P0128 on Cadillac Lyriq: Symptoms, Causes & DIY Fix
Official technical diagnostic wire for fault code P0128 (P0128: Coolant Temperature Below Thermostat Regulating Temperature) specifically affecting Cadillac Lyriq vehicles equipped with Ultium Dual Motor AWD (500HP / 450 lb-ft) / Single Motor RWD (340HP).
1. Symptoms of P0128 on Cadillac Lyriq
When the Engine Control Module (ECM/PCM) on your Cadillac Lyriq (Ultium Dual Motor AWD (500HP / 450 lb-ft) / Single Motor RWD (340HP)) logs fault code P0128, drivers commonly observe:
2. Most Likely Causes on Cadillac Lyriq
Historical warranty claims and technical repair bulletins for Cadillac Lyriq point to the following failure distribution:
3. Step-by-Step DIY Repair Guide for Cadillac Lyriq
Ensure coolant is topped up when engine is completely cold. on the Cadillac Lyriq equipped with Ultium Dual Motor AWD (500HP / 450 lb-ft) / Single Motor RWD (340HP).
Read live coolant temp; should reach 195°F-215°F within 10 minutes. on the Cadillac Lyriq equipped with Ultium Dual Motor AWD (500HP / 450 lb-ft) / Single Motor RWD (340HP).
Drain 1 gallon coolant, unbolt thermostat housing, install new OEM thermostat with jiggle valve up. on the Cadillac Lyriq equipped with Ultium Dual Motor AWD (500HP / 450 lb-ft) / Single Motor RWD (340HP).
Use spill-free funnel to burp air bubbles out of the heater core and engine block. on the Cadillac Lyriq equipped with Ultium Dual Motor AWD (500HP / 450 lb-ft) / Single Motor RWD (340HP).
Clear codes and confirm heater blows 130°F+ hot air at idle. on the Cadillac Lyriq equipped with Ultium Dual Motor AWD (500HP / 450 lb-ft) / Single Motor RWD (340HP).
4. Cadillac Lyriq P0128 FAQs
What causes code P0128 to trigger on a Cadillac Lyriq?
On the Cadillac Lyriq (Ultium Dual Motor AWD (500HP / 450 lb-ft) / Single Motor RWD (340HP)), P0128 is most frequently caused by Thermostat stuck open or Faulty Engine Coolant Temperature (ECT) sensor. Factory sensor degradation or wiring harness wear near the engine bay are common factors.
Can I safely drive my Cadillac Lyriq with code P0128?
Yes, but heater may blow lukewarm and fuel mileage will drop. Given that P0128 carries a Low severity rating on Cadillac vehicles, we advise performing diagnostic scanning via the OBD-II port located under driver steering column beneath knee bolster before extended highway driving.
Where is the diagnostic port located on the Cadillac Lyriq?
The 16-pin OBD-II DLC connector on the Cadillac Lyriq is situated under driver steering column beneath knee bolster. It requires no special tools to plug in a standard scanner.
How much does it cost to fix P0128 on a Cadillac Lyriq?
DIY replacement parts typically range between $120 - $300 (DIY: $25 - $60). If serviced at an authorized Cadillac dealership, expect an additional $120–$250 in professional labor based on their $165/hr hourly rate.