Wiring harness chafing, corrosion, or loose connector pins at the bypass position sensor or PCM; Internal wear or contamination in the EGR cooler bypass valve causing the sensor to read erratically; A failing bypass position sensor itself, especially due to heat and carbon exposure; Carbon buildup or mechanical binding in the bypass valve linkage that prevents smooth movement; Poor electrical ground for the sensor circuit; PCM software calibration issues or internal driver faults (less common)

DTC codes / P24xx · Auxiliary emission controls / P249B

P249BEGR Cooler Bypass Position Sensor Circuit Intermittent or Erratic Bank 2

P249B indicates that the powertrain control module has detected an intermittent or erratic signal from the EGR cooler bypass position sensor on bank 2. The sensor reports the physical position of the bypass valve that routes exhaust gas around the cooler, and the PCM sets this code when the signal jumps, drops out, or otherwise fails to behave predictably.

SystemPowertrain

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyIntermediate

AreaAuxiliary emission controls

Symptoms you may notice

Illumination of the check engine light, often with no other noticeable symptoms
Intermittent rough idle or hesitation during light acceleration
A slight decrease in fuel economy over time
Occasional surging or stumbling when the engine is cold
In rare cases, a noticeable lack of power or poor throttle response
Inspection or emissions readiness monitors may not complete

Common causes, most likely first

Wiring harness chafing, corrosion, or loose connector pins at the bypass position sensor or PCM
Internal wear or contamination in the EGR cooler bypass valve causing the sensor to read erratically
A failing bypass position sensor itself, especially due to heat and carbon exposure
Carbon buildup or mechanical binding in the bypass valve linkage that prevents smooth movement
Poor electrical ground for the sensor circuit
PCM software calibration issues or internal driver faults (less common)

How to diagnose and fix it

Inspect the bypass position sensor connector and wiring for damage, corrosion, or loose pins, and repair as needed
Check the sensor's reference voltage and ground circuits with a multimeter to confirm they are stable
Back-probe the sensor signal wire and monitor its voltage while manually moving the bypass valve to check for erratic output
Remove and clean the bypass valve and linkage to eliminate carbon binding, then retest
If the signal remains erratic, replace the bypass position sensor and clear the code
Verify the repair by running the EGR monitor and confirming the code does not return

What the bypass sensor signal really tells you

A common misdiagnosis is replacing the EGR cooler bypass valve assembly when the actual fault is a chafed wire near the exhaust manifold that only acts up when hot.
The PCM may set P249B even when the bypass valve moves freely, because the sensor's internal potentiometer can develop dead spots that only appear at certain temperatures.
Carbon buildup from extended oil change intervals or a failing EGR cooler can cause the bypass valve to stick intermittently, mimicking a sensor failure.
Bank 2 refers to the side of the engine that does not contain cylinder number one, so on V6 and V8 engines the sensor is often harder to reach and more exposed to heat.
A weak battery or poor ground can cause erratic sensor readings that set this code without any physical fault in the bypass system.

Can I keep driving with P249B?

You can usually continue driving with P249B, but avoid heavy acceleration and long uphill pulls until the fault is repaired, because an erratic bypass valve can affect emissions and, in some cases, engine performance. If the check engine light starts flashing or you notice severe power loss, stop driving and seek service immediately.

Tools that help

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