing codes after repair to verify readiness, though it is not specific to P0991.

Practical Tips

• Start with electrical/fuel-safety checks: focus on the sensor's wiring harness for the transmission fluid pressure sensor and its ground/supply.
• Don't overlook connector cleanliness and moisture intrusion; small amounts of oxidation can cause intermittent contact.
• Keep a log of live data during testing, including sensor voltage, ground integrity, and pressure readings, to identify intermittent anomalies that aren't obvious during static checks.
• If a sensor swap is performed, ensure the correct calibration and torque specs in the vehicle's service documentation to avoid leaks or sensor misalignment.

Summary

• P0991 indicates an intermittent signal issue in the transmission fluid pressure sensor circuit (sensor E). The most common root cause is intermittent electrical/wiring problems or a marginal sensor, with ECU interpretation or a rare hydraulic issue as secondary possibilities. A methodical electrical, sensor, and connector-focused diagnostic approach-complemented by live data correlation and, if needed, a sensor swap-offers the most reliable path to diagnosis and repair.

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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