What to do about a P20B9 fault code
Ignore P20B9 into the winter and the AdBlue can freeze solid, which starts the no-restart countdown on many Euro 6 vehicles. The code means the heater that keeps AdBlue liquid has an open circuit, and we can read the system wherever the vehicle is parked.
Can you keep using it with P20B9?
Fine for now
In mild weather the AdBlue stays liquid without the heater, so the vehicle usually works as normal. Use that window to get the fault diagnosed.
Get it booked this week
Once frosty nights arrive, a dead heater can't thaw the tank or lines, and dosing stops. Book a check before the cold sets in, or straight away if a countdown has appeared.
Stop and call us
There's no immediate danger in using the vehicle, but call us if the countdown is close to zero. Once it runs out, the engine won't restart after it's switched off.
Signs that go with P20B9
- The engine light and an AdBlue warning, often first seen on a cold morning
- A no-restart countdown on the dash
- AdBlue faults that clear themselves as the weather warms up
What P20B9 means
The official definition is Reductant heater A control circuit/open. Reductant is AdBlue, and heater A is usually the one in the tank. The ECU has found a break in the heater's control circuit, so it can't warm the fluid when the temperature drops towards the point where AdBlue freezes, around minus 11°C.
Common causes
A failed heater element
The heater can burn out over time. On many vehicles it's built into the tank sender or pump unit, so the whole unit is replaced.
Broken or corroded wiring
The loom runs close to the tank and takes road spray, so a corroded joint or a chafed wire can open the circuit.
A poor connector or blown fuse
Water in the heater's plug corrodes the pins, and a failed fuse or relay makes a healthy heater read as open.
What to do right now
- Check for a countdown on the dash and note the figure.
- Keep the AdBlue topped up with fresh fluid from a sealed container, so a low level doesn't add another fault.
- Don't keep clearing it. The heater test runs again every start and the code comes back.
- Book a check before the countdown runs low.
How we sort P20B9 where the vehicle is parked
We read the codes and AdBlue status, then check the heater circuit's supply and wiring to show whether the heater, a wire or the fuse is at fault. Diagnostics is £100 + VAT. We don't repair or replace AdBlue heaters, tanks or pumps, so any repair goes to a dealer or SCR specialist with our findings.
An AdBlue delete (£330 + VAT) is the other option. If the vehicle can't be moved, tell us when you book.
Reviews
Related AdBlue heater codes
P20BA, officially Reductant heater A control circuit performance, means the same tank heater has power but isn't heating as the ECU expects. P20BD is listed as Reductant heater B control circuit/open, the same open-circuit fault on the second heater, often the one warming the lines or pump.
P20C9 is listed as Reductant control module requested MIL illumination. The AdBlue control module has found a fault of its own and asked the ECU to light the warning, so its stored codes need reading. All three are diagnosed in the same visit as P20B9.
Will P20B9 put the car into limp mode?
Not usually on its own. If the AdBlue freezes and dosing stops, the countdown to no-restart is the bigger worry.
How long can I leave P20B9 before it gets worse?
In summer it may sit quietly for months. The first hard frost is when it causes trouble, so book before then.
Can P20B9 damage the AdBlue system?
Frozen AdBlue expands, so repeated freezing with no heater can strain the pump and lines.
Will my car pass its MOT with P20B9 showing?
No. A lit engine management light is an MOT fail whatever the code behind it.
Is P20B9 something I can fix myself?
Checking the fuse is a fair first step if you know where it is. Heater and wiring tests need a meter and live data, whether it's a Ford, a Mercedes or another make.
Can an AdBlue delete be used instead of a new heater?
Only on a vehicle used off-road or being exported, as a delete isn't permitted on UK roads. Our guide to whether an AdBlue delete is legal covers it.
Get P20B9 looked at where the vehicle is parked
Send us the registration and any countdown figure, and we'll come out and read the AdBlue heater circuit before the cold weather catches you out.
More fault codes explained
Eccles, Manchester M30 0SF