The P0651 code in your 2020 Acura TLX indicates an open circuit in the sensor reference voltage A circuit. The ECM provides stable reference voltages (typically 5 volts) to various sensors throughout the engine. When this reference circuit is open, sensors that depend on it cannot function properly, affecting multiple engine management functions.
Understanding Reference Voltage
Your TLX's ECM supplies precise 5-volt reference power to sensors like the throttle position sensor, manifold absolute pressure sensor, and others. These sensors use this reference voltage as a baseline for their measurements. If the reference circuit is open, sensors receive no voltage and cannot provide accurate readings to the ECM.
Impact of an Open Reference Circuit
Multiple sensors may report zero or out-of-range readings simultaneously. The ECM may set multiple codes for various sensors, all stemming from the common reference circuit failure. Engine operation becomes erratic as the ECM receives no useful data from affected sensors. The engine may enter a severe limp mode or fail to start depending on which sensors are affected.
Common Causes
A broken wire in the reference voltage circuit is a common cause. Corroded or disconnected connectors at the ECM or sensor connections create opens. A failed ECM internal reference voltage regulator can't supply voltage to the circuit. Physical damage to wiring from engine work, rodent damage, or vibration wear causes opens.
Diagnostic Approach
Using Honda HDS or a compatible scanner, check which sensors are reporting abnormal readings. If multiple sensors on the same reference circuit show issues, the reference circuit is the likely culprit. Measure voltage at sensor reference pins—you should see 5 volts. If 0 volts, trace the circuit back toward the ECM to find the open location.