P20BB🌫 Emissions (DEF heater, diesel exhaust aftertreatment)Stop and fix promptly

P20BB: Reductant Heater A Control Circuit Low (Meaning, Causes, Fixes)

Reviewed 2026-10-24 · Based on 2 cited source(s)

What P20BB means

Diesels with SCR inject DEF into the exhaust so the reduction catalyst can turn NOx into nitrogen and water. DEF can freeze, so heaters are built into the DEF tank and often the supply line to the DEF injector. OBD-Codes.com says P20BB is stored when the powertrain control module (PCM) detects insufficient voltage on the control circuit for the heater labeled A, which is used when there's more than one heater; your service manual tells you which heater is A. RepairPal lists the same title (Control Circuit Low) but describes the trigger more broadly as the reductant heater not performing as requested. It's part of a family: OBD-Codes.com uses P20B9 for no voltage (an open) and P20BA for voltage outside the expected range on the same circuit, and RepairPal lists P20BC as the Control Circuit High version.

Severity: can I keep driving?

Stop and fix promptly

Get it fixed soon. RepairPal rates P20BB as Urgent, and OBD-Codes.com calls it severe, warning that the SCR (selective catalytic reduction) system could be disabled and the catalyst could be damaged if the cause isn't fixed in a timely manner. Many diesel trucks and vans also limit speed or power when the DEF system stays faulty, and the check engine light will likely fail an emissions inspection.

Common symptoms

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

  1. A failed reductant (DEF) tank heater (both sources)
  2. Open or shorted wiring or a bad connector in the heater control circuit (both sources)
  3. A faulty reductant temperature sensor (OBD-Codes.com)
  4. A bad SCR controller or PCM, or a programming error (OBD-Codes.com)
  5. A failed glow plug control module (RepairPal)

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

  1. Read all stored codes and freeze frame data and write them down. Related DEF and SCR codes help show where the fault is.
  2. Look up technical service bulletins for your year, make, model and engine; OBD-Codes.com notes they often speed up diagnosis.
  3. Inspect the DEF heater wiring and connectors at the DEF tank and supply line for burnt, chafed or corroded wiring and loose pins. Repair any damage before going further.
  4. Check power and ground to the SCR system. OBD-Codes.com recommends testing fuses with the circuit loaded to avoid misdiagnosis.
  5. Clear the code and drive. If it doesn't come back right away, the fault may be intermittent and harder to pin down.
  6. With a capable scan tool and a multimeter, command the heater on and check the control circuit voltage. If voltage is low, OBD-Codes.com points to the controller or its programming; if it's good, measure the heater's resistance against your service manual's specification.

Typical fixes

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

  • Your truck or van shows a DEF warning, a speed-limit countdown or reduced power
  • You need a scan tool that can command the DEF heater on
  • The DEF tank or heater assembly has to come off the vehicle
  • The vehicle may still be under emissions warranty

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Is P20BB serious?

Treat it as urgent. RepairPal rates it Urgent and OBD-Codes.com calls it severe, because the SCR system could be disabled and the catalyst could be damaged if the cause isn't fixed.

What's the difference between P20B9, P20BA and P20BB?

All three point to reductant heater A's control circuit. OBD-Codes.com describes P20B9 as no voltage (an open), P20BA as voltage outside the normal range, and P20BB as insufficient voltage (low). The causes and tests are much the same.

Can P20BB show up after replacing the batteries?

A Duramax owner on OBD-Codes.com's forum reported P20BB and a service emissions warning right after changing the batteries on a 2021 truck, but neither source says battery work causes it. Check the batteries, cables and grounds, clear the code, and see whether it returns before replacing parts.

Do the sources agree on P20BB?

Mostly. OBD-Codes.com says the code is set when the PCM sees insufficient voltage on the heater A control circuit, while RepairPal says it sets when the heater does not perform as requested. Both name the heater and wiring. OBD-Codes.com adds the DEF temperature sensor and the controller, and RepairPal alone lists a glow plug control module failure, so confirm against your vehicle's service information before replacing that.

How much does it cost to fix P20BB?

Neither source gives a repair cost. It ranges from a wiring or fuse repair to a new DEF heater or tank assembly, so check emissions warranty coverage and get a written estimate after diagnosis.

Sources

  1. OBD-Codes.com: P20BB Reductant Heater A Control Circuit Low (Stephen Darby, ASE Certified Technician)
  2. RepairPal: P20BB Reductant Heater "A" Control Circuit Low (rated Urgent)

Last reviewed 2026-10-24. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.