P20BB code meaning
Reductant Heater A Control Circuit Low
Powertrain system Generic code P20xx codes Check engine light
What P20BB means
A low reading on the heater A control circuit usually means the output or its wire is grounded, or the element pulls far more current than it should. GM's 2025 DEF module checks heater 1 (tank heater 1) in both states: switched on, a low-side drain voltage above 0.45 V or a load current above 95 A counts; switched off, the high-side voltage sitting in a low band near 0.5–0.6 V. Two seconds of failures give a Type A fault. Ford logs P20BB on circuit 1 above 15 A and lets the glow plug module report it within 250 ms; VW uses a signal voltage under 2.97 V on the 2014 Touareg. On 2020–2022 Silverado and Sierra diesels GM found the test itself setting P20BB falsely and released new DEF module calibrations (22-NA-112). Expect a warning lamp and, later, a speed limit.
P20BB at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyMedium
- Typical labor0.5 to 4 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P20BB
- Malfunction indicator lamp, possibly with a blown heater fuse
- Message that speed will be reduced if the system is not serviced
- No DEF heating, so dosing is delayed in frost
Common causes of P20BB
- Most likelyHeater A wire chafed through and touching ground
- PossibleInternal short in the heater element, drawing excessive current
- PossibleDEF control module calibration setting the code falsely (GM 22-NA-112)
- Less likelyMoisture bridging heater pins to ground in the connector
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Reductant line heater a
What each part of P20BB 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. |
| BB | The exact fault in that area. |
What to do next
- Note P20BB 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 P20BB
What does P20BB mean?
P20BB means "Reductant Heater A Control Circuit Low". A low reading on the heater A control circuit usually means the output or its wire is grounded, or the element pulls far more current than it should. It is a generic code, so it means the same on every make.
Is it safe to drive with P20BB?
Drive gently and only to a workshop. With P20BB the car can go into limp mode (reduced power) to protect itself.
Will P20BB fail an emissions test?
Yes, it can. P20BB 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 P20BB to fix?
Difficulty: medium. A typical repair takes 0.5 to 4 hours of work. This is a job for a workshop.
Will P20BB clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.