P0873 on 2001-2008 GMC Sierra 2500: Transmission Pressure Switch 'C'/'D' Circuit High Fixes

What's Unique About the 2001-2008 Gmc SIERRA 2500

The GMC Sierra 2500 from this era used robust transmissions like the Allison 1000 (typically with Duramax diesels) and the 4L80-E (with gasoline engines). For the Allison, P0873 is a very common code pointing directly to the internal Pressure Switch Manifold (PSM), a known failure point. The failure mechanism is well-documented: heat cycles cause the plastic housing and integrated seals of the PSM to degrade, swell, and stretch, leading to internal hydraulic leaks that trigger the code. This often manifests as a problem only after the transmission warms up. The manufacturer even released a technical service bulletin (PIP4379N) that lists P0873 among a group of codes that can appear together due to these types of transmission issues. This makes diagnosis on the Sierra more straightforward than on vehicles where the cause is less documented.

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 GMT800 generation (2001-2006 and 2007 'Classic') and the start of the GMT900 generation (2007-2008). While the body style changed, the powertrain options, including the Allison 1000 and 4L80-E transmissions, remained largely consistent. The primary cause and fix are the same across these years. However, the Allison transmission was updated from a 5-speed to a 6-speed in 2006, which included changes to internal components and solenoids.

Symptoms You May Notice

• Check Engine Light or transmission warning light is on
• Harsh or erratic shifting
• Transmission is stuck in one gear (limp mode)
• Delayed gear engagement
• Increased fuel consumption
• Symptoms appear or worsen only after the transmission warms up (15-20 minutes of driving).
• 'Range Shift Inhibited' message on the driver information center (DIC).
🎬 Watch: How to fix the Range Shift Inhibited message.

Most Likely Causes

1. Failed Transmission Fluid Pressure Switch Manifold (PSM) 🔴 High Probability → Shop Transmission Assembly The internal pressure switch manifold on the Allison 1000 is a notorious failure point. Over many heat cycles, the plastic housing and integrated O-ring seals degrade, swell, and stretch, causing internal hydraulic leaks that the TCM interprets as an electrical fault.
   How to confirm: A key diagnostic clue is if the code and limp mode only appear after the truck has been driven for 15-20 minutes, pointing to a heat-related failure. A professional scan tool can monitor the live data from each pressure switch. If the switch for the corresponding circuit shows a constant high voltage regardless of gear, the PSM has failed. Upon removal, the old part's seals may be visibly stretched or swollen compared to the new part.
   Typical fix: Drop the transmission oil pan and replace the entire Pressure Switch Manifold assembly. A new filter and fluid should be installed at the same time.
   Est. part cost: $100-$200
2. Wiring Harness Issue 🟡 Medium Probability The wiring harness can be damaged in two main ways: 1) The internal harness insulation degrades from constant exposure to hot transmission fluid, leading to shorts between wires. 2) The external 'pass-through' connector on the side of the transmission can leak fluid through its seals, contaminating the pins and causing a poor connection. 🎬 See this video for a simple fix for intermittent connections. Additionally, on GMT800/900 trucks, the engine harness is known to chafe against the ECM/TCM bracket, upper control arm, and shock tower, which can short out various transmission-related circuits.
   How to confirm: Inspect the main round transmission connector for any signs of fluid leakage. If fluid is present, the internal harness may need replacement. If the external connector is dry and the problem persists, the internal harness must be checked for continuity, shorts, and degraded insulation after removing the oil pan. For 5-speed Allison models, pins backing out of the TCM connector due to harness strain is a common issue causing intermittent connections.
   Typical fix: Repair the damaged section of the harness or replace the internal/external harness if damage is severe. Using a genuine OEM or high-quality aftermarket harness is critical to prevent repeat failures. If pins have backed out of the TCM connector, they must be reseated.
   Est. part cost: $50-$300
3. Low or Contaminated Transmission Fluid ⚪ Low Probability → Shop Transmission Assembly While less likely to cause a 'circuit high' fault (which is typically electrical), very low or dirty fluid can cause erratic transmission behavior and trigger various codes. A leaking seal between the transmission and transfer case can cause fluid to move into the transfer case, lowering the transmission fluid level without an external leak.
   How to confirm: Check the transmission fluid level and condition with the engine running and transmission at operating temperature. If the fluid is dark, burnt-smelling, or low, it needs to be addressed. Check the transfer case fluid level; if it is overfull and smells like transmission fluid, the seal has failed.
   Typical fix: Perform a transmission fluid and filter change. This is often done as part of the PSM replacement. If the transfer case seal is leaking, it must be replaced.
   Est. part cost: $75-$150

Rare But Worth Checking

• Faulty Transmission Control Module (TCM): → Shop Transmission Assembly This is rare. On these trucks, the TCM is often mounted in the engine bay on the fan shroud and can be susceptible to heat and vibration. However, it should only be considered a potential cause after the pressure switch manifold and all related wiring (including grounds and connectors) have been thoroughly tested and confirmed to be good.
• Faulty Ignition Switch: A failing ignition switch can cause low voltage to be supplied to the TCM, leading to communication errors and seemingly random transmission faults, including 'Range Shift Inhibited' messages. This is an unusual but documented fix when other common causes have been ruled out.

Diagnosis Steps

1. Check the transmission fluid level and condition. Ensure it is full and not burnt.
2. Use a professional-grade OBD-II scanner to confirm P0873 and check for any other transmission-related codes.
3. Monitor the live data stream for the transmission pressure switches. Observe the voltage for the switch indicated by the code. A reading that is stuck high (typically near 5V or battery voltage) confirms the electrical fault.
4. Crucially, note if the fault only appears after the vehicle has warmed up for 15-20 minutes. This strongly suggests a heat-related failure of the PSM.
5. Inspect the main transmission wiring harness 'pass-through' connector for fluid contamination or pushed-out pins.
6. Inspect the main engine wiring harness for chafing, especially near the ECM/TCM bracket, upper control arm, and shock tower.
7. For 5-speed models (2001-2005), inspect the TCM connectors for pins that have backed out, causing a poor connection.
8. If the fault persists and/or is heat-related, the next step is to drain the fluid (approx. 9 quarts) and drop the transmission oil pan (typically 13mm bolts).
9. Visually inspect the internal wiring harness for brittle or flaking insulation. Inspect the Pressure Switch Manifold, comparing its seals to a new part; look for swelling or stretching.
10. If wiring is confirmed to be good, the Pressure Switch Manifold is the faulty component and should be replaced. The PSM is typically held on by 8mm or 5/16" bolts.

Parts You'll Likely Need

• Transmission Pressure Switch Manifold (Allison 1000) (OEM #29546972) — This is the most common failure point for code P0873 on Sierra 2500s with the Allison 1000 transmission. Part #29546972 is an updated 4-switch design that supersedes the original 6-switch design on 2001-2005 models and fits up to 2019 models.
  Trusted brands: ACDelco, Allison (Genuine)
  OEM price range: $100-$150
  Aftermarket price range: $70-$120
• Transmission Pressure Switch Manifold (4L80-E) (OEM #24222077) — For gasoline engine models with the 4L80-E transmission, this is the corresponding part that fails and causes pressure switch codes.
  Trusted brands: ACDelco, Standard Motor Products
  OEM price range: $60-$100
  Aftermarket price range: $40-$70
• Transmission Filter Kit (with Pan Gasket) — The filter and pan gasket must be replaced whenever the transmission oil pan is removed for service.
  Trusted brands: ACDelco, Allison, Wix
  OEM price range: $40-$60
  Aftermarket price range: $25-$40
• Automatic Transmission Fluid — The fluid lost when dropping the pan must be replaced. Use fluid that meets the correct specification. DEXRON-VI is the modern, backward-compatible fluid for GM transmissions.

Related Codes That Often Appear With This One

• P0701 — A general transmission control system fault code that often accompanies more specific codes like P0873.
• P0872 — This code is for 'Transmission Fluid Pressure Sensor/Switch C Circuit Low', and seeing it alongside P0873 could indicate an intermittent wiring problem or a failing PSM that is fluctuating between states.
• P0877 — This code is for 'Transmission Fluid Pressure Sensor/Switch D Circuit Low', which points to a similar fault on a different pressure circuit, often within the same switch assembly.
• P0751, P0756, P0761, etc. — These are solenoid performance codes. As noted in TSB PIP4379N, multiple transmission electrical faults can be triggered at once, pointing to a systemic issue like a failing switch manifold or harness.

Technical Service Bulletins (TSBs) & Recalls

• PIP4379N: Mentions that P0873 can be found with a host of other transmission DTCs, suggesting a common underlying cause that should be diagnosed systematically, starting with P0701 if present.

Platform-Specific Known Issues

• A very common owner experience involves the truck driving perfectly when cold. After about 15-20 minutes of operation, as the transmission reaches full operating temperature, it will suddenly shift hard, the check engine light will come on, and the transmission will be forced into 'limp mode' (stuck in 3rd gear). This heat-soak failure is a classic symptom of the internal seals on the Pressure Switch Manifold failing, a well-documented issue on the Allison 1000 transmission.

Mechanic-Grade Diagnostic Values

• Transmission Fluid Pressure (TFP) Switch Signal Voltage — expected: The switch provides a ground to the TCM. When fluid pressure is present and the switch is closed, voltage should be near 0V. When there is no pressure and the switch is open, it should read the reference voltage (typically 5V or 12V).. Failure: A 'Circuit High' code like P0873 is set when the TCM sees a constant high voltage (stuck near 5V or battery voltage) when it expects to see 0V, indicating an open circuit.
• TCM Power Supply Voltage — expected: 12.6 Volts. Failure: Operating voltage below 11.5 Volts can cause the Allison TCM to malfunction, potentially causing 'Range Shift Inhibited' messages and other codes.

Wiring & Ground Locations

• TCM Ground (G105) — For 2001 models, the TCM ground wires (pins 1 and 5) are spliced at S103 and terminate at ground point G105, located on the right front of the engine block, above the oil pan.. A poor ground connection at this point can cause a floating ground, leading to incorrect voltage readings and a variety of electrical fault codes, including P0873.
• Engine Harness Chafe Points — The main engine wiring harness is known to rub against the ECM/TCM mounting bracket, the driver's side upper control arm, and the shock tower bolt.. Chafing at these points can wear through the insulation of wires for the transmission sensors or controls, causing a short to voltage or an open circuit, which directly triggers codes like P0873.
• TCM Connectors (5-Speed Allison) — The TCM is mounted on the driver's side fan shroud. The wiring harness connects with a tight 90-degree bend.. Vibration and the tight angle of the harness can cause individual pins to back out of the TCM connector, leading to intermittent open circuits and transmission faults. This is a common, often-overlooked issue before condemning the TCM itself.
• Transmission Fluid Pressure Switch C Signal Wire — On some 2006 models with the 4L80-E, the signal wire for pressure switch C is a RED wire at pin 25 of the C1 (Blue) PCM connector.. This specific pin can be tested for continuity and shorts to voltage/ground when diagnosing a P0873 code without dropping the transmission pan.

Real Owner Repair Stories

• Reddit user in r/Duramax (2005 GMC Sierra 2500HD (LLY)) — 'Range Shift Inhibited' message on the dash, transmission going into limp mode.
  ❌ Tried (didn't work) Replacing the NSBU switch, Replacing the internal wiring harness, Replacing the Pressure Switch Manifold (PSM)
  ✅ What actually fixed it The ignition switch had an internal failure, causing low voltage to the TCM. Replacing the $35 ignition switch resolved the issue after months of misdiagnosis.
• YouTube video by 'that tranny guy' (Duramax with Allison Transmission) — 'Range Shift Inhibited' message, erratic shifting, not wanting to upshift.
  ❌ Tried (didn't work) Replacing the internal wiring harness (this fixed the upshift issue but not the 'Range Shift Inhibited' message), Replacing the Neutral Safety Back-up (NSBU) switch, Replacing the ignition switch, Replacing the TCM (could not be programmed because the shop couldn't communicate with it)
  ✅ What actually fixed it The vehicle speed sensor (VSS) in the transfer case had failed. The TCM was seeing a mismatch between input and output speeds, interpreting it as a slip, and triggering the fault. Replacing the VSS in the transfer case fixed the problem.
• DuramaxDiesels.com forum user (2003 Duramax (LB7)) — 'Range Shift Inhibited' message, especially when shifting from Reverse to Drive.
  ❌ Tried (didn't work) Replacing internal wiring harness, Replacing both transmission filters, Replacing NSBU switch, Adding new Transynd fluid
  ✅ What actually fixed it The seal between the transmission and the transfer case failed, allowing transmission fluid to leak into the transfer case. This caused the transmission to be intermittently starved of fluid, triggering the fault. Replacing the transfer case input seal resolved the fluid transfer and the code.

OEM Part Supersession History

• Unknown (6-switch design) → 29546972 (4-switch design) — The original 6-switch PSM used in 2001-2005 models was updated to a more reliable 4-switch design.

Model Year Variations Within This Range

• 2001-2005: These models used a 5-speed Allison 1000 transmission with an external Neutral Start Back-up (NSBU) switch and a 6-switch design for the original Pressure Switch Manifold.
• 2006-2008: Starting in 2006, the Allison 1000 became a 6-speed. The NSBU switch was moved inside the transmission (becoming an Internal Mode Switch), and different solenoids were used. This can change diagnostic procedures slightly, though the P0873 code still commonly points to the PSM.