P29EA

NOx Sensor Heater Control Circuit Low (Bank 1 Sensor 1)

P29EA is a generic OBD-II powertrain diagnostic trouble code: NOx Sensor Heater Control Circuit Low (Bank 1 Sensor 1). 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
P29EA
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P29EA means

P29EA is set when the ECM detects that the heater control circuit for the upstream NOx sensor (Bank 1, Sensor 1) is reading a lower voltage than expected when the heater is commanded on. The NOx sensor requires an integral heater to bring the sensing element up to its operating temperature quickly and to maintain accurate NOx measurement across a wide range of exhaust temperatures. Without proper heater function, the sensor output is unreliable, particularly at low exhaust temperatures during cold starts or light load operation.

A circuit low condition on the heater circuit typically points to a short to ground in the heater supply wire, an open in the heater power feed, or a failed heater element inside the sensor itself. The ECM monitors the current or voltage on the heater output to detect these conditions. When the fault is confirmed, the ECM may disable NOx sensor feedback and revert to open-loop SCR dosing, which reduces efficiency and can increase real-world NOx emissions.

Because NOx sensor heaters draw significant current, it is important to verify that the power supply and relay circuits for the heater are intact before replacing the sensor. A blown fuse or relay failure can produce a heater circuit low fault without any fault in the sensor itself.

Common causes

Most-frequently reported root causes when P29EA is logged.

  • 1
    Short to ground in the NOx sensor heater supply wire.
  • 2
    Failed heater element inside the NOx sensor with low resistance causing excess current draw.
  • 3
    Blown heater circuit fuse causing zero voltage at the heater terminal.
  • 4
    Failed heater relay preventing voltage supply to the sensor heater.
  • 5
    Corroded or damaged heater circuit connector reducing effective voltage.
  • 6
    Open circuit in the heater ground return path.
  • 7
    ECM driver output fault affecting heater circuit control.

Symptoms drivers notice

MIL illuminated with NOx heater circuit DTC stored.
NOx sensor possibly reporting inaccurate values at low exhaust temperatures.
SCR dosing may revert to open-loop strategy reducing system efficiency.
Possible companion NOx sensor performance or efficiency DTC.
No immediately noticeable driveability change in most cases.

How to diagnose P29EA

A typical diagnostic flow when this code is present.

  1. 1
    Read all DTCs and freeze frame data; note any companion NOx sensor performance codes.
  2. 2
    Check the heater circuit fuse and relay for the NOx sensor heater supply.
  3. 3
    Measure heater supply voltage at the NOx sensor connector with the ignition on; low or zero voltage with a good fuse points to a wiring or relay fault.
  4. 4
    Measure heater element resistance at the sensor connector; compare to specification.
  5. 5
    Inspect the heater circuit wiring harness for chafing, shorts, or corrosion.
  6. 6
    Verify heater ground circuit continuity back to the chassis ground point.
  7. 7
    Replace NOx sensor if heater element resistance is out of specification and circuit supply voltage is confirmed correct.

Vehicles where we've handled P29EA

Platforms in our catalogue with confirmed P29EA coverage.

BMW 320D
2016

Related powertrain codes

Frequently asked questions

Does a heater fault mean the NOx sensor element is also bad?

Not necessarily. The heater and sensing element are separate components within the sensor. A heater fault does not always mean the sensing element is defective.

Can I drive with P29EA?

Driving is possible but NOx sensor accuracy may be degraded, particularly when the exhaust is cold. SCR efficiency can decrease as a result.

Will P29EA cause the DEF to stop being injected?

The ECM typically continues DEF injection in open-loop mode, but the feedback control that optimizes dosing based on NOx sensor data is lost.

What causes a NOx sensor heater to fail early?

Thermal cycling, vibration, and moisture ingestion are common causes of premature heater failure. Sensors mounted near exhaust joints or in locations prone to water splash may fail sooner.

Disabling P29EA in software

RaceTune can permanently disable P29EA — 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 P29EA 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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