Comprehensive diagnostic guide for OBD-II code P0843

What This Code Means

• P0843 is defined as a transmission fluid pressure sensor/switch A circuit high. This corresponds to the "A" circuit reading high are used by the vehicle's powertrain control module to monitor parameters and set codes when faults are detected.
• For context, Wikipedia's OBD-II pages describe how diagnostic trouble codes are used by modern vehicle systems to monitor parameters and alert the driver when issues are detected, with powertrain codes being one category of DTCs.

Important Notes

• Manufacturer variation exists: While P0843 generally indicates a high signal from TFPS A, some makes/models may phrase the fault or use different sensor labeling. Always cross-check with the factory service information for the exact sensor location and circuit description on the specific vehicle.
• This guide emphasizes safety due to hydraulic transmission systems and high-pressure oil. If transmission service is required, follow proper procedures for depressurizing and handling transmission fluid.
• If the vehicle is equipped with more than one transmission fluid pressure sensor (e.g., sensors A and B), P0843 refers to sensor A; other related codes (e.g., P0844, etc.) may refer to other sensors or circuit conditions.

Symptoms

• Check Engine Light / MIL illuminated with P0843 stored or pending.
• Transmission behavior changes: harsh or irregular shifts, slipping, delayed engagement, or a limp/limited-drive mode (in some vehicles, the PCM protects the transmission and enters a reduced-power state).
• Transmission fluid leaks or contamination signs (less direct, but can accompany sensor wiring issues).
• Diagnostic scan may show high TFPS A readings or a fault in the TFPS circuit even if the transmission pressure is actually within spec.

Probable Causes

Note: The probabilities below are qualitative estimates from ASE-type diagnostic experience rather than published NHTSA statistics, since do not include complaint data.

• Faulty transmission fluid pressure sensor A or its signal circuit (wiring, connector, or sensor stuck/high reading) - about 40-60%
• Wiring harness or connector problems in the TFPS A circuit (damaged insulation, corrosion, loose/dirty connectors, shorts to power or ground) - about 25-35%
• PCM/ECM input issue or sensor reference voltage problems (software/communication fault or a faulty reference) - about 5-10%
• Actual hydraulic/transmission issues that falsely trigger the high reading (rare in the context of "circuit high" unless the sensor is reading true high pressure due to a fault in the circuit interpretation) - about 5-10%
• Other sensor interaction or related circuit faults (e.g., ground fault affecting TFPS signal) - small probability

Pre-Diagnostic Checks

• Confirm the code and gather freeze-frame data and any related codes. Note the vehicle's transmission mode (automatic vs. CVT), fluid type and level, and whether any symptoms are currently present or intermittent.
• Inspect the TFPS A circuit visually: sensor connector, wiring harness, and grounds for damage, corrosion, abrasion, oil contamination, or looseness.
• Check power, ground, and signal references for the TFPS A circuit with a multimeter or scan tool:
  • Verify the reference voltage from the PCM (commonly 5 V on many sensors).
  • Verify the TFPS A signal line voltage when the engine is running and under varying conditions.
  • Check for a clean ground reference and absence of excessive resistance or impedance in the sensor ground path.
• If available, compare TFPS A data with actual transmission hydraulic pressure measurements using a known-good pressure test method or tool. This helps determine if the fault is sensor/circuit-based or a true hydraulic/pressure fault.
• Look for related trouble codes that might affect the TFPS circuit or transmission operation (e.g., other pressure sensor codes, circuit faults, or trans-related codes). If the vehicle has a history of transmission or ECU/software issues, consider those as contributing factors.

Recommended diagnostic procedure (step-by-step)

1) Verify and document

• Use an OBD-II scanner to confirm P0843 is current (not merely historical) and retrieve freeze-frame data. Note engine RPM, transmission temperature, vehicle speed, throttle position, fluid condition, and any live sensor data related to TFPS A.
• Check for any other DTCs that may illuminate or influence transmission behavior.

2) Visual and physical inspection

• Locate TFPS A (per service manual) and inspect the sensor, its connector, and wiring harness.
• Look for signs of contamination (oil/fluids), chafing, pin corrosion, bent pins, or loose connectors. Inspect the ground strap/ground circuit for corrosion or looseness.
• Confirm transmission fluid level and condition; extremely low level or degraded fluid can affect sensor readings or transmission operation, though a circuit-high fault is primarily electrical.

3) Electrical checks (circuit integrity)

• With the ignition ON (engine OFF) and key in run position, perform a resistance/continuity check on the TFPS A circuit wires between the sensor and the PCM connector to detect shorts or opens.
• Measure the TFPS A reference voltage at the PCM side and at the sensor side. Typical expectation is a stable reference (often around 5 V) with minimal resistance.
• Check the TFPS A signal circuit by measuring the sensor output voltage with engine cranked/started. Look for an abnormally high voltage or a voltage that does not respond to throttle or engine load changes.
• Check for shorts to battery voltage or to other circuits (protect against 12 V shorts if the system uses a different reference than 5 V).
• Inspect for ground faults by measuring sensor ground continuity and resistance to chassis ground.

4) Sensor health check

• If practical, test the TFPS A sensor in-circuit using a known-good sensor or a calibrated test procedure (swap with a known-good unit if available) to see if the high reading persists.
• Monitor TFPS A voltage live data under different operating conditions (idle, rpm rise, warm-up) to determine if the signal behaves erratically or remains stuck high.

5) Hydraulic/pressure consideration (if sensor/circuit tests are inconclusive)

• If possible, verify actual transmission fluid pressure with a mechanical gauge or factory service equipment. Compare the measured pressure to the sensor reading to determine if the sensor is accurate or if there is an actual hydraulic pressure issue.
• If actual pressure is correct but TFPS A reads high, focus on sensor/circuit or PCM interpretation issues.

6) ECM/PCM and software considerations

• Check for any available factory service bulletins or software updates related to TFPS or transmission control logic for the vehicle.
• If the sensor circuit tests are normal but P0843 persists, consider a PCM/ECU fault or software issue as a possible cause, and verify if a programming update is available.

7) Related codes and cross-checks

• Check for any other DTCs that could influence transmission logic or sensor readings (e.g., other pressure sensor codes, circuit faults, or transmission control-related codes).
• Some vehicles may have multiple TFPS sensors; ensure you're testing the correct circuit (A) and not confusing it with another sensor circuit (B or others).

Repair approaches (based on findings)

• If the TFPS A sensor is faulty or its circuit shows consistent high readings with no hydraulic pressure issue:
  • Replace the TFPS A sensor and, if applicable, replace or re-pin the sensor connector; repair damaged wiring as needed.
  • Recheck the circuit and confirm that the new sensor yields normal readings and the fault does not recur.
• If wiring/connectors are damaged or corroded:
  • Repair or replace the affected wiring harness segments; clean and re-seat connectors; apply proper dielectric/insulating sealant as appropriate for the vehicle.
  • Ensure proper grounding and secure connector retention.
• If a PCM/ECU issue or software fault is suspected:
  • Perform software/firmware updates per the manufacturer's service information.
  • If advised by the OEM, reprogram or reflash the ECU; recheck after update.
• If a hydraulic/transmission issue is diagnosed (less common for a circuit-high fault, but possible if pressure is actually high due to pump/VBV problems):
  • Address any hydraulic issues per transmission service procedures (fluid replacement, valve body inspection, pump, or related components as indicated by diagnostics).
• After any repair, clear the DTCs and test-drive the vehicle to ensure P0843 does not return and that transmission performance is restored.

Safety Considerations

• Transmission systems operate under hydraulic pressure; use proper PPE, avoid opening hot transmission components unless you have depressurized and properly supported equipment.

• When testing electrical circuits, disconnecting or removing components may cause unintended side effects; use proper lock-out/tag-out practices and follow OEM procedures.

• Use appropriate personal protective equipment and follow fire safety practices when working around transmission fluid and high-heat components.

• Code definition: Transmission Fluid Pressure Sensor/Switch A Circuit High (P0843) - Open Source OBD2 definitions (MIT). This source provides the basic definition used in this guide. )

• Context on DTCs and their role: OBD-II - Diagnostic Trouble Codes; OBD-II - Powertrain Codes. These pages describe how DTCs are generated and used by powertrain systems, giving context for P0843 as a powertrain code monitored by the PCM.

• Emphasis on diagnostic use and monitoring: OBD-II context sections highlight how modern systems monitor parameters and trigger codes when faults are detected.

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.

---