P20B8
Reductant Heater Control Circuit - HighP20B8 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.
What P20B8 means
P20B8 is set when the ECM detects an unexpectedly high voltage or an open-load condition on the control circuit of the reductant heater. The reductant heater is responsible for thawing frozen DEF fluid in the tank, lines, and pump during cold ambient conditions. The ECM monitors the heater driver circuit and logs this fault when the feedback signal remains high when it should be low, indicating a short to voltage or loss of ground path.
Because the reductant heater circuit carries relatively high current to generate heat quickly, wiring faults are often associated with heat-damaged insulation near the exhaust tunnel or undercarriage routing. Connector pin corrosion from road salt or moisture ingress is also a common contributor on vehicles operating in cold climates where the heater sees frequent use.
A non-functional reductant heater does not prevent SCR operation in warm weather but renders the system unable to thaw frozen DEF in cold conditions, causing SCR unavailability and potential regulatory non-compliance from the first sustained cold soak. Many OEMs will log additional low-pressure or no-dosing faults alongside P20B8 in winter months.
Common causes
Most-frequently reported root causes when P20B8 is logged.
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1
Short to voltage on the reductant heater control or power supply wiring.
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2
Open ground path for the heater element causing the driver circuit to detect a high-side fault.
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3
Corroded or water-contaminated heater connector terminals.
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4
Failed heater element with an internal open circuit causing loss of load detection.
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5
Damaged wiring insulation in the undercarriage harness creating a voltage leak.
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6
Failed ECM heater driver circuit providing incorrect output voltage.
Symptoms drivers notice
How to diagnose P20B8
A typical diagnostic flow when this code is present.
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1
Retrieve and record all DTCs and freeze-frame data before clearing codes.
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2
Locate the reductant heater circuit in the OEM wiring diagram and trace the harness from the ECM to the heater assembly.
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3
Inspect the harness for damaged insulation, especially near heat sources or undercarriage attachment points.
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4
Disconnect the heater connector and measure voltage on the control wire with ignition on to check for an unintended supply voltage indicating a short to power.
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5
Measure resistance of the heater element across its terminals and compare to the OEM specification; an open reading confirms a failed heater element.
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6
Inspect connector terminals for corrosion, moisture intrusion, or pushed-back pins; clean or replace as needed.
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7
If the harness, connector, and element all test good, test the ECM driver output with an oscilloscope or test lamp during a commanded heater-on cycle.
Vehicles where we've handled P20B8
Platforms in our catalogue with confirmed P20B8 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
Will P20B8 affect me in summer?
In warm weather, the heater is not needed and its failure will not affect SCR dosing. The problem becomes critical in sub-zero temperatures when DEF can freeze and block the entire reductant system.
Can a blown fuse cause P20B8?
A blown heater circuit fuse can remove the load, causing the driver circuit to detect an abnormal condition. Always check the relevant fuse before proceeding to harness and component testing.
Is P20B8 the same as the heater being stuck on?
No. P20B8 specifically indicates a high-circuit condition, meaning the driver sees more voltage than expected. A heater stuck on would typically set a different, low-side or overload fault code.
Does the reductant heater only heat the tank?
No. Most systems heat the tank, the supply line between the tank and pump, and sometimes the pump head itself. All segments must function for reliable cold-weather SCR operation.
Disabling P20B8 in software
RaceTune can permanently disable P20B8 — 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 P20B8 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
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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