P2A73

NOx Sensor Circuit Range/Performance Bank 1 Sensor 2

P2A73 is a generic OBD-II powertrain diagnostic trouble code: NOx Sensor Circuit Range/Performance Bank 1 Sensor 2. It is logged by the engine control unit when the powertrain monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P2A73
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P2A73 means

P2A73 is set when the powertrain control module (PCM) or diesel exhaust aftertreatment control module detects that the downstream NOx sensor signal on Bank 1 (Sensor 2, post-SCR catalyst) is out of the expected operating range or is not performing as anticipated under current exhaust conditions. The NOx sensor is a critical feedback element for the selective catalytic reduction (SCR) system, monitoring the conversion efficiency of the catalyst and the adequacy of reductant dosing.

The sensor generates a voltage or CAN signal proportional to the NOx concentration in the exhaust stream. When the module determines that the output is implausible relative to engine load, exhaust temperature, or cross-checked against other sensors (such as the upstream NOx sensor or oxygen sensor), it flags this code. Common triggers include a deteriorated sensor element, contamination from reductant crystallization, or wiring harness faults causing offset readings.

Continued operation with this fault may result in non-compliant NOx emissions and can mask SCR catalyst degradation. The malfunction indicator lamp (MIL) is illuminated and a freeze-frame is stored. Addressing root cause promptly is important to maintain SCR efficiency and avoid potential catalyst damage from uncorrected dosing errors.

Common causes

Most-frequently reported root causes when P2A73 is logged.

  • 1
    Faulty or deteriorated downstream NOx sensor (Bank 1 Sensor 2).
  • 2
    NOx sensor element contaminated by reductant (DEF/AdBlue) crystallization.
  • 3
    Open, short, or high resistance in the NOx sensor wiring harness or connector.
  • 4
    Corroded or damaged NOx sensor connector pins.
  • 5
    Failed or degraded SCR catalyst skewing sensor plausibility checks.
  • 6
    PCM or aftertreatment control module software fault or calibration error.
  • 7
    Exhaust leaks upstream of the sensor altering NOx concentration readings.

Symptoms drivers notice

MIL (check engine light) illuminated.
Possible increase in NOx tailpipe emissions.
SCR system may operate in open-loop mode, increasing reductant consumption.
No noticeable drivability change in most cases.
Diagnostic freeze-frame data stored in the control module.

How to diagnose P2A73

A typical diagnostic flow when this code is present.

  1. 1
    Connect a scan tool, retrieve all stored DTCs, and record freeze-frame data.
  2. 2
    Inspect the downstream NOx sensor connector and wiring harness for corrosion, damage, or reductant crystallization.
  3. 3
    Check NOx sensor live data and compare upstream vs. downstream values under steady cruise conditions.
  4. 4
    Verify exhaust system integrity; check for leaks between the SCR catalyst and the sensor bung.
  5. 5
    Inspect the SCR catalyst condition and check for reductant deposit buildup in the dosing system.
  6. 6
    If wiring and exhaust integrity are confirmed good, replace the downstream NOx sensor and retest.
  7. 7
    Clear codes, perform a drive cycle, and confirm no re-occurrence.

Related powertrain codes

Frequently asked questions

Can I drive with P2A73 active?

The vehicle is generally driveable, but NOx emissions will be elevated and SCR dosing accuracy is compromised. Repair promptly to avoid catalyst damage.

Is P2A73 always caused by a bad sensor?

Not always. Wiring faults, exhaust leaks, and reductant system contamination are common causes that should be checked before replacing the sensor.

Will P2A73 cause limp mode?

Typically not immediately, but if combined with other SCR faults it can trigger a torque reduction or reductant warning sequence depending on the vehicle manufacturer.

How do I distinguish a sensor fault from a catalyst fault?

Compare upstream and downstream NOx sensor readings. If the upstream sensor is plausible but downstream reads unexpectedly high or low, the catalyst or dosing system should also be evaluated before condemning only the sensor.

Disabling P2A73 in software

RaceTune can permanently disable P2A73 — 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.

Permanent
The monitor is disabled in the ECU itself — not just cleared. It cannot return.
Tailored to your file
Each patch is matched to your specific software version — never a one-size-fits-all file.
Reversible
The original file is always preserved. Reflash the stock to return the ECU to factory state.

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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