What it means
Reductant heater A (typically the supply line heater nearest the tank) control circuit is open — that segment of DEF line will not be kept above freezing in winter.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Reductant heater A warms one section of the DEF line, typically the supply line nearest the tank. P20B9 indicates an open in heater A's control circuit, so that line segment may not be heated when the emissions system requests it.
Common causes
Most likely first.
- Heater winding open
- Pin pushed back out of connector body
- Cracked solder joint inside the module or sensor
Symptoms
- Check engine light or NOx warning on
- AdBlue/DEF level or quality warning
- Possible no-start countdown after ignore period
- Failed emissions inspection (NOx)
How to diagnose it
- Save freeze-frame data and check for other DEF line heater codes to determine whether the fault is limited to heater A.
- Inspect heater A's connector and the wiring along the tank-side DEF supply line for looseness, corrosion, or damage.
- Check that the heater A harness has not been pulled, rubbed, or pinched along its route from the tank area.
- With the circuit isolated according to the manufacturer's procedure, test heater A's element and wiring continuity for an open.
- If the heater and wiring are intact, check the module connector and commanded output using the manufacturer's diagnostic procedure.
Common fixes
- Reconnect and secure heater A's loose connector.
- Repair an open or damaged section of heater A wiring.
- Replace a damaged terminal or connector at the tank-side line heater.
- Replace heater A or its line assembly if the element is open to specification.
Can I keep driving?
The vehicle may be drivable in warm conditions, but the tank-side DEF line can freeze in cold weather and interrupt dosing, potentially leading to emissions warnings or a no-start countdown. Avoid prolonged use in freezing conditions until heater A works.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.