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DTC codes / U06xx · Network communication / U0651

U0651Lost Communication With EGR Sensor A

This code means the engine control module has stopped receiving valid data from the exhaust gas recirculation sensor on the 'A' circuit, which is typically the sensor that reports EGR valve position or EGR flow. The module sets the code after detecting missing or implausible messages from that sensor over a defined number of ignition cycles, not necessarily a hard failure of the sensor itself.

SystemNetwork
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaNetwork communication

Symptoms you may notice

  • Illumination of the check engine light, often with no other noticeable symptoms
  • Rough idle or stumbling when the engine is cold, especially if the EGR valve is stuck open
  • A slight hesitation or surge during light-throttle acceleration
  • Increased nitrogen oxide emissions that may cause a failed emissions inspection
  • In some vehicles, reduced fuel economy or a mild loss of power
  • Intermittent warning light that comes and goes with vibration or temperature changes

Common causes, most likely first

  1. Damaged, corroded, or disconnected wiring or connector pins at the EGR sensor or its harness
  2. Failed EGR sensor itself, such as an internal open circuit or worn potentiometer
  3. Poor electrical ground or reference voltage supply shared with other sensors on the same circuit
  4. Carbon buildup or mechanical binding in the EGR valve that prevents the sensor from reporting a plausible position
  5. Faulty engine control module software or a module that has lost communication on the sensor bus
  6. Aftermarket electrical accessories or poor previous repairs that disturbed the sensor circuit

How to diagnose and fix it

  1. Connect a scan tool and record all stored and pending codes, then check live data for the EGR sensor signal while wiggling the harness
  2. Visually inspect the EGR sensor connector, pins, and wiring for corrosion, chafing, or loose terminals, and repair as needed
  3. Measure reference voltage and ground at the sensor connector with a multimeter to confirm the circuit is complete
  4. Check the EGR valve for carbon binding and clean or replace it if the sensor cannot move through its full range
  5. Substitute a known-good EGR sensor or perform a resistance sweep test on the existing sensor to confirm failure
  6. Clear codes and perform a relearn or drive cycle to verify the communication fault does not return
  7. If the circuit and sensor test good, inspect the engine control module connectors and software for communication issues

What the EGR sensor code hides

  • A loose or corroded ground wire shared with the EGR sensor circuit often causes this code even when the sensor itself tests perfectly on the bench.
  • Carbon holding the EGR valve partially open can make the sensor report an implausible position, so the module flags a communication loss rather than a range fault.
  • This code can be set by a low battery or a jump-start event that momentarily drops module voltage, and it may not return after clearing.
  • Aftermarket alarm or remote-start wiring tapped into the sensor harness is a frequent hidden cause that visual inspection of the factory loom will miss.
  • Do not replace the EGR sensor before checking the reference voltage and ground at the connector, because a bad ground will destroy a new sensor's signal just as quickly.

Can I keep driving with U0651?

The vehicle is generally safe to drive with this code because it does not disable the engine, but avoid long trips or heavy loads until the fault is repaired. Continued driving with a stuck-open EGR valve can cause rough running and increased emissions, so schedule diagnosis soon.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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