P20BD

Reductant Heater B Control Circuit/Open

P20BD is a generic OBD-II powertrain diagnostic trouble code: Reductant Heater B Control Circuit/Open. It is logged by the engine control unit when the scr/adblue monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.

Code
P20BD
Group
Powertrain
System
SCR/AdBlue
Severity
Warning (MIL on)
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What P20BD means

P20BD is an SAE generic powertrain code set by the ECM/PCM when it detects an open circuit condition in the control circuit for Reductant Heater B — the secondary heating element within the SCR (Selective Catalytic Reduction) AdBlue/DEF (Diesel Exhaust Fluid) system. The reductant heater is responsible for preventing the urea-based fluid from freezing in cold ambient temperatures, ensuring the SCR catalyst receives properly conditioned reductant for NOx conversion. The "B" designator distinguishes this circuit from the primary heater (covered by the P20B9 family), and typically refers to a second heater element, zone, or supply line heater depending on the vehicle architecture.

An open circuit fault means the ECM commanded the heater on but detected no current flow through the circuit — indicating a break in the wiring, a failed heater element presenting infinite resistance, a blown fuse, or a faulty heater control relay. This fault is part of the P20BD–P20C0 family: P20BD (open), P20BE (range/performance), P20BF (low), P20C0 (high), each targeting a different electrical failure mode on the same heater B circuit.

Because the SCR system cannot guarantee proper DEF delivery in sub-zero conditions without the heater, the ECM will log this code, illuminate the MIL, and may restrict vehicle operation if the emissions system is deemed non-functional. On Euro 6 and EPA Tier 4 platforms the fault typically triggers a warning lamp immediately; repeated or persistent faults can escalate to a derate or torque-limiting limp mode on commercial vehicles with stricter OBD enforcement.

Common causes

Most-frequently reported root causes when P20BD is logged.

  • 1
    Open circuit or broken wire in the reductant heater B supply or ground wiring harness
  • 2
    Failed reductant heater B element (internal open — element burned out or corroded open)
  • 3
    Blown fuse or fusible link in the reductant heater B circuit
  • 4
    Faulty heater control relay not passing current to the heater element
  • 5
    Corroded, loose, or damaged connector pins at the heater B connector or ECM harness
  • 6
    Short to ground upstream of the heater causing a protective fuse to blow
  • 7
    Defective ECM/PCM output driver for the heater B control circuit (rare)
  • 8
    Frozen or cracked DEF tank/pump module damaging the integrated heater wiring

Symptoms drivers notice

MIL (check engine light) illuminated
AdBlue/SCR system warning light or message on instrument cluster
DEF fluid may freeze in very cold temperatures causing SCR system faults
Increased NOx emissions (SCR efficiency reduced when reductant is not properly conditioned)
Possible torque derate or limp mode on heavy-duty or Euro 6 vehicles with strict SCR enforcement
OBD freeze frame data showing low ambient temperature at time of fault

How to diagnose P20BD

A typical diagnostic flow when this code is present.

  1. 1
    Retrieve all stored DTCs and freeze frame data; note ambient temperature at fault set and check for companion SCR/DEF codes (P20BE, P20BF, P20C0, P207F, P2BAD)
  2. 2
    Inspect the reductant heater B fuse and relay — replace any blown fuse and retest; if fuse blows again, suspect a short to ground in the heater circuit
  3. 3
    Visually inspect the wiring harness and connector to the reductant heater B element for chafing, corrosion, melted insulation, or backed-out pins; repair as needed
  4. 4
    Measure resistance of the heater B element across its terminals (disconnect harness first) — compare to manufacturer spec; an open (OL/infinite) reading confirms a failed heater element
  5. 5
    With harness disconnected at heater and at ECM, perform a continuity check on the supply wire and ground wire for the heater B circuit to isolate any open in the harness
  6. 6
    Check for battery voltage at the heater B connector supply wire when the ECM commands the heater on (cold soak test or scan tool actuator test); absent voltage points to relay or ECM driver fault
  7. 7
    Clear DTCs, perform a cold-start drive cycle (or freeze frame conditions), and confirm the fault does not return before releasing the vehicle

Vehicles where we've handled P20BD

Platforms in our catalogue with confirmed P20BD coverage.

AUDI A4 20D
VW TRANSPORTER
AUDI A6
2015
AUDI A6 30D
2015
AUDI A7
AUDI A7 30D

Related powertrain codes

Frequently asked questions

Can I drive with P20BD set?

Short-term driving is generally possible, but you should not ignore it. The MIL will remain on and the vehicle will fail an emissions inspection. In very cold weather, DEF fluid may freeze without the heater, causing the SCR system to shut down and potentially triggering a torque derate on commercial or Euro 6 vehicles. Have the fault diagnosed promptly.

Is P20BD the same as P20BF or P20BE?

No — they are related codes targeting the same Reductant Heater B circuit but for different electrical conditions. P20BD = open circuit (no current path); P20BE = range/performance (heater is present but not meeting expected output); P20BF = circuit low (signal or current below threshold); P20C0 = circuit high (signal or current above threshold). Diagnosis starts with the specific fault type indicated.

How much does it cost to fix P20BD?

Cost varies by root cause. A wiring repair or connector clean/re-pin may cost $50–$150 in labour. A replacement reductant heater element or integrated DEF pump/heater module typically runs $150–$600 in parts plus 1–2 hours of labour. An ECM driver fault is rare and expensive. Always confirm the root cause with resistance and continuity tests before replacing the heater module.

Could a bad DEF quality sensor cause P20BD?

No. P20BD is strictly a control circuit electrical fault (open circuit) for the heater B element. DEF quality or concentration faults set different codes (e.g. P203F, P204F). However, both conditions can co-exist if the DEF system has been neglected, so always read all stored DTCs and address them in logical order.

Disabling P20BD in software

RaceTune can permanently disable P20BD — 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 P20BD 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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