P2A79
NOx Sensor Circuit Low Bank 1 Sensor 2P2A79 is a generic OBD-II powertrain diagnostic trouble code: NOx Sensor Circuit Low 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.
What P2A79 means
P2A79 is set when the ECU detects that the signal from the downstream NOx sensor on Bank 1 (Sensor 2, post-SCR catalyst) is below the minimum expected voltage or signal level. This is a hard circuit fault indicating the sensor output has dropped outside the electrically valid range, as opposed to a rationality or slow-response fault.
A low circuit signal can result from a short to ground in the signal wire, an open circuit in the sensor supply voltage wire, internal sensor failure causing the output to collapse, or a damaged sensor heater that prevents the sensing element from reaching operating temperature and producing a stable signal. Water intrusion into the connector is a frequent contributor on post-catalyst sensors that are exposed to condensate.
With the downstream NOx sensor inoperative, the ECU cannot monitor SCR catalyst conversion efficiency or confirm that tail-pipe NOx is within compliance limits. The MIL is illuminated, OBD catalyst monitoring is disabled for this bank, and the vehicle may accumulate derate trigger conditions depending on calibration. Prompt repair is required for both emissions compliance and regulatory OBD monitor readiness.
Common causes
Most-frequently reported root causes when P2A79 is logged.
-
1
Short to ground on the sensor signal wire causing the output to read at or near zero.
-
2
Open circuit in the sensor reference voltage supply wire collapsing the signal.
-
3
Internal sensor failure with the sensing element or signal conditioning circuit producing no output.
-
4
Water or moisture intrusion into the connector causing a leakage path to ground.
-
5
NOx sensor heater failure preventing element warm-up and stable signal generation.
-
6
Chafed wiring harness grounding the signal conductor against the exhaust or chassis.
-
7
Corroded sensor connector pins causing high resistance or open circuit on the signal line.
Symptoms drivers notice
How to diagnose P2A79
A typical diagnostic flow when this code is present.
-
1
Connect a scan tool and verify the downstream NOx sensor live data reads at minimum or zero with ignition on.
-
2
Inspect the sensor connector and wiring harness for moisture intrusion, corrosion, or physical damage.
-
3
Measure signal wire voltage at the connector with the sensor disconnected to identify a short to ground.
-
4
Check the sensor reference supply voltage and ground circuit continuity.
-
5
Test the sensor heater resistance and confirm the heater circuit is receiving proper voltage.
-
6
Disconnect the sensor and recheck circuit voltages to determine whether the fault is in the sensor or the wiring.
-
7
Replace the downstream NOx sensor if wiring and connector checks are satisfactory and the fault remains sensor-side.
Vehicles where we've handled P2A79
Platforms in our catalogue with confirmed P2A79 coverage.
Related powertrain codes
- P2A00 — O2 Sensor Circuit Range/Performance, Bank 1 Sensor 1
- P2A01 — O2 Sensor Circuit Range/Performance - Bank 2 Sensor 1
- P2A02 — Sensor Circuit Range/Performance Bank 1 Sensor 3
- P2A03 — Sensor Circuit Range/Performance Bank 2 Sensor 1
- P2A04 — Sensor Circuit Range/Performance Bank 2 Sensor 2
- P2A05 — Sensor Circuit Range/Performance Bank 2 Sensor 3
Frequently asked questions
Can moisture in the connector cause P2A79?
Yes. Post-catalyst sensors are exposed to condensate and road spray. Moisture in the connector can create a leakage path to ground, pulling the signal low and setting this code.
Is P2A79 the same as P2A77?
No. P2A77 is a slow-response performance fault for Bank 1 Sensor 2, while P2A79 is a circuit-low fault for the same sensor. Both affect the downstream sensor but for different reasons.
Will a failed heater cause the signal circuit to read low?
An unheated NOx sensor may produce an unstable or low signal because the sensing element is below operating temperature. Heater faults can thus indirectly cause or accompany a signal-low code.
Can I use live data to confirm P2A79?
Yes. A genuine circuit-low fault will show the sensor reading stuck at zero or minimum in live data even with the engine at normal operating temperature and NOx present in the exhaust.
Disabling P2A79 in software
RaceTune can permanently disable P2A79 — 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.
ECUs with a P2A79 disable in our catalogue
Confirmed coverage from our recipe database — we support many more families. Upload your file and our identifier will match it automatically.
- Bosch EDC17C50 verified 1 software version
- Bosch EDC17C56 verified 1 software version
- Bosch MD1CP002 verified 1 software version
- Bosch MD1CS001 verified 1 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.
Got P2A79 in your scan?
Upload your ECU file — we'll identify the exact software version and confirm whether a disable is available for your car.
Upload your file