P0848 on 2006-2010 GMC Sierra: Transmission Pressure Sensor 'B' Circuit High Causes and Fixes

What's Unique About the 2006-2010 Gmc SIERRA

The 2006-2010 GMC Sierra model years span a critical transition in transmission technology. Earlier models and lighter-duty trucks often used the 4-speed 4L60-E or 4L65-E transmission, while later models (especially from 2009 onwards) and heavier-duty trucks used the 6-speed 6L80/6L90 or Allison transmissions. The fix for P0848 is vastly different between them. 4-speed models have a simple pressure switch manifold on the valve body. In contrast, 6-speed 6L80/6L90 models have the switches integrated into a complex Transmission Electro-Hydraulic Control Module (TEHCM), which is known to fail when small, rubber-like laminate discs covering the pressure sensor ports rupture or delaminate from heat and pressure cycles. Correctly identifying the transmission is the most important step in diagnosis.

Diagnostic Flowchart

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

Generation note: This range covers the end of the GMT800 generation (sold as the 'Classic' in 2007) and the bulk of the GMT900 generation (starting 2007). The key difference for this code is the transmission used. 4-speed automatics (4L60E/4L65E) were common in earlier 1500 models, while 6-speed automatics (6L80) became standard around 2009. Heavy Duty models used the Allison transmission.

Symptoms You May Notice

• Check Engine Light or Transmission Warning Light is on
• Harsh, erratic, or delayed shifting
• Transmission stuck in one gear (fail-safe or "limp mode"), often 3rd gear
• Failure to shift or engage gears properly
• Reduced fuel economy
• On cold starts, the code may not appear if the vehicle is allowed to warm up for several minutes before driving.
• Transmission may go into neutral unexpectedly, with a 'Shift Range Inhibit' message.

Most Likely Causes

1. Faulty Transmission Fluid Pressure Switch Manifold (4-Speed 4L60E/4L65E Models) 🔴 High Probability → Shop Transmission Assembly This is a very common wear item on GM 4-speed automatics. The plastic switch assembly is constantly exposed to heat and fluid, leading to internal electrical failure over time.
   How to confirm: After verifying external wiring is good, drop the transmission pan and test the switch assembly for proper electrical function according to the service manual. The component is bolted directly to the valve body.
   Typical fix: Replace the entire pressure switch manifold assembly 🎬 Watch: Step-by-step guide to replacing the 4-speed pressure switch manifold., which is bolted to the valve body. Also replace the transmission filter and pan gasket.
   Est. part cost: $70-$150
2. Faulty Transmission Electro-Hydraulic Control Module (TEHCM) (6-Speed 6L80/6L90 Models) 🔴 High Probability → Shop Transmission Assembly On 6L80/6L90 transmissions, the pressure switches are integrated into the TEHCM inside the oil pan. A widely known failure point is the rupture or delamination of thin laminate discs covering the pressure sensor ports. This causes internal pressure leaks and incorrect sensor readings.
   How to confirm: Diagnosis requires a high-end scan tool to monitor TEHCM data. If external wiring is confirmed good, the TEHCM is the presumed failure. Visual inspection after removing the TEHCM can reveal the torn rubber diaphragms.
   Typical fix: The official fix is to replace the entire TEHCM assembly, which must be programmed to the specific vehicle's VIN. However, a common and more affordable aftermarket repair involves replacing only the failed laminate plate/seal assembly, which may not require reprogramming.
   Est. part cost: $300-$800 for a full TEHCM, or under $100 for a seal plate kit.
3. Damaged Wiring or Connectors 🟡 Medium Probability The internal transmission harness can become brittle from heat cycles, causing breaks or shorts. The external harness on GMT900 trucks is known to chafe against sharp metal edges, particularly at the ECM/TCM mounting bracket, the driver's side upper control arm, and the shock tower, leading to a short to voltage.
   How to confirm: Visually inspect the external harness at known chafe points for damage. Test for continuity and shorts between the TCM and the transmission case connector using a multimeter.
   Typical fix: Repair the damaged section of wire or replace the affected harness (internal or external).
   Est. part cost: $70-$250
4. Low or Degraded Transmission Fluid ⚪ Low Probability → Shop Transmission Assembly While not a direct cause of an electrical 'circuit high' code, low or burnt fluid can cause overheating and erratic operation, stressing electronic components and leading to their eventual failure.
   How to confirm: Check the transmission fluid level and condition using the dipstick. The fluid should be at the correct level, red/pink in color, and not smell burnt.
   Typical fix: Perform a transmission fluid and filter change using the correct DEXRON-VI fluid.
   Est. part cost: $50-$100

Rare But Worth Checking

• Rodent Damage to Wiring Harness: Mice and other rodents can build nests in the engine bay and chew through wiring. A user on DuramaxDiesels.com found that multiple transmission codes, including P0848, were caused by chewed wires in the harness under the underhood fuse block.
• Faulty Transmission Control Module (TCM) or Powertrain Control Module (PCM): → Shop Engine Control Module (ECM) This is very rare. The module should only be suspected after all other possibilities (sensor, wiring, TEHCM) have been definitively ruled out.

Diagnosis Steps

1. Check and correct the transmission fluid level and condition.
2. Use an OBD-II scanner to check for any other related trouble codes. If P0701 is present, follow diagnostics for it first as per TSB PIP4379K.
3. Identify the specific transmission in the vehicle (e.g., 4L60E, 6L80, Allison). This is the most critical step.
4. Thoroughly inspect the main external engine wiring harness for damage, especially at known chafe points near the ECM bracket, upper control arm, and shock tower.
5. If external wiring is okay, the next step involves dropping the transmission pan.
6. Inspect the fluid in the pan for excessive metal debris, which would indicate a more severe mechanical failure.
7. For 4-speed models, visually inspect the internal wiring and replace the pressure switch manifold.
8. For 6-speed (6L80) models, inspect the TEHCM for ruptured laminate discs. Decide whether to replace the entire TEHCM (requires programming) or attempt a repair with an aftermarket seal plate kit.
🎬 See how to reseal 6L80 pressure switches without replacing the TEHCM.

Parts You'll Likely Need

• Transmission Fluid Pressure Switch Manifold (OEM #24229849) — This is the most common failure for this code on models with the 4L60E/4L65E transmission.
  Trusted brands: ACDelco, Dorman, Rostra
  OEM price range: $100-$150
  Aftermarket price range: $70-$120
• Transmission Electro-Hydraulic Control Module (TEHCM) (OEM #24256861 / 24256863 (for approx. 2010+ models, supersedes earlier versions)) — This is the most common failure for this code on models with the 6L80/6L90 transmission, as the pressure switches are integrated into it. Must be programmed to the vehicle's VIN.
  Trusted brands: ACDelco, GM Genuine, Sonnax (Remanufactured), Dorman (Remanufactured)
  OEM price range: $400-$800
  Aftermarket price range: $300-$600
• TEHCM Pressure Switch Laminate Plate / Seal Kit — An aftermarket solution for 6L80 models that allows for the replacement of just the failed rubber diaphragms on the TEHCM, avoiding the cost and programming of a full TEHCM replacement.
  Trusted brands: Various aftermarket kits
  OEM price range: N/A
  Aftermarket price range: $40-$90
• Transmission Filter and Pan Gasket Kit — Required for any repair that involves dropping the transmission pan, such as replacing the TEHCM or pressure switch manifold.
  Trusted brands: ACDelco, Dorman, Wix
  OEM price range: $50-$100
  Aftermarket price range: $30-$70

Related Codes That Often Appear With This One

• P0700 — This is a general transmission fault code, indicating the TCM has requested the Check Engine Light. It almost always appears with specific codes like P0848.
• P0701 — Indicates a transmission control system performance issue. The manufacturer TSB specifically advises diagnosing P0701 first if it is present with P0848. [PIP4379K, 5]
• P0843, P0873, P0877 — These are codes for other pressure switches ('A', 'C', etc.) in the same assembly. Seeing multiple pressure switch codes together strongly points to a failed pressure switch manifold, TEHCM, or a shared wiring/power issue.

Technical Service Bulletins (TSBs) & Recalls

• This GM Technical Service Bulletin and its later revisions (e.g., PIP4379M/N) address a wide range of transmission codes, including P0848. It suggests that when multiple codes appear together, they may stem from a single internal hydraulic or electronic fault. The bulletin specifically advises technicians to follow diagnostics for code P0701 first if it is present, as it indicates a general control system performance issue that could be the root cause.

Platform-Specific Known Issues

• On GMT900 trucks with the 6L80 transmission, failure of the TEHCM is a widely documented problem. The internal design, where the control module and its integrated switches are submerged in hot transmission fluid, leads to a high failure rate.
• Wiring harness chafe points are a known issue on the GMT900 platform, where the harness can rub against brackets near the control modules or suspension components, causing shorts that can trigger codes like P0848.

Mechanic-Grade Diagnostic Values

• TFP Sensor/Switch 'B' Signal Voltage (Key On, Engine Off) — expected: Typically 0 volts when the switch is closed (pressure applied) and 5V or 12V when open (no pressure).. Failure: For P0848, the voltage is stuck high (near 5V or 12V) when it should be low.
• TFP Sensor Reference Voltage (at sensor connector, unplugged) — expected: ~5.0 V.. Failure: Absence of 5V reference points to a wiring or TCM/PCM issue, not the sensor itself.
• Sensor Ground Circuit Resistance (from sensor connector to battery negative) — expected: Near 0 Ohms (low resistance).. Failure: High resistance indicates a poor or open ground connection, which can cause a 'Circuit High' code.

Scan Tool Commands That Help

• Tech2 / GDS2: Transmission Data Display — To monitor the live data state (e.g., 'High' or 'Low', 'On' or 'Off') of TFP Switch 'B' (often labeled as PS2) while commanding different gears. This helps confirm if the switch is electrically stuck in the high state regardless of the commanded hydraulic state.
• High-End Scan Tool: Wiggle Test Monitoring — While monitoring the live voltage or state of the TFP 'B' circuit, physically wiggle the wiring harness at the transmission connector and along known chafe points. A jump or change in the reading indicates an intermittent short or open in the harness.

Wiring & Ground Locations

• Main Transmission Connector (6L80) — A 16-pin round connector on the passenger side of the transmission case.. This is the main interface between the external harness and the internal TEHCM. Pin 5 is the ground and Pin 4 is Battery+. Corrosion or damage here can cause P0848 and other codes.
• Engine Block to Chassis Ground — A ground strap running from the rear of the engine block (passenger side) to the firewall/chassis.. This is a primary ground path for the engine and transmission electronics. Corrosion or breakage of this strap is common and can cause numerous, seemingly unrelated electrical issues, including erratic transmission behavior and codes.
• PCM/TCM Ground — A ground wire from the main engine harness that bolts to the engine block, often near the back.. A poor connection at this specific ground can cause the PCM/TCM to malfunction, leading to incorrect sensor readings and codes like P0848. Some owners add a redundant ground wire from this point to the firewall as a preventative measure.
• Radiator Support Grounds — Located on the radiator support on both the driver and passenger sides, where the negative battery cable connects to the frame.. These are critical chassis grounding points. Corrosion here can degrade the ground quality for the entire vehicle's electrical system, contributing to intermittent and hard-to-diagnose faults.

Real Owner Repair Stories

• SilveradoSierra.com forum user (2011 Chevrolet Silverado with 6.2L V8 and 6L80E transmission) — Check Engine Light with codes P0848 and P0873.
  ❌ Tried (didn't work) Initial diagnosis suggested a complete TEHCM replacement.
  ✅ What actually fixed it The owner dropped the pan and found the laminated plate with its integrated rubber seals on the TEHCM had delaminated and was leaking. Replacing only this aftermarket plate/seal assembly resolved both codes without needing to replace or reprogram the entire TEHCM.

OEM Part Supersession History

• 24241350 / 24252114 → 24256861 / 24256863 — Internal design improvements, including updated solenoids and a better pressure manifold gasket to address known failure points.
  Heads up: Early (pre-2010) and late TEHCMs are generally not interchangeable due to hardware and internal calibration differences. A replacement TEHCM must be correct for the model year and requires VIN programming.
• 24256861 → 19431596 — Further component and manufacturing process upgrades by GM. This part was on a long backorder, indicating a high failure rate of the original.
  Heads up: This is the newest service replacement for many 6L80/Allison applications that originally used 24256861. It still requires VIN-specific programming by a dealer or qualified shop.

Model Year Variations Within This Range

• 2007-2009: The first-generation 6L80E TEHCMs used in these early GMT900 trucks are known to have a higher failure rate, particularly related to pressure switch membrane cracking and solenoid issues.
• 2009+: Starting in 2009, the 6-speed 6L80E became the much more common transmission behind the popular 5.3L V8, largely replacing the 4-speed 4L60E. A V8 Sierra from this period is highly likely to have the 6L80E, making the torn TEHCM diaphragm the most probable cause of P0848.