P0873 on 2001-2008 Chevrolet Silverado 2500: Transmission Pressure Switch Fixes

What's Unique About the 2001-2008 Chevrolet SILVERADO 2500

Most Silverado 2500s in this year range use the robust Allison 1000 transmission. For this specific transmission, the pressure switches are not individual sensors but are integrated into a single component called the Pressure Switch Manifold (PSM), which is bolted to the valve body inside the transmission. While the generic code is 'C', Allison technical documentation refers to this specific circuit as pressure switch 'E'. Failure of this internal PSM is a well-documented issue and the most common cause for this code on these trucks, as confirmed by GM Technical Service Bulletins.

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 (2001-2007 Classic) and the start of the GMT900 (2007.5-2008) generations. The Allison transmission evolved from a 5-speed to a 6-speed around 2006. However, parts suppliers indicate that the same updated Pressure Switch Manifold (part #29546972) fits both versions across a wide range of years (2001-2018), simplifying the repair.

Symptoms You May Notice

• Check Engine Light is on
• Transmission is stuck in one gear (Limp Mode)
• Harsh or erratic shifting
• Delayed engagement when shifting into Drive or Reverse
• Transmission warning light may be illuminated on the dash
• Symptoms are often worse or only appear after the transmission has warmed up (e.g., after 15-20 minutes of driving).

Most Likely Causes

1. Failed Transmission Pressure Switch Manifold (PSM) 🔴 High Probability → Shop Transmission Assembly The internal pressure switch manifold on the Allison 1000 is a known failure point. Over time, heat cycles can cause the plastic housing and integrated O-rings to degrade, swell, and fail, leading to internal hydraulic leaks that trigger the code. TSB PIP4379N includes P0873 in a list of codes that can point to this type of failure.
   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. This points to a heat-related failure of the PSM seals, rather than a persistent electrical short. When the pan is dropped, a visual inspection of the old part may reveal swollen or stretched O-rings. Given the high probability, replacement is the most common confirmation.
   Typical fix: Replace the Pressure Switch Manifold. This requires draining the fluid, dropping the pan, unbolting the old manifold from the valve body, and installing the new one. 🎬 Watch: This walkthrough shows how to replace the manifold and fix limp mode.
   Est. part cost: $100-$150
2. Wiring Harness Issue 🟡 Medium Probability The wiring harness can be damaged either externally (at the main round connector on the side of the transmission) or internally (inside the oil pan). A short to voltage on the signal wire will cause a 'Circuit High' code. Pins can also become loose or corroded in the main TCM connector.
   How to confirm: Inspect the external connector for corrosion or pushed-out pins. Inspect the internal harness for damage when the pan is off. A multimeter can be used to check for a short between the signal wire and a power source. If the code appears immediately at startup, 🎬 See how to diagnose limp mode issues that occur on cold starts. an electrical issue is more likely than a heat-related PSM failure.
   Typical fix: Repair the damaged section of the harness or replace the internal/external harness if damage is severe.
   Est. part cost: $50-$250
3. Stuck Valve in Valve Body ⚪ Low Probability → Shop Transmission Valve Body Debris in the transmission fluid can cause control valves within the valve body to stick, leading to a genuine high-pressure condition that the sensor correctly reports.
   How to confirm: This is difficult to confirm without disassembly and is usually suspected only after a new PSM and wiring check fail to resolve the issue. A technician may be able to identify a stuck valve upon inspection of the valve body bores.
   Typical fix: Remove and clean the valve body, freeing any stuck valves. In some cases, the entire valve body may need to be replaced.
   Est. part cost: $0 (if cleaning works) - $500+

Rare But Worth Checking

• Faulty Transmission Control Module (TCM): → Shop Transmission Assembly A failing TCM can send erroneous codes. This is rare and should only be considered after all other possibilities, especially the PSM and wiring, have been eliminated. The TSB PIP4379N suggests diagnosing code P0701 (a TCM code) first if it is present.

Diagnosis Steps

1. Use an OBD-II scanner to confirm P0873 and check for any other codes, especially P0700 or other P08xx codes.
2. Note whether the code appears immediately or only after the transmission warms up. A heat-dependent issue strongly suggests the PSM.
3. Check the transmission fluid level and condition. Ensure it is at the correct level and does not smell burnt or contain excessive debris.
4. Visually inspect the main transmission electrical connector on the outside of the case for any signs of damage, corrosion, or fluid intrusion.
5. If external checks are clear, drain the transmission fluid and remove the oil pan.
6. Visually inspect the internal wiring harness and the Pressure Switch Manifold (PSM) bolted to the valve body.
7. The most common repair at this stage is to replace the PSM. Inspect the O-rings on the old part for swelling or damage upon removal.
8. If a new PSM does not resolve the code, further diagnosis of the wiring harness for a short-to-voltage or inspection of the valve body for a stuck valve is required.

Parts You'll Likely Need

• Transmission Pressure Switch Manifold (OEM #29546972) — This is the most common failure point for code P0873 on the Allison 1000. It contains the pressure switch that generates the fault. Part number 29546972 is an updated 4-switch design that replaces the original 6-switch design on 2000-2005 models and is used on later models as well.
  Trusted brands: Allison (Genuine), ACDelco
  OEM price range: $100-$150
  Aftermarket price range: $70-$120

Related Codes That Often Appear With This One

• P0700 — P0700 is a generic code indicating the TCM has requested the check engine light. It will almost always be present alongside a more specific transmission code like P0873.
• P0872, P0877, P0843, etc. — These are other pressure switch circuit codes. The provided TSB (PIP4379N) lists a large family of these codes that can appear together, often pointing to a common failure of the pressure switch manifold or a related electrical issue.

Technical Service Bulletins (TSBs) & Recalls

• PIP4379N: Mentions P0873 as part of a group of potential transmission codes and advises on diagnostic priority.

Platform-Specific Known Issues

• The NHTSA Technical Service Bulletin #PIP4379N, which applies to this vehicle range, directly lists P0873 as part of a group of potential transmission codes that may occur.
• A very common owner-reported symptom is that the truck drives normally when cold and only enters limp mode and sets the code after 15-20 minutes of driving, once the transmission fluid reaches operating temperature.

Mechanic-Grade Diagnostic Values

• PSM 'E' Switch Circuit Continuity — expected: Below 1 Ω. Failure: Resistance higher than 1 Ω indicates a bad wire or connection between the PSM connector and the TCM.
• PSM 'E' Switch Circuit to Ground — expected: 10 kΩ or higher (OL). Failure: A low resistance reading indicates a short to ground in the signal wire.
• Shift Solenoid C, D, or E Resistance — expected: 20 - 30 Ω. Failure: A reading outside this range indicates a faulty solenoid.
• Trim Solenoid A or B Resistance — expected: 5.5 - 8.0 Ω. Failure: A reading outside this range indicates a faulty trim solenoid.
• Speed Sensor (Engine, Turbine, Output) AC Voltage — expected: 150mV - 15V AC, varying with RPM. Failure: No voltage or voltage that doesn't change with RPM suggests a failed sensor or wiring issue.

Scan Tool Commands That Help

• Tech 2 / GDS2: Fast Adapt Relearn — After replacing the transmission, valve body, or Pressure Switch Manifold to allow the TCM to learn the hydraulic characteristics of the new components. Requires transmission temp to be 105°F or warmer.
• Tech 2 / GDS2: Live Data > Pressure Switch Status — To monitor the ON/OFF state of each pressure switch (PS1/C, PS2/D, PS3/E, PS4/R) with the engine on and off to see if the reported state matches the expected state. With key-on-engine-off, three switches should be open and one closed; this should invert when the engine is started.
• Push-Button Shifter: Check / Clear DTCs — To check and clear transmission codes without a scan tool. To check, turn key to ON (engine off) and press the UP and DOWN arrow buttons simultaneously twice. To clear, press and hold the MODE button for 10 seconds.

Wiring & Ground Locations

• TCM Connector J2, Pin 3 — On the Transmission Control Module (TCM). This is the RED connector of the two main TCM connectors.. This pin is the specific input for the Pressure Switch Manifold 'E' circuit (which corresponds to DTC P0873). The wire is typically white. Probing this pin is essential for end-to-end circuit diagnosis.
• TCM Connector J2, Pin 20 — On the Transmission Control Module (TCM), in the same RED connector.. This is the sensor ground (Analog Ground) for the Pressure Switch Manifold. A poor connection here can affect all pressure switch readings. The wire is typically green.
• G101 — Typically located at the left front of the engine, on the lower block.. This is a primary ground point for the Engine Control Module (ECM) and Transmission Control Module (TCM). A corroded or loose connection here can cause a wide range of powertrain issues, including erroneous transmission codes.
• G103 — Located at the left rear of the engine compartment on the cowl, often above the brake booster.. This ground serves the Data Link Connector (DLC). A poor ground here can cause issues with scan tool communication, hindering diagnosis.

OEM Part Supersession History

• 24226580 (or similar 6-switch design) → 29546972 (4-switch design) — The original 6-switch Pressure Switch Manifold used on 2000-2005 models was updated to a more reliable 4-switch design.
  Heads up: The new 4-switch manifold (29546972) is the correct service replacement for the older 6-switch design and is compatible with both 5-speed (2001-2005) and 6-speed (2006+) transmissions within this vehicle's range.

Model Year Variations Within This Range

• 2001-2005: These models use the Allison 1000 5-speed transmission and originally came with a 6-switch Pressure Switch Manifold. The valve body is specific to the 2000-2003 or 2004-2011 year breaks and is not interchangeable with later versions.
• 2006-2008: These models use the Allison 1000 6-speed transmission. The Neutral Start Back-up (NSBU) switch was moved to be an internal component (Internal Mode Switch) instead of an external one. The valve body design also differs from the early 5-speed models.