P2B68
NOx Sensor Circuit High Voltage Bank 1 Sensor 2P2B68 is a generic OBD-II powertrain diagnostic trouble code: NOx Sensor Circuit High Voltage 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 P2B68 means
P2B68 is stored when the aftertreatment or engine control module detects that the signal voltage from the downstream NOx sensor on Bank 1 (Sensor 2, post-SCR) is higher than the maximum calibrated threshold. This is the complementary fault to P2B67 and indicates that the measured signal on the sensor output circuit has risen above the expected operating range, making the reading implausible under current exhaust conditions.
A high voltage condition on the NOx sensor signal circuit is most commonly caused by a short to a reference or supply voltage on the signal wire, an open in the sensor signal return (ground) path causing the signal to float high, or a failed sensor that is internally producing an out-of-range output. It can also be caused by a wiring harness defect where the signal wire contacts a higher-voltage source such as an ignition or battery supply wire.
With the sensor signal stuck high, the module cannot correctly evaluate post-catalyst NOx levels and will typically suspend closed-loop SCR feedback control. This may result in suboptimal reductant dosing and elevated NOx tailpipe emissions. The MIL is illuminated and a freeze-frame is stored. Diagnosis should begin with a thorough inspection of the sensor harness and connector before the sensor itself is replaced.
Common causes
Most-frequently reported root causes when P2B68 is logged.
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1
Short to reference or supply voltage on the NOx sensor signal wire.
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2
Open in the sensor signal return (ground) circuit causing the signal to float high.
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3
Failed NOx sensor producing an internally saturated high output.
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4
Chafed signal wire contacting an ignition or battery power source.
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5
Damaged or corroded connector allowing unintended voltage on the signal pin.
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6
Incorrect sensor installation or substitution with an incompatible part number.
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7
Failed sensor controller unit generating an out-of-range output.
Symptoms drivers notice
How to diagnose P2B68
A typical diagnostic flow when this code is present.
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1
Retrieve all DTCs and freeze-frame data; note any correlated codes.
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2
Inspect the downstream NOx sensor connector and harness for signs of contact with voltage sources or corrosion.
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3
With the sensor connected and ignition on, measure signal circuit voltage; a reading above specification indicates a short to voltage or sensor failure.
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4
Disconnect the sensor and re-measure the signal circuit at the harness; if voltage drops to zero or reference level, the sensor is internally sourcing the high voltage.
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5
If voltage persists with the sensor disconnected, trace the signal wire for contact with a power supply.
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6
Verify the sensor ground return circuit is continuous and not open.
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7
Replace the sensor or repair wiring as appropriate, clear codes, and confirm normal signal range during engine operation.
Related powertrain codes
Frequently asked questions
Can P2B68 and P2B67 occur simultaneously?
These two codes represent opposite faults (signal too high vs. too low) and would not normally be active at the same time on the same sensor circuit. Concurrent storage may indicate intermittent wiring.
Could using the wrong replacement sensor cause P2B68?
Yes. Fitting an incompatible NOx sensor with a different output characteristic can generate out-of-range signals that trigger this code.
Does this fault affect the upstream NOx sensor?
No. P2B68 is specific to Bank 1 Sensor 2 (downstream, post-SCR). Bank 1 Sensor 1 (upstream) would have its own distinct fault codes.
Is this code emissions-related or mechanical?
It is an electrical/sensor circuit fault. The underlying issue is in the sensor or its wiring, not in a mechanical SCR or dosing component.
Disabling P2B68 in software
RaceTune can permanently disable P2B68 — 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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