U05A3
Lost Communication With NOx Sensor Module BU05A3 is a generic OBD-II network diagnostic trouble code: Lost Communication With NOx Sensor Module B. It is logged by the engine control unit when the network monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What U05A3 means
U05A3 is set when the ECM or emissions control module detects that expected network messages from NOx sensor module B have not been received within the required time period. NOx sensor module B typically refers to the downstream (post-SCR catalyst) NOx sensor, which is used to measure the SCR system conversion efficiency by comparing outlet NOx concentration to the upstream input measured by sensor A.
The downstream NOx sensor plays a critical role in closed-loop SCR control and catalyst efficiency monitoring. Loss of this signal prevents the ECM from performing catalyst efficiency calculations, which will cause companion codes for SCR catalyst performance to set after a number of drive cycles without valid downstream NOx data.
The diagnostic approach mirrors that used for U04A4. Technicians should confirm power and ground at the sensor, inspect the communication harness which passes through or near the hot rear exhaust area, and verify scan tool communication with the sensor before considering sensor replacement. Thermal damage to connectors in the tailpipe area is a particularly common physical cause for downstream sensor communication faults.
Common causes
Most-frequently reported root causes when U05A3 is logged.
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1
Open or short circuit in the NOx sensor B CAN or SENT communication wire.
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2
Loss of power or ground at the downstream NOx sensor connector.
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3
Heat damage to the connector or harness near the rear silencer or exhaust.
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4
Corroded terminals at the NOx sensor B connector.
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5
Failed downstream NOx sensor module not transmitting.
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6
CAN bus segment fault affecting the downstream sensor node.
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7
Sensor incompatibility or missing adaptation after sensor replacement.
Symptoms drivers notice
How to diagnose U05A3
A typical diagnostic flow when this code is present.
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1
Retrieve all DTCs from all modules and compare to see whether U05A3 is isolated or part of a wider network fault.
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2
Verify battery voltage is within specification and check the fuse and relay for the downstream NOx sensor power supply.
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3
Inspect the NOx sensor B connector and the harness run in the rear exhaust area for heat damage, melted insulation, or corrosion.
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4
Attempt to access NOx sensor B live data via the scan tool to evaluate whether any communication is present.
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5
Measure continuity and check for shorts on the sensor communication wire between the sensor and the ECM or gateway module.
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6
If the circuit is electrically sound and power is present, substitute a correctly specified NOx sensor and retest.
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7
After repair, clear DTCs and run a complete drive cycle including highway cruise to confirm the fault does not return.
Related network codes
- U055A — Lost Communication With Windshield Wiper Control Module
- U0585 — Invalid Data Received From Active Grille Air Shutter Module "A"
- U0586 — Invalid Data Received From Active Grille Air Shutter Module "B"
- U058C — Invalid Data Received From Active Grille Air Shutter Module "C"
- U058D — Invalid Data Received From Active Grille Air Shutter Module "D"
- U0599 — Invalid Data Received From DC to DC Converter Control Module "A"
Frequently asked questions
Is U05A3 more or less serious than U04A4?
Both are communication faults of similar severity. U04A4 (upstream) affects dosing accuracy directly; U05A3 (downstream) affects efficiency monitoring. Either can eventually lead to deration if SCR performance cannot be confirmed.
Why is the downstream sensor more prone to heat damage?
The downstream NOx sensor is located after the SCR catalyst closer to the tailpipe. Exhaust temperatures in this area can still be high, and the sensor harness is often routed near the underfloor exhaust which experiences radiant heat during extended high-load driving.
Can I clear U05A3 without fixing it and pass an emissions inspection?
Clearing codes will reset readiness monitors. Without a completed drive cycle confirming no fault return, the SCR readiness monitor will show incomplete, which fails most OBD emissions checks.
Do all diesel vehicles have two NOx sensors?
Not all. Single NOx sensor setups exist on some systems where dosing is purely open-loop or estimated from other parameters. Dual-sensor closed-loop SCR systems are more common on Euro 6 and later vehicles due to tighter NOx limits.
Disabling U05A3 in software
RaceTune can permanently disable U05A3 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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