P20BB–P20C7 Reductant Heater A Control Circuit Low

Which code is which

Same fault, different cylinder or bank — diagnosed the same way. Several set together usually means one shared cause.

The definition

P20BB: Reductant Heater A Control Circuit Low.

P20BB 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:

1. Unplug the sensor. If the reading jumps to full scale, the wiring is fine and the sensor has failed low.
2. Check the sensor's 5V supply and its earth before condemning the sensor itself.

What actually causes this on a VAG

An AdBlue heater circuit fault on a diesel, this time meaning the control unit sees the heater's circuit pulled down to earth - a short, or more current flowing than there should be. That is an electrical fault in the wiring or the heater itself, not a fluid or a dosing problem. The heater's job is to keep the exhaust fluid from freezing, so this typically shows itself in cold weather and may not have troubled you all summer. The car generally still drives, but the fault needs sorting rather than clearing and forgetting.

Most likely causes, in order

1. Chafed wiring under the car. On most models the loom to the AdBlue tank and delivery module runs in the road spray and takes stones, salt and anything that snags on it. A wire rubbed through to the underbody or a heat shield earths the circuit and gives exactly this code. Get the car safely raised and follow the harness looking for worn insulation, especially where it is clipped to the body, crosses a bracket or passes through a grommet. It is a leading cause and, unlike the alternatives, it is repairable.
2. Water or corrosion in the connector. A wet or corroded plug creates a leakage path to earth that behaves like a short. Unplug the connectors at the tank and delivery module and look for damp, green or white deposits, and a perished seal. Clean, dry and protect them properly rather than just pushing them back together.
3. Heater element shorted internally. The element itself can break down to its casing. The way to separate that from a harness fault is to disconnect the heater and see whether the code still sets with the plug hanging free - if it does, the fault is upstream in the wiring; if it only appears with the heater connected, the heater is at fault. Expect an open-circuit code while it is unplugged; that is normal.
4. Control module or relay driver. Uncommon, and worth considering only once the loom and the heater have each been individually cleared. Bear in mind that a long-standing short can damage whatever was driving it, so fix the wiring properly before assuming the module was the original problem.

Worth knowing: Look under the car before you look at prices. A shorted wire in the underfloor loom is a repair, whereas an AdBlue tank or delivery module is one of the more expensive parts on the vehicle - and a shop that condemns the module without having traced the harness end to end has skipped the cheap half of the job. Ask what was actually measured at the module connector before agreeing to anything.

Common questions

What does P20BB mean?

P20BB means Reductant Heater A Control Circuit Low. The voltage on this circuit is sitting near zero. Typically a short to earth, or a sensor that has failed to its low end.

Can I drive with P20BB?

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.

Related reading

• 2.0 TDI EA288 problems: the owner's and buyer's guide
• DPF regeneration on a VAG diesel: what it is and what blocks it
• Why the same fault code has completely different causes