P29EB

Reductant Heater Control Circuit High

P29EB is a generic OBD-II powertrain diagnostic trouble code: Reductant Heater Control Circuit High. 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
P29EB
Group
Powertrain
System
Powertrain
Severity
Warning (MIL on)
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What P29EB means

P29EB indicates that the control module has detected a higher-than-expected voltage or current on the control circuit for the reductant (DEF/AdBlue) heating element. The reductant heater is critical in cold climates to prevent the urea solution from freezing, since DEF freezes at approximately -11 degrees Celsius. The ECM or SCR module monitors the heater circuit for proper electrical operation and sets this fault when the circuit voltage or resistance is outside acceptable limits in the high direction.

A circuit-high condition typically points to an open circuit in the low-side driver path, a failed heater control relay stuck open, wiring harness damage causing loss of ground, or an internal fault in the SCR control module itself. It can also result from a connector that has corroded or pulled apart on the heater ground path, preventing the module from pulling the circuit low when the heater is commanded on.

In cold operating environments this fault is particularly problematic because it may prevent DEF from liquefying after a cold soak, causing the system to enter a no-dosing state and eventually triggering NOx-related inducement. Diagnosis should focus on the heater circuit wiring and relay before suspecting the module.

Common causes

Most-frequently reported root causes when P29EB is logged.

  • 1
    Open circuit or broken wire in the reductant heater low-side control path.
  • 2
    Failed heater control relay stuck in the open position.
  • 3
    Corroded or disconnected connector at the reductant tank heater or module.
  • 4
    Loss of heater circuit ground causing a false high-voltage reading.
  • 5
    Faulty SCR/reductant control module with internal driver fault.
  • 6
    Damaged wiring harness from heat, abrasion, or rodent damage near the reductant tank.

Symptoms drivers notice

MIL illuminated, especially noted after cold-weather operation.
DEF may remain frozen in cold temperatures, causing SCR system to disable dosing.
Possible additional SCR or NOx DTCs stored if system cannot dose.
No obvious drivability symptoms in warm weather when heater is not needed.
Heater inoperative when tested via scan tool bi-directional output test.

How to diagnose P29EB

A typical diagnostic flow when this code is present.

  1. 1
    Record all DTCs and freeze frame data with a scan tool.
  2. 2
    Inspect the reductant heater connector and wiring harness for corrosion, damage, or disconnection.
  3. 3
    Check for proper ground continuity at the heater circuit ground point.
  4. 4
    Test the heater control relay for proper operation and verify it is receiving a command signal.
  5. 5
    Measure heater circuit voltage with the heater commanded on to confirm high-side voltage is present.
  6. 6
    Perform a resistance check of the heater element itself to verify it is within specification.
  7. 7
    If wiring and relay check out, suspect the SCR control module and consult OEM service data.

Vehicles where we've handled P29EB

Platforms in our catalogue with confirmed P29EB coverage.

BMW 320D
2016

Related powertrain codes

Frequently asked questions

Does P29EB affect driving in warm weather?

In warm weather the heater is not needed, so there may be no immediate drivability impact, but the MIL will remain on and the system will be unprotected in cold conditions.

What is the reductant heater for?

DEF freezes at about -11 degrees C. The heater melts frozen DEF so the system can resume dosing after a cold soak.

Can a bad relay cause P29EB?

Yes. A relay stuck open prevents the module from completing the heater circuit, which the module interprets as a high-circuit condition.

Is P29EB the same as P29EA?

No. P29EA typically refers to a circuit-low condition on the reductant heater, while P29EB refers to a circuit-high condition, indicating different electrical faults.

Disabling P29EB in software

RaceTune can permanently disable P29EB — 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 P29EB 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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