U0073 on 2008-2009 Pontiac G8: Control Module Communication Failure Causes and Fixes

What's Unique About the 2008-2009 Pontiac G8

The Pontiac G8, based on the Australian Holden Commodore (VE), has a specific electrical architecture. While U0073 is a standard GM network code, G8 owners have reported it in conjunction with other codes like P0700 (TCM requesting MIL), pointing towards potential issues with modules or wiring harnesses specific to this platform. A GM technical document, #PIT4730, specifically lists the 2008-2009 G8 and links U0073 to symptoms like intermittent no-starts, door locks cycling, and the 'Service Stabilitrak' message. The TSB notes that intermittent shorts in the data circuits can cause the BCM, in particular, to appear offline. Forum discussions on G8board.com and LS1tech.com confirm these symptoms are common precursors to a U0073 code.

Symptoms You May Notice

• Check Engine Light is on
• "Service Stabilitrak" message on the driver information center (DIC)
• Transmission may shift erratically or get stuck in one gear (limp mode)
• Instrument cluster gauges may freeze or drop to zero simultaneously
• Vehicle may not start or may be difficult to start
• Power door locks may cycle on their own
• ABS and Traction Control warning lights may illuminate
• Phantom battery drain, where the battery dies over a couple of days

Most Likely Causes

1. Weak or Failing Battery 🔴 High Probability → Shop Vehicle Battery Control modules are sensitive to voltage. A battery that is old, has a low state of charge, or has corroded terminals can cause modules to drop off the communication network, especially during startup. This is a very common first-check for any 'U' code on GM vehicles.
   How to confirm: Test the battery voltage with a multimeter. A healthy, fully charged battery should read approximately 12.6 volts with the engine off and 13.7-14.7 volts with the engine running. Have the battery load-tested at an auto parts store, as a battery can show good voltage but fail under load.
   Typical fix: Charge or replace the battery. Ensure battery terminals and ground connections are clean and tight.
   Est. part cost: $150-$250
2. Damaged or Corroded Wiring Harness 🟡 Medium Probability The GMLAN communication wires (a twisted pair) can become chafed, corroded, or pinched. GM TSB #PIT4730 mentions a potential for a pinched harness near the instrument panel (I/P) carrier mount on the G8's left side. Owners on forums have also reported issues with corrosion in connectors.
   How to confirm: Visually inspect wiring harnesses for damage, especially around the engine bay, transmission, and under the dashboard near the steering column. A professional would use a multimeter to check for 60 ohms of resistance across pins 6 and 14 of the OBD-II port (with the battery disconnected) to verify the integrity of the CAN bus terminating resistors. 🎬 Watch: A guide to testing CAN bus resistance with a multimeter. A reading of 120 ohms suggests a break in the circuit or a problem with one of the two terminating resistors (located in the ECM and EBCM), while a reading near 0 ohms indicates a short circuit.
   Typical fix: Repair the damaged section of the wiring harness. This may involve soldering, heat-shrinking, and re-securing the harness.
   Est. part cost: $10-$50 for repair materials
3. Faulty Control Module 🟡 Medium Probability Any module on the high-speed network can fail, shorting out the bus or simply stop communicating. On the G8 and its platform mates, the Body Control Module (BCM) and Electronic Brake Control Module (EBCM/ABS) are common points of failure that can bring down the network. A failing BCM can also cause phantom battery drains.
   How to confirm: This requires a process of elimination using a high-end scan tool. A technician will view the network status and unplug modules one by one until communication is restored to the remaining modules. This isolates the faulty unit. Sometimes a burnt electronic smell can be detected near the failed module.
   Typical fix: Replace the failed module. The new module (e.g., BCM, EBCM, TCM) will require programming by a dealer or a qualified shop with GM software to match the vehicle's VIN.
   Est. part cost: $200-$800 depending on the module
4. Poor Ground Connection ⚪ Low Probability Corroded or loose ground straps for the engine, body, or specific modules can create electrical noise and communication faults. A G8 forum member on GRRRR8.net identified several key ground locations, including on the cylinder heads and front chassis rails, that should be inspected. Some owners have added a secondary ground wire from the cylinder head to the frame to resolve electrical issues.
   How to confirm: Inspect all major ground points between the engine, chassis, and battery. Key locations on the G8 include the right front chassis rail (G101/G102), the EBCM bracket (G103), the jump start post (G104), and grounds on both cylinder heads (G110/G112). Clean any corrosion and ensure all connections are tight.
   Typical fix: Clean the ground connection point to bare metal and tighten the fastener.
   Est. part cost: $1-$5 for cleaning supplies

Rare But Worth Checking

• Aftermarket OBD-II Devices:

Diagnosis Steps

1. Check Battery Health: Ensure the battery is fully charged (12.6V+) and the charging system is working correctly (13.7-14.7V). A weak battery is a common cause of communication codes.
2. Scan for All Codes: Use a high-quality OBD-II scanner to read codes from all available modules (Engine, Transmission, ABS, Body). Note all U-codes present, as they can help pinpoint which module is offline.
3. Visual Inspection: Thoroughly inspect the main wiring harnesses in the engine bay, under the dash, and near the transmission for any signs of damage, chafing, or corrosion. Pay close attention to the area near the I/P carrier mount as noted in TSB #PIT4730.
4. Check Fuses: Inspect all fuses related to the various control modules (ECM, BCM, TCM, ABS) in both the under-hood and trunk-mounted fuse boxes.
5. Check CAN Bus Resistance (Advanced): Disconnect the battery. Use a multimeter to measure the resistance between Pin 6 (CAN High) and Pin 14 (CAN Low) on the OBD-II port. A healthy network should read approximately 60 ohms. A reading of 120 ohms indicates an open circuit or a missing terminating resistor (the G8 has terminating resistors in the ECM and EBCM). A reading near 0 ohms indicates a short circuit between the two CAN wires.
6. Isolate the Faulty Module (Professional Level): If the resistance is incorrect, a technician will disconnect modules one by one from the GMLAN network until the resistance returns to 60 ohms. The last module disconnected is likely the source of the fault. This requires knowledge of module locations and connectors.
7. Check Ground Connections: Inspect and clean the main ground points, particularly G103 on the EBCM bracket and the grounds on the cylinder heads.

Parts You'll Likely Need

• Body Control Module (BCM) (OEM #15948438, 25910474, 25892622) — The BCM is a central communication hub on the G8 and acts as a gateway between networks. Its failure is a common cause for bringing down the entire GMLAN network and can cause many strange electrical
🎬 See this walkthrough on how to install a new BCM.

Related Codes That Often Appear With This One

• P0700 — This code means the Transmission Control Module (TCM) has detected a fault and requested the Check Engine Light. It often appears with U0073 when the TCM loses communication with the ECM.
• U0100 — Lost Communication with ECM/PCM. This is a more specific version of U0073, indicating the ECM is the module that cannot be reached.
• U0121 — Lost Communication with Anti-Lock Brake System (ABS) Control Module. This often points to a fault with the EBCM or its wiring, a known issue on this platform.
• U0140 — Lost Communication With Body Control Module (BCM). This is a very common companion code, as the BCM is a central hub for many network communications and a known failure point.

Technical Service Bulletins (TSBs) & Recalls

• PIT4730 / PIT4730G: 'Additional Diagnostic Information For U0140 Or U0073'. This TSB covers multiple GM vehicles, including the 2008-2009 G8. It describes symptoms like intermittent no-start, cycling door locks, and frozen gauges linked to these codes. It specifically points to intermittent shorts in the GMLAN data circuits or a pinched harness near the instrument panel carrier mount as potential causes.

Platform-Specific Known Issues

• Real Owner Experience: Intermittent No-Start and P0700/U0073: A user on GRRRR8.net reported their 2009 G8 was having trouble starting and eventually wouldn't turn over, throwing codes P0700 and U0073. They had the battery tested, which came back fine despite being a little low on charge (12.3V). This highlights that even a slightly weak battery can trigger these codes, and a simple voltage check isn't always enough; a load test is crucial.
• Known Wiring Chafe Point from GM TSB #PIT4730: A well-documented issue covered in GM TSB #PIT4730 points to a pinched wiring harness as a cause for U0073 and U0140. The harness can be chafed between the Instrument Panel (I/P) carrier and the body mount, about 14 inches from the display on the left side of the dash. Technicians are advised to remove the instrument panel cover to inspect this specific area for damage when these codes are present with symptoms like cycling door locks or gauges freezing.
• Forum Deep Dive: Importance of Ground Locations: A detailed thread on GRRRR8.net, with input from Australian users familiar with the Holden platform, lists critical ground locations. User 'Crazy Paul' identifies G103 (on the EBCM bracket) and grounds on the cylinder heads (G110/G112) as essential engine/system grounds. Another user in a separate thread mentioned that checking the ground from the passenger side cylinder head to the body had solved a previous problem for them. This corroborates that poor grounds are a valid and specific area to check on the G8.

Mechanic-Grade Diagnostic Values

• GMLAN Bus Resistance — expected: ~60 Ω (measured between DLC pins 6 and 14 with battery disconnected). Failure: A reading of ~120 Ω indicates an open circuit or one of the two terminating resistors is offline. A reading near 0 Ω indicates the CAN High and CAN Low wires are shorted together.
• GMLAN Bus Voltage (Recessive State / Idle) — expected: Both CAN High (Pin 6) and CAN Low (Pin 14) wires should be at ~2.5 V.. Failure: Significant deviation from 2.5 V on either line at idle indicates a short to power or ground.
• GMLAN Bus Voltage (Dominant State / Communicating) — expected: CAN High (Pin 6) toggles to ~3.5 V while CAN Low (Pin 14) toggles to ~1.5 V.. Failure: Failure to reach these approximate voltages during communication (viewed on an oscilloscope) points to a wiring issue, interference, or a faulty module.

Scan Tool Commands That Help

• GM Tech 2 / GDS2: Module Status / Network Test — This is the first step to see which modules are actively communicating on the network and which are offline. This helps narrow the search for the source of the U0073 code.
• GM Tech 2 / GDS2: Brake Pedal Position (BPP) Sensor Calibration — This is a mandatory procedure after installing a new or used BCM to ensure the brake lights and stability control systems function correctly.
• GM Tech 2 / GDS2: SDM Primary Key Relearn — This is a mandatory procedure after installing a new or used BCM. It syncs the new BCM with the airbag control module (SDM). Failure to perform this will result in an airbag warning light and a disabled SRS system.
• GM SPS (Service Programming System): Module Programming — When replacing a major control module like the BCM or EBCM, it must be programmed with the vehicle's VIN and correct software calibration. This function loads the necessary files into the new module.

Wiring & Ground Locations

• DLC (Data Link Connector) — Under the driver's side dashboard, to the left of the steering column.. This is the primary access point for diagnosis. High-speed GMLAN is on Pin 6 (CAN High) and Pin 14 (CAN Low). Resistance and voltage tests are performed here.
• Terminating Resistors — There are two 120-ohm resistors. One is internal to the Engine Control Module (ECM). The other is at the opposite end of the bus, typically within the Electronic Brake Control Module (EBCM/ABS module) on this platform.. These resistors are required for the network to function. If one is offline due to a module failure or wiring break, the total bus resistance will change from 60 ohms to 120 ohms, causing communication failures.
• GMLAN Wires — The high-speed GMLAN wires are a twisted pair, typically a Dark Green wire and a Tan or Tan/Black wire, running between all high-speed modules.. The twisted pair design helps cancel out electromagnetic interference. Any damage, shorts, or opens in this pair will bring down the network.
• C3 (X3) Connector — Located on the back of the left instrument panel junction block (fuse box area).. This is a professional diagnostic point. Disconnecting this connector splits the GMLAN bus into two halves. This allows a technician to determine which half of the vehicle's network contains the fault, dramatically speeding up diagnosis.

Real Owner Repair Stories

• YouTube channel 'Anything Auto' (2009 Pontiac G8) — No start after an alternator replacement, multiple warning lights, and a U0140 code (Lost Communication with BCM). The scan tool could not communicate with the BCM at all.
  ❌ Tried (didn't work) Checking all related fuses in the engine bay and trunk fuse boxes.
  ✅ What actually fixed it The Body Control Module (BCM) was faulty. Replacing it with a pre-programmed used unit restored communication with the scan tool and allowed the vehicle to run properly again. The technician noted the BCM is located behind the driver's side lower dash and is held in by clips, not bolts.

OEM Part Supersession History

• 15948438, 15872388, 25892622, 10382479 → Multiple part numbers exist and may have been superseded over time. — Part revisions and software updates.
  Heads up: The specific part number is less important than the programming. A replacement BCM, whether new or used, MUST be programmed with the vehicle's VIN using GM's Service Programming System (SPS). Failure to program will result in a no-start condition or multiple system failures.

Model Year Variations Within This Range

• 2008-2009: No significant electrical or module variations have been documented between the 2008 and 2009 model years that would alter the diagnosis or common causes of a U0073 code. The BCM, EBCM, and network architecture are consistent across this range.

Diagnostic Flowchart

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: For expensive electronic modules like the Body Control Module (BCM) or Electronic Brake Control Module (EBCM), a used part from a reputable salvage yard or online reseller is a very smart, cost-effective choice. However, it is critical that the module is programmed to your vehicle's VIN.

Donor-vehicle mileage cap: roughly under 150000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• Purchase from a seller who offers VIN-programming as a service before shipping.
• Ensure the seller offers a warranty or guarantee of functionality.
• Visually inspect the received part for any signs of water damage, case damage, or corrosion on the connector pins before installation.

OEM-only on this vehicle (don't cheap out):

• Body Control Module (BCM)
• Electronic Brake Control Module (EBCM)
• Engine Control Module (ECM)

Aftermarket brands forum-validated for this vehicle:

• For control modules, there are no widely trusted aftermarket brands; using genuine new or used/reprogrammed GM parts is strongly recommended to ensure compatibility and avoid further network issues.

Brands owners have reported issues with on this vehicle:

• Avoid unbranded, 'plug-and-play' modules from marketplaces that do not require VIN programming. These are highly likely to be incompatible and cause system conflicts.

Real Owner Stories

Aggregated from forums and TSBs cited above. Mileages and costs reflect what owners reported in those sources.

Documented NHTSA Reports

Related OBD-II Codes

• U0140 — Lost Communication With Body Control Module. This code is specifically mentioned in TSB #PIT4730 as appearing alongside U0073, often caused by the same pinched wiring harness issue. The BCM is also a common module to fail and bring down the network.
• P0700 — Transmission Control System Malfunction. This is a request from the TCM to turn on the Check Engine Light. It frequently accompanies U0073 when the TCM loses communication, leading to symptoms like erratic shifting or limp mode.
• U0100 — Lost Communication With ECM/PCM. The Engine Control Module (ECM) contains one of the two critical 120-ohm terminating resistors for the CAN bus. If it fails or loses connection, the entire network can go down, triggering U0073 in other modules.
• U0121 — Lost Communication With Anti-Lock Brake System (ABS) Control Module. The EBCM (ABS module) is a common failure point on the G8 and contains the second terminating resistor. Its failure will cause a network fault, triggering U0073 and illuminating the ABS and Traction Control warning lights.
• U0155 — Lost Communication With Instrument Panel Cluster (IPC) Control Module. This code is directly related to the symptom of instrument cluster gauges freezing or dropping to zero, which occurs when the IPC module drops off the communication bus.

Frequently Asked Questions