U05A5: Invalid Data Received From NH3 Sensor

What it means

A receiving module detected implausible data from the ammonia (NH3) slip sensor downstream of the SCR. Concentration values fail plausibility checks and are rejected.

At a glance

• Usually lights the warning light of the system that lost communication
• Affects emissions; the car can fail an emissions test

How it works

The NH3 slip sensor measures ammonia downstream of the SCR catalyst so the vehicle can monitor how well the exhaust aftertreatment is controlling ammonia. This code sets when a receiving module rejects the sensor's reported concentration as implausible, so ammonia monitoring may be unreliable.

Common causes

Most likely first.

1. Module internal fault or sensor input drift
2. Software version mismatch after replacement
3. Gateway module routing CAN messages incorrectly

Symptoms

• Multiple warning lights may illuminate
• Affected modules may go into limp mode or shut down
• Specific functions inoperative depending on module
• Diagnostic tools may show communication faults

How to diagnose it

1. Save freeze-frame data and scan all modules for SCR, NH3 sensor, and communication codes to identify when the invalid reading occurred.
2. Compare live NH3 concentration and related SCR or exhaust data with operating conditions, and note whether the values are missing or implausible.
3. Inspect the downstream NH3 sensor connector, harness, and nearby CAN wiring for heat damage, corrosion, loose pins, or poor grounds.
4. Check related SCR and exhaust sensor faults that could make the reported NH3 concentration appear inconsistent.
5. Verify that the receiving module, NH3 sensor, and gateway have compatible software and configuration, particularly after replacement.
6. If the readings, wiring, and configuration appear sound, use manufacturer-approved network tests to distinguish a sensor control-unit fault from a receiving-module or gateway fault.

Common fixes

• Repair damaged or corroded NH3 sensor or CAN wiring and connector faults.
• Resolve related SCR or exhaust sensor faults that are distorting the NH3 plausibility check.
• Apply the correct software or configuration to a replaced sensor, receiving module, or gateway.
• Replace the NH3 sensor control unit, gateway, or receiving module only after testing confirms the fault.

Can I keep driving?

You can usually drive cautiously if the vehicle runs normally, but arrange diagnosis soon because SCR ammonia monitoring may be degraded and emissions warnings or restrictions can follow. Avoid extended or heavy-load trips if power is limited or additional exhaust warnings appear.