P20BC–P20C8 Reductant Heater A Control Circuit High
The voltage on this circuit is sitting near battery voltage. Typically a short to a positive feed, or a disconnected sensor floating high.
Can you drive it? The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.
Which code is which
Same fault, different cylinder or bank — diagnosed the same way. Several set together usually means one shared cause.
| Code | Definition | Severity |
|---|---|---|
P20BC | Reductant Heater A Control Circuit High | Investigate soon |
P20C0 | Reductant Heater B Control Circuit High | Investigate soon |
P20C4 | Reductant Heater C Control Circuit High | Investigate soon |
P20C8 | Reductant Heater D Control Circuit High | Investigate soon |
The definition
P20BC: Reductant Heater A Control Circuit High.
P20BC is a manufacturer-specific powertrain code (the engine or gearbox). Its meaning is set by the carmaker rather than by the SAE standard, so a generic OBD-II reader will often show it as an unknown code or misread it.
What to check first
Cheapest and fastest checks first:
- Unplug the sensor and check whether the signal falls. If it stays high, suspect the wiring rather than the sensor.
- Check the earth path - a missing earth makes a sensor read high.
What actually causes this on a VAG
Your diesel uses AdBlue, a urea solution, to clean up its exhaust, and AdBlue freezes at about minus 11 Celsius - so the tank, the pump and the delivery lines are heated to keep it liquid through winter. This code says the control unit's driver for one particular heater is sitting higher than it should. On a normally driven heater circuit that points to an open circuit - the module cannot see the heater on the end of the wire - though a short to battery positive on the same line reads the same way. Which heater matters enormously here. On VAG applications the AdBlue tank heater has its own separate codes, and so does the metering and delivery unit heater, which means heater 'A' is a different element again, normally one of the heated AdBlue lines. Read the manufacturer's own text with a VAG-capable tool before buying anything, because it names the element. The car may drive perfectly in warm weather and only misbehave when it turns cold - do not let that lull you, because on many EU6 diesels an unresolved AdBlue fault eventually starts a countdown that ends with the car refusing to restart.
Most likely causes, in order
- Fuse or supply feeding the heater circuit. The cheapest possible cause and worth two minutes before anything else. Check the fuse for the AdBlue or emissions heating circuit and any associated relay. A blown fuse also raises the question of what blew it, so if it goes again there is a short somewhere in the loom and you need to find it rather than keep fitting fuses.
- Corroded or damaged wiring and connectors under the car. The AdBlue tank, the delivery module and the heated lines usually live under the vehicle, exposed to salt, water and stone chips. Connectors corrode, plugs work loose over speed bumps, and the loom chafes where it is clipped to the tank or subframe. Get the car up and unplug the connectors at each end of the heated line and at the tank and delivery module, looking for green pins, damp inside the plug or a rubbed-through cable. This is where most of these faults are found.
- The heating element itself has gone open circuit. Once the supply and the wiring are proven, the element on the end of the circuit is the answer. For a heated AdBlue line that usually means replacing the line assembly, with the heater built into it. Confirm which element the fault text names before ordering, because the tank heater and the delivery module heater are entirely different parts with their own codes, and they are the expensive end of this.
- Control module output stage. Rare, and only a candidate once the fuse, the loom, the connectors and the heater itself have been proven good.
Worth knowing: Never top up with anything other than proper AdBlue to the correct standard. Water, screenwash or a contaminated container will crystallise or corrode inside the system and can destroy the pump and the injector, turning a wiring fault into a far bigger repair. Do not simply clear this one and forget it either - a heater fault found in summer will come back the first cold week. And if the dash is showing a restart countdown, treat it as urgent: once it reaches zero the engine will not start again until the fault is fixed and the system reset with the proper equipment.
Reading the code is the easy part
A code names the circuit that complained. What tells you why is watching the sensor behind it while the engine runs — and that is where a code reader stops and a diagnostic tool starts.
VAGPULSE reads every module in a VW, Audi, Skoda or SEAT — not just the engine — shows the live measuring blocks behind this fault, gives you the freeze-frame conditions it was logged under, and clears it once you have fixed it. It also does what a generic OBD reader cannot on a VAG: service resets, coding, adaptations, and Stage 1–3 flashing with your original file saved first.
$249 once. No subscription, unlimited cars. Windows 10/11.
See VAGPULSE — $249 Look up another codeSoftware downloads instantly · cable ships September 30 · cable guarantee — refund if it won't connect
Prefer to work on your phone? The VAGPULSE Android app is a free download and its demo mode runs against a sample car with no hardware at all, so you can see exactly what it reads before spending anything. Reading your car with it needs the VAGPULSE cable, which comes with the licence.
Common questions
What does P20BC mean?
P20BC means Reductant Heater A Control Circuit High. The voltage on this circuit is sitting near battery voltage. Typically a short to a positive feed, or a disconnected sensor floating high.
Can I drive with P20BC?
The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.