P27CC

NOx Sensor Circuit Low (Bank 1 Sensor 2)

P27CC 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.

Code
P27CC
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P27CC means

P27CC is set when the ECM detects that the output signal voltage from the downstream NOx sensor (Bank 1, Sensor 2) has fallen below the minimum expected threshold for the operating conditions. The NOx sensor produces a variable signal representing the concentration of nitrogen oxides in the exhaust stream downstream of the SCR catalyst, and a persistently low signal is interpreted as either a sensor failure or a circuit fault.

A low circuit condition can originate from a short to ground in the signal wire, an open in the sensor reference or power supply circuit, internal sensor element failure, or poor connector contact causing intermittent signal dropout. The ECM uses this downstream sensor signal to regulate DEF injection into the SCR system; a low or absent signal disrupts closed-loop NOx control.

When P27CC is active the ECM typically defaults DEF dosing to an open-loop strategy, which may over-dose or under-dose DEF, potentially causing ammonia slip or elevated NOx emissions respectively. If the fault persists across multiple drive cycles, an engine power derate may be commanded depending on vehicle manufacturer calibration.

Common causes

Most-frequently reported root causes when P27CC is logged.

  • 1
    Short to ground on the NOx sensor signal circuit wire.
  • 2
    Open circuit in the sensor supply voltage or reference wire.
  • 3
    Failed NOx sensor with an internally shorted sensing element.
  • 4
    Corroded or backed-out terminal in the NOx sensor connector.
  • 5
    Chafed wiring harness causing an intermittent ground fault.
  • 6
    Excessive DEF crystallization bridging connector terminals.
  • 7
    ECM internal fault affecting the NOx sensor input circuit.

Symptoms drivers notice

MIL illuminated with P27CC stored.
DEF system or SCR warning indicator active.
No or very low NOx reading shown on scan tool live data for Sensor 2.
Possible ammonia smell from the exhaust due to DEF over-dosing in open-loop.
Potential engine power reduction after prolonged fault duration.

How to diagnose P27CC

A typical diagnostic flow when this code is present.

  1. 1
    Retrieve all DTCs and note freeze frame data with a scan tool before any repairs.
  2. 2
    Check the downstream NOx sensor live data value; a reading at or near zero when exhaust is hot is suspect.
  3. 3
    Inspect the sensor connector and wiring harness for corrosion, damage, or DEF crystal contamination.
  4. 4
    Measure signal voltage at the connector with the sensor connected; compare to manufacturer specification.
  5. 5
    Perform a resistance check on the signal wire between ECM and sensor for continuity and shorts to ground.
  6. 6
    Verify sensor heater operation by checking heater resistance and supply voltage.
  7. 7
    Replace the NOx sensor if the circuit checks pass and the sensor output remains low.

Vehicles where we've handled P27CC

Platforms in our catalogue with confirmed P27CC coverage.

BMW 320D
2016

Related powertrain codes

Frequently asked questions

Is P27CC the same as a completely dead NOx sensor?

A low circuit code often indicates an electrical fault in the circuit, which may or may not include a failed sensor. The circuit must be checked before condemning the sensor.

Can DEF crystallization cause P27CC?

Yes. DEF deposits can bridge connector terminals or corrode pins, creating a partial short that pulls the signal low.

Does P27CC affect DEF consumption?

It can. With the closed-loop NOx feedback unavailable, the ECM may over-dose DEF in open-loop mode, increasing consumption.

What tools are needed to diagnose this code?

A scan tool capable of live NOx sensor data, a digital multimeter for circuit voltage and resistance tests, and a wiring diagram for the NOx sensor circuit are essential.

Disabling P27CC in software

RaceTune can permanently disable P27CC — 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.

ECUs with a P27CC 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

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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