P27B9

NOx Sensor Heater Control Circuit High Bank 1 Sensor 2

P27B9 is a generic OBD-II powertrain diagnostic trouble code: NOx Sensor Heater Control Circuit High 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
P27B9
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P27B9 means

P27B9 is stored when the control module detects a high-voltage condition on the heater control circuit for the NOx sensor located downstream of the SCR catalyst on bank 1 (sensor 2 position). NOx sensors contain an internal heater element to bring the sensing cell to its operating temperature rapidly, and the module actively controls this heater via a dedicated driver circuit.

A high-circuit condition indicates that the voltage on the heater control line is elevated beyond the expected level when the heater should be commanded off, or that the heater drive voltage is consistently above specification during operation. This typically points to a short to power in the control wiring, a faulty heater relay, or a problem within the NOx sensor module itself, as many NOx sensors incorporate internal electronics that manage the heater independently.

While the NOx sensor may continue to provide some output, unreliable heater operation can slow warmup and degrade the accuracy of NOx readings used for SCR closed-loop dosing. Extended operation with this fault may result in accumulated SCR efficiency codes and eventual regulatory derate.

Common causes

Most-frequently reported root causes when P27B9 is logged.

  • 1
    Short to voltage on the NOx sensor heater control circuit wiring (bank 1, sensor 2).
  • 2
    Faulty heater relay supplying uncontrolled power to the sensor heater.
  • 3
    Internal NOx sensor electronic module fault causing heater driver anomaly.
  • 4
    Corroded or damaged connector creating an unintended voltage path.
  • 5
    PCM or SCR module internal heater driver circuit fault.
  • 6
    Aftermarket wiring modification introducing voltage to the heater circuit.

Symptoms drivers notice

MIL illuminated.
Possible SCR efficiency or NOx sensor performance codes if heater malfunction affects sensor readings.
DEF warning or NOx system alert on the instrument cluster in some vehicles.
No direct driveability symptom in most cases.
Potential torque derate if SCR system cannot confirm emissions compliance.

How to diagnose P27B9

A typical diagnostic flow when this code is present.

  1. 1
    Read all DTCs; note any companion NOx sensor performance or SCR efficiency codes.
  2. 2
    Inspect the NOx sensor (bank 1, sensor 2) wiring harness and connector for chafing, corrosion, or short to power.
  3. 3
    With the heater circuit commanded off by the scan tool, measure voltage on the heater control wire; voltage present confirms a short to power in the harness.
  4. 4
    Check the NOx sensor heater relay for proper operation and verify it opens when de-energized.
  5. 5
    Monitor the NOx sensor heater current PID live to see if the heater is drawing current when it should not be.
  6. 6
    If harness and relay are confirmed good, test the NOx sensor module supply and ground circuits.
  7. 7
    Replace the NOx sensor if the internal heater driver is confirmed faulty after ruling out external wiring issues.

Vehicles where we've handled P27B9

Platforms in our catalogue with confirmed P27B9 coverage.

BMW 320D
2016

Related powertrain codes

Frequently asked questions

Is the NOx sensor heater circuit the same as the O2 sensor heater circuit?

They are similar in concept but separate circuits. NOx sensors operate at higher temperatures and often include onboard electronics that manage the heater, making diagnosis slightly more complex than for a conventional oxygen sensor.

Can I drive with P27B9 active?

Generally yes in the short term, but if the NOx sensor heater fails completely, cold SCR system operation will degrade NOx conversion accuracy, potentially triggering additional faults and derate.

What is bank 1 sensor 2 for a NOx sensor?

Bank 1 is the bank containing cylinder 1. Sensor 2 is the downstream sensor, located after the SCR catalyst, used to monitor aftertreatment efficiency.

Will replacing the NOx sensor fix this code?

Only if the fault is internal to the sensor module. If the wiring has a short to power, replacing the sensor will not resolve the fault and the new sensor could be damaged.

Disabling P27B9 in software

RaceTune can permanently disable P27B9 — 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 P27B9 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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