P20BB

Reductant Heater A Control Circuit Low

P20BB is a generic OBD-II powertrain diagnostic trouble code: Reductant Heater A Control Circuit Low. 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
P20BB
Group
Powertrain
System
SCR/AdBlue
Severity
Warning (MIL on, possible limp mode)
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What P20BB means

P20BB is a SAE generic powertrain code that sets when the Engine Control Module (ECM) detects an abnormally low voltage on the control circuit for Reductant Heater A — the heating element responsible for keeping Diesel Exhaust Fluid (DEF/AdBlue) from freezing in cold ambient conditions. The heater is typically integrated into the DEF tank unit or supply module and is switched by the ECM through a relay or high-side driver. A low-circuit reading means the measured voltage has dropped below the expected operating threshold, pointing to a short to ground, excessive resistance, or a driver fault rather than the open-circuit or high-voltage conditions flagged by sibling codes P20B9 and P20BC respectively.

The SCR (Selective Catalytic Reduction) system depends on a consistent DEF supply to reduce NOx emissions. When the heater circuit fault is present the ECM cannot confirm proper heater operation, which may prevent DEF from thawing and flowing to the injector in freezing temperatures. Depending on the manufacturer's calibration strategy, the ECM will illuminate the MIL and may initiate a NOx-emission-based torque derate or a countdown to a speed-limited limp mode if the fault persists across multiple drive cycles without correction.

P20BB is electrically diagnosed rather than chemically diagnosed: the fault is in the heater control wiring or driver stage, not in DEF quality or dosing quantity. Repairs therefore centre on inspecting the wiring harness from the ECM to the DEF tank heater, testing the heater element resistance, checking fuses and relays in the reductant heater supply circuit, and verifying ECM output driver integrity before considering module replacement.

Common causes

Most-frequently reported root causes when P20BB is logged.

  • 1
    Short to ground in the wiring harness between the ECM and the reductant heater element
  • 2
    Damaged, chafed, or corroded connector pins at the DEF tank / supply module connector
  • 3
    Failed or open reductant heater element (internal short pulling circuit low)
  • 4
    Blown fuse or failed relay in the reductant heater power supply circuit
  • 5
    High-resistance or broken ground path for the heater control circuit
  • 6
    ECM internal output driver failure for the reductant heater channel
  • 7
    Water or freeze damage to the DEF tank wiring harness in the undercarriage

Symptoms drivers notice

MIL (check engine light) illuminated
AdBlue/DEF warning message or gauge anomaly on the instrument cluster
DEF does not thaw or flow in cold ambient temperatures, leading to SCR system unavailability
Potential torque reduction or speed-limited limp mode after multiple fault cycles (manufacturer-dependent)
Possible increased NOx emissions or failed emissions inspection
Companion SCR or reductant-system codes (P20B9, P20BD, P207F) may be present simultaneously

How to diagnose P20BB

A typical diagnostic flow when this code is present.

  1. 1
    Perform a full system scan and record all DTCs; note any companion reductant-system or SCR codes alongside P20BB
  2. 2
    Inspect the wiring harness and connector at the DEF tank / supply module for chafing, corrosion, pushed-back pins, or water intrusion; repair as needed
  3. 3
    With the circuit unplugged, measure resistance of the reductant heater element between its supply and ground terminals against the manufacturer's specification (typically 2–10 Ω); replace the DEF tank unit if open or shorted
  4. 4
    Check the reductant heater fuse and relay for continuity and correct operation; replace any failed component
  5. 5
    With the connector re-seated, back-probe the ECM output wire and verify supply voltage when the ECM commands the heater on; a reading near 0 V confirms a short to ground on that wire
  6. 6
    If wiring and heater element test good, use a factory-level scan tool to command the heater circuit and monitor ECM driver current; an internal driver fault that persists with a known-good harness may indicate ECM replacement is required
  7. 7
    Clear DTCs, perform a complete drive cycle in conditions that would trigger heater operation, and confirm no re-set of P20BB before returning the vehicle

Vehicles where we've handled P20BB

Platforms in our catalogue with confirmed P20BB coverage.

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

Related powertrain codes

Frequently asked questions

Can I drive with P20BB set?

Short-term driving is generally possible but not advisable. In warm weather the immediate drivability impact is minimal, but in cold climates the DEF heater failure can prevent AdBlue from thawing, causing the SCR system to stop dosing. Most manufacturers will initiate a torque or speed derate after several fault cycles without correction, so the fault should be diagnosed promptly.

Is P20BB the same as a DEF quality fault?

No. P20BB is a circuit-electrical fault on the heater control wiring and does not indicate contaminated or low-quality DEF. DEF quality and concentration faults are reported by separate codes such as P207F. Topping up or replacing DEF will not resolve P20BB.

What is the difference between P20BB, P20B9, and P20BC?

All three codes relate to the same Reductant Heater A control circuit but describe different electrical failure modes: P20B9 indicates an open circuit (no continuity), P20BB indicates a low-voltage / short-to-ground condition, and P20BC indicates a high-voltage condition. Identifying which code is present helps pinpoint whether the fault is a broken wire, a grounded wire, or a short to power.

Will clearing the code fix the problem?

No. Clearing the DTC without repairing the underlying wiring or heater fault will cause the code to return after the next drive cycle in which the ECM attempts to operate the heater. A lasting repair requires identifying and correcting the short-to-ground or failed heater element before the code is cleared.

Disabling P20BB in software

RaceTune can permanently disable P20BB — 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 P20BB 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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