P20B9 on 2011-2025 Chevrolet Silverado 3500HD — Reductant Heater "A" Control Circuit/Open
1 vehicles · 1 makes · $300-$2,500 repair
How to diagnose and fix P20B9: most-reported vehicles
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Chevrolet Silverado 3500HD · 2011–2025
The Diesel Exhaust Fluid (DEF) system is plagued with issues including DEF tank heater failure, injector clogging, quality sensor malfunctions, and SCR catalyst degradation. Any failure triggers a countdown timer that eventually limits the truck to 5 mph if not addressed. The system is sensitive to DEF quality and crystallization.
Common Symptoms
- Check engine light with DEF warning
- Reduced engine power message
- Speed limited to 55 mph then 5 mph
- DEF level sensor reading incorrectly
- Service exhaust system message
How to Fix
Diagnose the specific fault code to determine which DEF component failed. Common repairs include DEF injector replacement, tank heater element replacement, or DEF quality sensor replacement. Use only API-certified DEF fluid. Do not use non-certified DEF or add water. The dealer can reset the countdown timer after repairs.
Sources for this issue (3)
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P20B9 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P20B9 mean?
P20B9 stands for "Reductant Heater "A" Control Circuit/Open." This code applies to diesel vehicles with Selective Catalytic Reduction (SCR) and means the engine computer detected an open circuit in the control for reductant (Diesel Exhaust Fluid / DEF) heater "A". This heater keeps the DEF and its lines from freezing or crystallizing in cold weather so the emissions system can function. An open control circuit means the heater can't be commanded properly, which can cause DEF freezing, SCR faults, and eventually reduced power or limited engine starts to enforce emissions compliance. It should be addressed promptly, especially in cold climates.
What are the most common causes of P20B9?
The most common causes of P20B9 are: Open or broken wiring in the reductant heater circuit, Corroded or loose heater connector, Failed reductant (DEF) tank heater element, Blown fuse or relay for the heater circuit, Faulty DEF heater control module, PCM driver circuit fault (rare). The specific cause varies by vehicle.
How much does it cost to fix P20B9?
Repair costs for P20B9 range from $300 to $2,500, depending on the vehicle and root cause.
Which vehicles are affected by P20B9?
Au7o has documented P20B9 across 1 vehicle models from 1 manufacturers: Chevrolet.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.