P0843 on 2009-2010 GMC Sierra 2500: Transmission Pressure Sensor 'A' Circuit High Causes and Fixes

What's Unique About the 2009-2010 Gmc SIERRA 2500

The 2009-2010 GMC Sierra 2500 typically uses the heavy-duty 6L90 automatic transmission. In this specific transmission, the five pressure switches (including the one that triggers P0843) are not individual, externally-accessible sensors. Instead, they are integrated directly into the Transmission Electro-Hydraulic Control Module (TEHCM), which also contains the TCM and solenoids. This all-in-one design means that a single pressure switch failure, a very common issue, requires the replacement of the entire, more expensive TEHCM assembly located inside the transmission pan.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Harsh, delayed, or erratic shifting.
• Transmission stuck in 'limp mode' (usually a single gear to protect the transmission).
• No engagement into Drive or Reverse, or a long delay before engagement.
• Check Engine Light or Transmission Warning Light is on.
• Flashing 'D' (Drive) indicator on the dashboard.
• Increased fuel consumption.

Most Likely Causes

1. Failed Transmission Fluid Pressure (TFP) Sensor 'A' 🔴 High Probability → Shop Transmission Assembly The TFP sensors are integrated into the TEHCM, a component known to fail on GM's 6L80/6L90 transmissions. The internal laminated discs or diaphragms of the pressure switches crack or tear due to constant exposure to heat cycles and vibration inside the transmission, causing an open circuit. This is a well-documented failure point.
   How to confirm: A professional scan tool can monitor live data from the transmission pressure switches. The switch corresponding to code P0843 will show a constant high voltage 🎬 See this breakdown of P0843 causes and common fixes. (typically ~5V) or an unchanging 'OFF' state that does not change when gears are commanded. Technicians can also test continuity at the TEHCM pins after removal to confirm the internal open circuit.
   Typical fix: Replacement of the entire TEHCM assembly, which is mounted to the valve body inside the transmission. The new module must be programmed to the vehicle. Some specialized shops offer a repair service using kits (like those from TransGo) to rebuild the pressure switch diaphragms 🎬 Watch: How to rebuild the TEHCM pressure switches yourself. on the original TEHCM, which can be more cost-effective.
   Est. part cost: $400-$800 for a new or remanufactured TEHCM. Repair kits are cheaper at $40-$80 but require more labor.
2. Damaged Internal Wiring Harness 🟡 Medium Probability Like the TEHCM, the internal wiring harness is submerged in hot transmission fluid, which can make the plastic connectors and wire insulation brittle and prone to cracking, leading to an open or short circuit.
   How to confirm: After dropping the transmission pan, visually inspect the wiring harness that connects to the TEHCM for cracks, breaks, or corroded pins at the connector.
   Typical fix: Replace the internal transmission wiring harness. This is often done as a precaution whenever the TEHCM is replaced.
   Est. part cost: $50-$100
3. Low or Contaminated Transmission Fluid ⚪ Low Probability → Shop Transmission Assembly While less likely to cause a 'Circuit High' fault which is purely electrical, very low or dirty fluid can cause erratic operation, overheating, and contribute to setting various pressure-related codes. It generally won't be the root cause of P0843 alone but can indicate larger problems.
   How to confirm: Check the transmission fluid level and condition. The fluid should be at the correct level and red in color, not brown, black, or burnt-smelling.
   Typical fix: Perform a transmission fluid and filter change using the correct fluid type (DEXRON-VI). This is required as part of the TEHCM replacement anyway.
   Est. part cost: $100-$200

Rare But Worth Checking

• Faulty Transmission Control Module (TCM): → Shop Transmission Assembly Since the TCM is part of the TEHCM in this vehicle, this is usually diagnosed and fixed as a single unit. An independent TCM failure causing just this code is rare. GM TSB PIP4379N explicitly states that replacing the TCM is unlikely to correct this DTC.
• Damaged External Wiring Harness: The wiring between the main transmission connector and the vehicle's computers could be chafed or damaged, causing a short to power or an open circuit. This is uncommon but should be checked if other work was recently done on the vehicle or if there is visible damage.

Diagnosis Steps

1. Check the transmission fluid level and condition. Ensure it is full and the fluid is clean using DEXRON-VI fluid.
2. Use a professional scan tool to read all stored codes from the TCM. Note if other transmission codes are present, as suggested by TSB PIP4379N.
3. Monitor the live data for all Transmission Fluid Pressure Sensors. A reading for sensor 'A' that is stuck high (e.g., near 5 volts) or does not change state (ON/OFF) regardless of gear selection points to an electrical fault.
4. Inspect the main external transmission wiring harness and connector for any visible damage, corrosion, or loose pins.
5. If external checks are clear, the next step is to drain the fluid and remove the transmission pan.
6. Visually inspect the TEHCM and the internal wiring harness for any signs of damage, cracks, or discoloration.
7. If no visual fault is found, the TEHCM is still the most probable cause and should be replaced based on the diagnostic trouble code.
8. After replacement, refill with new DEXRON-VI fluid, install a new filter, and use a GM-compatible scan tool to program the new TEHCM to the vehicle's VIN. A relearn procedure must also be performed.

Parts You'll Likely Need

• Transmission Electro-Hydraulic Control Module (TEHCM) (OEM #Varies by year/VIN, e.g., ACDelco 24256861 (example, verify with VIN)) — This is the most common cause of code P0843, as it contains the integrated pressure switch that has failed. It is typically replaced as a complete unit.
  Trusted brands: ACDelco (OEM), Street Smart Transmission (Remanufactured), Sonnax (offers remanufactured units and repair kits)
  OEM price range: $500-$800
  Aftermarket price range: $400-$600 (Remanufactured)
• Automatic Transmission Fluid (DEXRON-VI) (OEM #ACDelco 10-9395 (example)) — The transmission fluid must be drained to access the TEHCM and will need to be replaced (approx. 7-9 quarts for a pan drop).
  Trusted brands: ACDelco, Valvoline, Castrol
  OEM price range: $10-$15 per quart
  Aftermarket price range: $8-$12 per quart
• Transmission Filter and Pan Gasket Kit (OEM #ACDelco 24226850 (Gasket for 6L90)) — The filter should always be replaced when the transmission pan is removed for service. A new gasket is required.
  Trusted brands: ACDelco, Wix, ATP
  OEM price range: $40-$70
  Aftermarket price range: $25-$50

Related Codes That Often Appear With This One

• P0700 — This is a general transmission fault code that simply means the TCM has stored a specific trouble code, like P0843. It is a request for the main engine computer to turn on the Check Engine Light.
• P0751, P0756, P0776 — These are shift solenoid performance codes. They often appear with pressure switch codes because the solenoids and switches are all part of the same TEHCM assembly, and a failing module can trigger multiple internal faults.
• P0842, P0848, P0873, P0877 — These are codes for the other transmission fluid pressure switches ('A' Circuit Low, 'D', 'B', etc.). Multiple pressure switch codes appearing together strongly point to a failure of the TEHCM or its internal harness.

Technical Service Bulletins (TSBs) & Recalls

• PIP4379N: Addresses delayed or no engagement into drive/reverse and range inhibit issues, listing P0843 as one of many possible related DTCs. It advises against simply replacing the TCM as a fix.

Platform-Specific Known Issues

• A known issue documented in GM Technical Service Bulletin #PIP4379N involves a combination of symptoms like delayed or no engagement into drive or reverse, often accompanied by a range of transmission codes including P0843. The bulletin guides technicians through a diagnostic process, emphasizing that P0701 should be diagnosed first if present.

Mechanic-Grade Diagnostic Values

• Transmission Fluid Pressure (TFP) Sensor 'A' Signal Voltage — expected: A variable voltage, typically between 0.5V and 4.5V, that changes with commanded gear and pressure.. Failure: A sustained reading above 4.5V, often stuck at a constant 5.0V, regardless of gear selection. This indicates an open or 'high' circuit fault.
• TCM Ground Circuit Resistance — expected: Less than 0.5 Ohms between the appropriate ground pins on the transmission connector and a known-good chassis ground.. Failure: High or infinite resistance, indicating a poor ground connection that can cause erratic electrical faults, including sensor circuit high codes.

Scan Tool Commands That Help

• GDS2 / Tech2: Service Programming System (SPS) — This function is mandatory after replacing the TEHCM. It flashes the new module with the correct VIN-specific software and calibrations. Failure to program the new TEHCM will result in a no-start or immediate limp-mode condition.
• GDS2 / Tech2: Service Fast Learn Adapts — This procedure must be performed after TEHCM replacement and programming. It allows the new TCM to learn the hydraulic characteristics of the individual clutches. The transmission fluid must be between 158-212°F (70-100°C) to initiate the procedure. Skipping this step will result in poor shift quality and may set other DTCs.

Wiring & Ground Locations

• G110 — Located at the right rear of the engine compartment, sometimes on a bracket at the back of the cylinder head.. This is a major engine and body harness ground point. A loose or corroded G110 ground can cause a floating ground reference for various sensors, including those in the transmission, potentially leading to incorrect high voltage readings and codes like P0843.
• Transmission Connector (X175 on some diagrams) — The main 16-pin round connector on the passenger side of the transmission case.. This is the sole electrical interface between the vehicle's main wiring harness and the internal TEHCM. All power, ground, and data signals for the transmission pass through this connector. Corrosion or pushed-out pins here can directly cause a 'Circuit High' fault.
• ECM Connector X1 — One of the main connectors at the Engine Control Module (ECM), typically located in the engine bay.. While the TCM is inside the transmission, it communicates with the ECM via the CAN bus. A fault at the ECM connector is unlikely to cause an isolated P0843 code but could be a factor if multiple unrelated codes are present. The Park/Neutral signal, for example, is on Pin 1 of the X1 connector.

Real Owner Repair Stories

• eBay Buyer Feedback (2009 Chevy 2500 LTZ (LMM Duramax)) — Transmission was shifting hard and going in and out of limp mode.
  ✅ What actually fixed it The owner purchased a brand new, pre-programmed TEHCM (TCM). After installing the new module, the truck was 'back to driving like Brand New Again'.

OEM Part Supersession History

• 24256861 → 19435614 — Part revision and update by the manufacturer.
  Heads up: The TEHCM must be programmed for the specific vehicle VIN. While mechanically interchangeable within certain year groups, the internal software and calibrations are different. Installing a TEHCM from a different year group without correct programming will cause communication errors or limp mode.

Model Year Variations Within This Range

• 2009 vs 2010+: GM made running changes to the TEHCM design. The early Gen 1 TEHCMs used through 2009 (like P/N 24241350) were known for frequent pressure switch issues. Starting around 2010, updated TEHCMs (like P/N 24256861) were introduced with improved solenoids and an updated pressure manifold gasket to address some of these failure points. While the P0843 code can occur on both, the specific internal failure mode may relate to the TEHCM generation.