P20B8 code meaning
Reductant Metering Unit Heater Control Circuit High
Powertrain system Generic code P20xx codes Check engine light
What P20B8 means
P20B8 marks the upper limit of the metering unit heater circuit. The 2014 Touareg CNRB table from Volkswagen sets it when the conductance measured at heating start exceeds 0.74 siemens, meaning the element is below about 1.35 ohms, with additional limits at 0.96 and 0.52 siemens. Such high conductance suggests shorted heater windings, wires touching each other or fluid bridging the terminals, and it implies excess current that strains the driver stage and wiring. The control unit will normally switch the heater off for protection, so in winter the outcome resembles an open heater (P20B5). Makers that judge their outputs by voltage may use the same code for a short to battery instead.
P20B8 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyHard
- Typical labor0.5 to 3 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P20B8
- Warning lamp as soon as heating is requested
- Heater switched off, DEF stays frozen longer
- Possibly a blown heater fuse
Common causes of P20B8
- Most likelyHeater element internally shorted (too low resistance)
- PossibleHeater wires chafed and touching each other
- Less likelyDEF or water bridging the heater terminals
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Reductant metering unit heater
What each part of P20B8 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 2 | Generic code (second SAE range): it means the same on every make. |
| 0 | Area: fuel and air metering and auxiliary emission controls. |
| B8 | The exact fault in that area. |
What to do next
- Note P20B8 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P20B8
What does P20B8 mean?
P20B8 means "Reductant Metering Unit Heater Control Circuit High". P20B8 marks the upper limit of the metering unit heater circuit. It is a generic code, so it means the same on every make.
Is it safe to drive with P20B8?
Drive gently and only to a workshop. With P20B8 the car can go into limp mode (reduced power) to protect itself.
Will P20B8 fail an emissions test?
Yes, it can. P20B8 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P20B8 to fix?
Difficulty: hard. A typical repair takes 0.5 to 3 hours of work. This is a job for a workshop.
Will P20B8 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.