OBD-II trouble code
U040B: Invalid Data Received From Exhaust Gas Recirculation Control Module 'A'
A module is receiving messages from exhaust gas recirculation (EGR) control module 'A', but the data inside them is implausible or out of range. The connection is alive — the content is wrong. Emissions-relevant.
Quick facts
- System
- Network
- Category
- Network Communication
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $100 – $900
- DIY difficulty
- Shop recommended
What does U040B mean?
Exhaust gas recirculation (EGR) reduces oxides of nitrogen (NOx) by routing a metered amount of inert exhaust gas back into the intake to lower peak combustion temperatures. On vehicles that use a dedicated EGR control module — common on diesels and some larger gasoline engines — that module drives the EGR valve, watches EGR valve position, flow, and temperature, and reports its status to the engine control module over the network. The 'A' designation identifies the primary EGR controller on engines with more than one. U040B sets when a receiving module is still hearing from EGR control module 'A', but the data in its messages is invalid: a value is out of range, implausible, or contradicts what other modules see. The link is alive; the content can't be trusted. That is the key difference from a lost-communication code, which means the module has gone completely silent.
Because the fault is bad data rather than a dead bus, the causes cluster around whatever makes the module broadcast wrong information. A failing input — an EGR position sensor, a differential-pressure (DPFE) sensor, or an EGR temperature sensor — can push the module into reporting values other modules reject. Low system voltage is a classic trigger, since module logic gets unreliable as voltage sags. Heavy carbon buildup in the EGR passages can make actual valve behavior disagree with commanded position, producing implausible feedback. The module's own software can be at fault if it is outdated, corrupted, or was never properly programmed after a replacement, and corroded connectors or chafed wiring in the hot engine-bay harness can corrupt otherwise-good messages in transit.
Symptoms depend on which data is invalid. You may see a check engine light, rough idle, hesitation, or a slight loss of power, and the vehicle will typically fail an emissions test because EGR is an emissions function. On diesels, EGR faults can interact with the wider emissions strategy. The vehicle usually remains driveable, but because this code is emissions-relevant it should not be left indefinitely. U040B is frequently a secondary code — read the full list, because a companion EGR flow, position, or temperature code (P0401, P0403, P040x) often names the real root cause.
Common causes
- Failing EGR position, differential-pressure (DPFE), or temperature sensor feeding the module bad data
- Heavy carbon buildup making actual EGR behavior disagree with commanded position
- Low system voltage or a weak battery/charging system
- Corroded connectors or chafed wiring in the EGR/engine-bay harness
- Outdated, corrupted, or mismatched EGR control module software
- EGR control module replaced without proper programming
- Electrical noise or damaged bus wiring corrupting messages in transit
- EGR control module internal fault
Symptoms
- Check engine light illuminated
- Rough idle, hesitation, or a slight loss of power
- Failed emissions test (EGR is an emissions function)
- Possible increase in NOx and, on diesels, interaction with the wider emissions strategy
- Companion EGR flow, position, or temperature codes stored alongside U040B
- Vehicle generally still driveable
Diagnostic steps
- 1.Read ALL stored codes first — U040B is often secondary to an EGR flow, position, or temperature code (P0401, P0403, P040x) that names the bad signal.
- 2.Check battery and charging system voltage; low voltage is a common cause of implausible module data.
- 3.Use live data to compare commanded EGR position against reported position and flow.
- 4.Inspect the EGR valve and passages for heavy carbon buildup that could cause implausible feedback.
- 5.Inspect the EGR/engine-bay harness and connectors for corrosion, chafing, and heat damage.
- 6.Verify the module has the correct, current calibration, especially after a recent replacement or flash.
- 7.Address any companion EGR codes before condemning the module itself; the underlying fault often clears U040B.
Repair cost
$100 – $900
Cost depends on what is producing the bad data. An EGR position, DPFE, or temperature sensor typically runs $100-$350 including diagnosis. EGR valve cleaning or replacement runs $150-$500. Correcting low voltage or a charging fault is $150-$600. A module reflash is usually $100-$250, and EGR control module replacement with programming is the expensive case at $400-$900 — but that should only follow thorough diagnosis, since U040B is frequently a secondary code.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with egr valve replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
Leave this one to a qualified shop. It typically involves emissions-critical components, refrigerant handling, or other work that requires manufacturer-grade tooling, training, or certification. DIY attempts often produce a more expensive problem than the original code.