P20B9🌫 Emissions (DEF heater, diesel exhaust aftertreatment)Stop and fix promptly

P20B9: Reductant Heater A Control Circuit/Open (Meaning, Causes, Fixes)

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

What P20B9 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 P20B9 is stored when the powertrain control module (PCM) detects no 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 describes it more broadly as the reductant heater not performing as requested. Either way, the computer can't confirm that heater A is working.

Severity: can I keep driving?

Stop and fix promptly

Get it fixed soon. RepairPal rates P20B9 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) heater (both sources)
  2. An open or shorted circuit in the heater control wiring or connector (OBD-Codes.com)
  3. A faulty reductant temperature sensor (OBD-Codes.com)
  4. A bad SCR controller or PCM, or a programming error (OBD-Codes.com)

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 the fuses for the SCR or DEF system. OBD-Codes.com recommends testing them 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 for voltage on its control circuit, then 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 P20B9 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.

Can P20B9 show up in warm weather?

Yes. The heater only matters in cold weather, but the computer checks its circuit anyway. OBD-Codes.com adds that if the code appears in warm weather, a programming error is a suspect.

Is it usually the heater or the wiring?

Both sources point to the heater itself, and OBD-Codes.com also lists open or shorted wiring, a bad DEF temperature sensor and the controller. Check fuses, wiring and heater resistance before buying a heater or tank assembly.

Do the sources agree on P20B9?

Mostly. OBD-Codes.com says the code is set when the PCM sees no voltage on the heater A control circuit, while RepairPal says it sets when the heater does not perform as requested. RepairPal names only the heater as the cause; OBD-Codes.com adds wiring, the DEF temperature sensor and the controller.

How much does it cost to fix P20B9?

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: P20B9 Reductant Heater A Control Circuit/Open (Stephen Darby, ASE Certified Technician)
  2. RepairPal: P20B9 Reductant Heater "A" Control Circuit/Open (rated Urgent)

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