U0672 on 2020-2024 Cadillac CT5: Coolant Pump Communication Loss Causes and Fixes

What's Unique About the 2020-2024 Cadillac CT5

The Cadillac CT5, along with a wide range of other GM vehicles from this era using the 2.0L LSY and 2.7L L3B engines, is prone to this specific communication failure. The Active Thermal Management system replaces a traditional belt-driven water pump and thermostat with a sophisticated, variable-speed electric pump. While efficient, its reliance on a LIN bus data connection makes it susceptible to electrical faults. GM has issued multiple Technical Service Bulletins (TSBs) acknowledging that the root cause is often not the pump itself, but a poor connection at the pump's electrical connector or a fault in the battery-top fuse block. This makes checking the wiring a mandatory first step before considering a costly pump replacement.

Diagnostic Flowchart

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

Symptoms You May Notice

• Check Engine Light (Malfunction Indicator Lamp) is on.
• Engine temperature gauge rising rapidly or reading high.
• Message on the driver information center about engine overheating or 'Engine Power Reduced'.
• Engine entering a reduced power or 'limp' mode.
• Loss of cabin heat as hot coolant is not circulated to the heater core.
• Cooling fans running at high speed continuously, even when the engine is not hot.

Most Likely Causes

1. Poor Connection at the Electric Engine Coolant Pump 🔴 High Probability → Shop Engine Water Pump This is a widely known issue acknowledged by GM in TSBs #PIP5863B and #PIP5863C. The connector terminals can suffer from poor tension, contamination (dirt, oil), or fretting corrosion, leading to an intermittent or total loss of communication.
   How to confirm: Visually inspect the 4-pin connector (GM schematic ID X327) at the pump for damage, dirt, or loose pins. A common diagnostic step, per the TSB, is to simply unplug and securely replug the connector ('cycling the connection'), clear the code, and see if it returns.
   Typical fix: Clean the connector terminals with electrical contact cleaner and apply dielectric grease to prevent future moisture intrusion. If the connection remains faulty or terminals have lost tension, the connector pigtail may need to be replaced.
   Est. part cost: $0-$40
2. Failed Battery-Mounted Fuse Block 🟡 Medium Probability → Shop Vehicle Battery The electric coolant pump is powered by a dedicated 40-amp fuse located in the fuse block that sits directly on top of the battery. This block can develop internal faults, especially after a battery replacement, causing a voltage drop or open circuit even if the fuse itself looks fine.
   How to confirm: With the engine running, carefully wiggle the fuse block and its main cables. A diagnostic video for a related GMC Sierra showed a massive 5-7 volt drop when the fuse block was wiggled, confirming an internal failure. If codes appeared right after a battery change, this part is highly suspect.
   Typical fix: Replace the entire battery-mounted fuse block assembly. The individual fuses within are often not serviceable.
   Est. part cost: $50-$150
3. Failed Electric Engine Coolant Pump 🟡 Medium Probability → Shop Engine Water Pump The control module (GM RPO code K61) is integrated into the pump assembly. If the module fails internally from an electrical short or circuit board defect, it will stop communicating and require replacement of the entire pump unit. An internal short can also blow the main fuse.
   How to confirm: If the wiring, fuse block, and connector are all confirmed to be good (power and ground are present at the connector) and the code persists after cycling the connection, the pump assembly is the next logical point of failure.
   Typical fix: Replace the electric engine coolant pump assembly and the corresponding gaskets.
   Est. part cost: $350-$700
4. Damaged Wiring Harness ⚪ Low Probability GM has noted issues with wire harness chafing in various models. TSB 21-NA-149 (for related trucks) warns that the harness can rub through where it crosses the camshaft carrier cover or near the generator bracket, causing shorts or open circuits that can trigger U0672.
   How to confirm: Perform a continuity test on the power, ground, and LIN bus communication wire running to the coolant pump using a multimeter and a vehicle-specific wiring diagram. Visually inspect the harness along its entire length for rub marks, especially near engine brackets and covers.
   Typical fix: Repair the damaged section of the wiring harness and protect it from future chafing using convoluted tubing or re-routing.
   Est. part cost: $5-$50

Diagnosis Steps

1. Read all Diagnostic Trouble Codes (DTCs) with a compatible OBD-II scanner. Note any other LIN bus codes like U1348.
2. Locate the Electric Engine Coolant Pump. It is electronically controlled, not belt-driven, and typically found on the lower side of the engine.
3. Inspect the battery-top fuse block. Check the dedicated 40-amp fuse for the coolant pump circuit. Be aware that the entire block can fail internally, so test for voltage drop if possible.
4. Following GM TSB #PIP5863C, locate and carefully inspect the pump's 4-pin electrical connector (X327) for any signs of corrosion, contamination, backed-out pins, or poor terminal tension.
5. Cycle the connector by unplugging it, cleaning the pins with electrical contact cleaner, and plugging it back in securely. Apply dielectric grease.
6. Clear the U0672 code with the scanner.
7. Start the engine and let it run to see if the code returns. If it does not, the connection or fuse block was the likely issue. GM authorizes this as a warranty repair.
8. If the code returns, inspect the wiring harness leading to the pump for any signs of chafing, melting, or physical damage, paying close attention to where it might rub against the camshaft carrier or engine brackets as noted in related TSBs.
9. If all wiring and connections are confirmed to be good, the Electric Engine Coolant Pump assembly is the most probable cause of the failure and should be replaced.

Parts You'll Likely Need

• Electric Engine Coolant Pump (OEM #12738954 (latest), supersedes 12724523) — If the integrated control module fails, communication is lost. This is the required fix after all wiring and connection issues have been ruled out.
  Trusted brands: ACDelco (GM Genuine Part)
  OEM price range: $350-$450
  Aftermarket price range: $250-$400
• Battery-Mounted Fuse Block — Can fail internally, causing an open circuit to the coolant pump even if the fuse appears intact. A common failure point after a battery replacement.
  Trusted brands: ACDelco (GM Genuine Part)
  OEM price range: $50-$150
  Aftermarket price range: $40-$100
• Electrical Connector Pigtail — If the connector itself is damaged or terminals have lost tension, replacing the pigtail is necessary to ensure a solid connection.
  Trusted brands: ACDelco
  OEM price range: $20-$40
  Aftermarket price range: $15-$30

Related Codes That Often Appear With This One

• U1348 — This code indicates a fault on 'LIN Bus 4'. Since the electric coolant pump communicates on a LIN bus, a failing pump module or a wiring issue can disrupt the entire bus, causing this code to appear alongside U0672.
• P26BB — This code relates to the coolant flow control valve, which is part of the same Active Thermal Management system. A communication failure with the main pump can sometimes lead to faults being flagged in related components.

Technical Service Bulletins (TSBs) & Recalls

• PIP5863C: Advises technicians that a Malfunction Indicator Lamp (MIL) with code U0672 is likely due to a connection issue at the Electric Engine Coolant Pump and to inspect and cycle it first. It lists a wide range of affected GM vehicles.
• PIP5863B: An earlier version of the same bulletin with the same advice for code U0672.
• 21-NA-149: While for trucks, it provides valuable insight into potential harness chafing locations on the 2.7L L3B engine that could cause this code.

Platform-Specific Known Issues

• GM has issued Technical Service Bulletin #PIP5863C specifically for code U0672 across numerous vehicles with LSY and L3B engines. It advises technicians to inspect the Electric Engine Coolant Pump connection and cycle it before performing any other diagnostics, as this is the most common fix.
• A separate TSB for related truck platforms (#21-NA-149) highlights known engine harness chafing points that can cause various electrical codes, including U0672.

Mechanic-Grade Diagnostic Values

• LIN Bus Voltage (at rest, key on) — expected: Approx. 11-12 Volts (about 1V less than battery system voltage).. Failure: A steady voltage of 0V (short to ground), or a voltage equal to full battery voltage (short to power).
• LIN Bus Voltage (communicating) — expected: Voltage rapidly toggling between a high of ~12V and a low of ~1V.. Failure: A steady, non-toggling voltage indicates a lack of communication from either the ECM or the pump module.
• Ground Circuit Resistance at Pump Connector — expected: Below 0.1 Ω.. Failure: Resistance higher than 0.1 Ω indicates a poor ground connection, which can cause communication errors.

Scan Tool Commands That Help

• GDS2 (Global Diagnostic System 2): Output Control / Bidirectional Control — After checking for codes, a technician can use this function to directly command the electric coolant pump on and off. If the pump activates with the scan tool but not during normal engine operation, it suggests a problem with the ECM's command signal or sensor inputs. If the pump does not respond to the command despite having good power and ground, the pump's internal control module has failed.
• GDS2 (Global Diagnostic System 2): Data Display / Module Status — Used to view the live data parameters from the coolant pump module. If the scan tool shows the module as 'Not Communicating' or the data fields are blank or nonsensical, it confirms the U0672 fault and points the diagnosis toward the LIN bus wiring, power, or ground for that module.

Wiring & Ground Locations

• X327 — The 4-pin electrical harness connector that plugs directly into the Electric Engine Coolant Pump assembly.. This is the single most common failure point for U0672. TSB #PIP5863C specifically instructs technicians to inspect and cycle this connection due to high instances of poor terminal tension, corrosion, or contamination causing the communication loss.
• Battery-Mounted Fuse Block — Sits directly on top of the positive terminal of the vehicle's battery.. This block contains the dedicated 40-amp fuse that provides main power to the electric coolant pump. The entire block can fail internally, creating an open circuit even if the fuse looks good. It is a high-suspicion part if the code appears after a battery replacement or terminal service.
• Engine Block Ground (G1xx series) — A ground wire or strap connecting the engine wiring harness to the engine block or cylinder head. GM ground points in the engine bay are typically designated with a 'G1' prefix (e.g., G101, G104).. The coolant pump requires a solid ground connection to function and communicate properly. A loose or corroded engine ground can cause a variety of electrical issues, including communication codes like U0672. Verifying ground resistance is a key diagnostic step.

Real Owner Repair Stories

• YouTube user 'PAULY'S AUTO' (GM Truck with 2.7L L3B Engine (same engine family and issue as CT5)) — Check Engine Light with LIN bus codes related to the coolant pump.
  ❌ Tried (didn't work) Initial assumption was a bad pump.
  ✅ What actually fixed it Diagnosed a massive 5-7 volt drop across the battery-top fuse block when it was wiggled. The internal fuse connection was bad, requiring replacement of the entire fuse block assembly to restore proper power to the pump.
• YouTube user 'Casi que motor' (2022 Chevrolet Silverado 1500 2.7L) — Check Engine Light with code U0672.
  ✅ What actually fixed it The electric water pump had an internal short to ground. This short caused the main fuse in the battery-top fuse block to blow. The final repair required replacing both the failed water pump and the battery-top fuse block.

OEM Part Supersession History

• 12724523, 55508938, 55510114, 55515496, 55515871 → 12738954 — Part revision and consolidation of previous numbers into a single updated component.