P202B
Tank Heater Control Circuit LowP202B is a generic OBD-II powertrain diagnostic trouble code: Tank Heater Control Circuit Low. 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 P202B means
P202B is a SAE generic powertrain diagnostic trouble code that sets when the Engine Control Module (ECM) or Selective Catalytic Reduction (SCR) control module detects a lower-than-expected voltage on the reductant (AdBlue/DEF) tank heater control circuit. The reductant tank heater is a resistive heating element that prevents the aqueous urea solution from freezing in cold ambient conditions; without it, the SCR system cannot deliver reductant to the catalyst, causing NOx emissions to exceed limits.
A "circuit low" fault typically indicates a short to ground, an open circuit in the heater element itself, or a wiring fault between the control module and the heater. The control module monitors return voltage when it commands the heater on; if the measured voltage falls below a calibrated threshold, it flags P202B and illuminates the MIL. Depending on the vehicle manufacturer's SCR failure strategy, the module may also begin a NOx reagent dosing inhibit countdown, eventually inducing a torque or speed derestriction as mandated by emissions regulations.
P202B is closely related to P202A (circuit high) and P202C (circuit open). Because the reductant tank heater is essential for cold-climate SCR operation, this fault should be diagnosed before the onset of freezing temperatures to avoid AdBlue delivery failure and potential limp-mode conditions mandated by on-board NOx monitoring.
Common causes
Most-frequently reported root causes when P202B is logged.
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1
Short to ground in the reductant tank heater supply or signal wiring
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2
Open circuit or broken wire in the heater control circuit harness
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3
Failed (open or shorted) reductant tank heater element
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4
Corroded, damaged, or loose connector at the reductant tank heater plug
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5
Faulty reductant tank assembly (integrated heater element non-serviceable on some platforms)
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6
Defective SCR control module or ECM output driver for the heater circuit
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7
Blown fuse or relay in the reductant heater power supply circuit
Symptoms drivers notice
How to diagnose P202B
A typical diagnostic flow when this code is present.
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1
Connect a scan tool and confirm P202B is stored; note any related SCR codes (P202A, P202C, P20EE, P203F) and freeze-frame data including ambient temperature at time of fault
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2
Inspect the reductant tank heater connector and wiring harness for corrosion, backed-out pins, chafing, or visible damage; repair any faults found and retest
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3
With the circuit unplugged, measure heater element resistance across its terminals against manufacturer specification (typically 2–10 Ω); an open reading (OL) or shorted reading (near 0 Ω) indicates a failed heater element
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4
Check for continuity and shorts to ground in the wiring between the heater connector and the control module; repair or replace damaged harness sections as required
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5
Verify fuse and relay integrity in the reductant heater power supply circuit; replace any blown fuses and recheck
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6
If wiring and heater element test within spec, perform a module-commanded heater activation with a factory-level scan tool while monitoring circuit voltage; a voltage drop below threshold with a known-good circuit points to a faulty ECM/SCR module output driver
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7
Clear codes, perform a complete drive cycle including a cold-start if possible, and confirm no return of P202B before returning the vehicle
Vehicles where we've handled P202B
Platforms in our catalogue with confirmed P202B coverage.
Related powertrain codes
- P2000 — NOx Adsorber Efficiency Below Threshold Bank 1
- P2001 — NOx Adsorber Efficiency Below Threshold Bank 2
- P2002 — Diesel Particulate Filter Efficiency Below Threshold Bank 1
- P2003 — Diesel Particulate Filter Efficiency Below Threshold Bank 2
- P2004 — Intake Manifold Runner Control Stuck Open Bank 1
- P2005 — Intake Manifold Runner Control Stuck Open Bank 2
Frequently asked questions
Can I drive with P202B stored?
In warm weather you can typically drive normally because the reductant tank heater is only needed to prevent freezing. However, in cold climates the AdBlue solution may freeze without a working heater, preventing SCR dosing. Some vehicles will enter a torque derate or speed limiter after a set number of drive cycles with an active SCR fault, so the fault should be repaired promptly.
Does P202B always mean the heater element needs replacing?
Not necessarily. A short to ground or an open in the wiring harness, a corroded connector, or a blown fuse can all trigger a circuit-low fault without the heater element itself being faulty. Always inspect the wiring and connector first and measure the circuit before condemning the heater or the tank assembly.
Is P202B the same as P202A or P202C?
No. All three codes relate to the reductant tank heater control circuit, but they indicate different fault types: P202A is circuit high (voltage above threshold), P202B is circuit low (voltage below threshold), and P202C is circuit open (no continuity detected). Diagnosis of each begins with a visual inspection but diverges based on the specific electrical fault direction.
Will clearing P202B with a scan tool fix the problem?
Clearing the code removes the stored fault but does not repair the underlying electrical fault. If the root cause (damaged wiring, failed heater element, etc.) is not corrected, the code will return within one or a few drive cycles once the ECM re-runs the heater circuit monitor.
Disabling P202B in software
RaceTune can permanently disable P202B — 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.
ECUs with a P202B 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 EDC17C74 verified 2 software versions
- Bosch EDC17CP44 verified 1 software version
- Bosch MD1CP004 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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