Emissions control: EGR, DPF, catalytic converter and AdBlueGeneric OBD-II codeUrgency: Garage soon
Code P20B9: adBlue heater: open control circuit
P20B9 means: reductant heater “A”: open control circuit.[1][2][3] The reductant is AdBlue (urea solution) in the SCR system of diesel engines. The tank includes a heating element that prevents the liquid from freezing (RACE). P20B9 is stored when the control unit detects that the control circuit of heater “A” is open: it sees no voltage on it, or the heater does not respond as commanded.[2][3][4][5]
What code P20B9 means
The reductant is AdBlue (urea solution) in the SCR system of diesel engines. The tank includes a heating element that prevents the liquid from freezing (RACE). P20B9 is stored when the control unit detects that the control circuit of heater “A” is open: it sees no voltage on it, or the heater does not respond as commanded.[2][3][4][5]
The “A” only serves to distinguish heaters when the car has more than one, and CarParts warns that the definition may vary by manufacturer. PartsAvatar describes, in systems that use a combined element, a heater that also acts as a temperature and level sensor for the reductant; when the sensor detects that the liquid is close to freezing, the control unit powers the heater to thaw it.[2][4]
CarWiki places it in the electrical connection or in the components of the reductant heating system. RACE adds that when the electrical part of the tank module fails, the usual remedy is to replace the whole assembly. It is a generic code of the OBD-II standard (it starts with P2), with the same definition on any brand.[1][5][6]
Original definition (SAE J2012): Reductant Heater "A" Control Circuit/Open[1][2][3]
Causes of P20B9
From most to least likely, according to the sources.
- 1Heater circuit wiring or connectors damaged, cut, short-circuited or in poor contact.[1][2][4]
- 2Faulty reductant heater.[1][2][4]
- 3Blown or interrupted fuse in the control circuit.[1]
- 4Faulty heater control module, SCR or engine control unit, or a programming error.[1][2]
- 5Faulty reductant temperature sensor.[2]
- 6Control unit terminal or earth in poor condition.[1]
Symptoms
- Engine fault warning light on.[2][4]Engine warning light: faults that cause it
- Warning of a fault in the exhaust or emissions control system.[1]Pollution control faulty: faults that cause it
- Irregular power or reduced engine performance.[1][2]Less power: faults that cause it
- Higher fuel consumption.[1][2]Uses more fuel: faults that cause it
- Black smoke from the exhaust, according to CarParts.[2]Black smoke: faults that cause it
- Diesel particulate filter regeneration may not take place.[2][4]
How to diagnose P20B9
In order: from the most likely and cheapest to check to the most expensive.
- 1Read all the system codes. If there are other SCR or AdBlue codes, resolve them before continuing with P20B9.[2][4]
- 2Visually inspect the heater and its wiring: breaks, damage or corrosion, and burnt or loose connectors. Repair anything damaged.[4]
- 3Check the fuse, and the power supply and earth of the control circuit.[1]
- 4Measure the heater harness looking for an open circuit or a short circuit, and check the heater itself.[2][4]
- 5Check the reductant temperature sensor.[1][2]
- 6If everything above is good, check the heater control module and the control unit, and check whether there is a software update.[1][4]
- 7If the heater is integrated into the AdBlue tank module, the repair is usually to replace the assembly, according to RACE. After repair, clear the code and check that it does not return.[4][5]
Can I keep driving with P20B9?
Garage soon
The sources do not agree on the severity. CarParts rates it as severe because it can disable the SCR and damage the catalytic converter, and advises not driving with the code and taking it to a workshop soon. PartsAvatar mentions only the warning light and that the particulate filter cannot regenerate; CarWiki speaks of an exhaust system working less effectively. RACE adds that faults in the SCR system can give warnings on the instrument panel and even prevent a restart. The prudent course is to diagnose it soon.[1][2][4][5]
Documented faults related to P20B9
Known faults of specific engines and cars, with their repair cost, the affected years and their sources.
AdBlue (SCR) system and NOx sensor: the most serious faults by engine and car
- Medium severityFrequency: Very frequent1.5 BlueHDiAdBlue tank and pump failure1.5 BlueHDi engines built up to August 2020 can suffer failures of the AdBlue tank pump module, with a "no engine start" countdown. The whole tank is usually replaced and Stellantis contributes to the cost up to 8 years.Repair: €1,000–€1,500
- Medium severityFrequency: Very frequent1.6 HDiAdBlue tank and pump fault (1.6 BlueHDi)The 1.6 BlueHDi engines (2013-2019) suffer from the AdBlue tank defect of Stellantis diesels: an emissions warning appears with a countdown until starting is prevented. Replacing the tank, with the integrated pump, costs over €1,000 (price in Spain), but there is an extended warranty of up to 8 years.Repair: €1,000–€1,200
- Medium severityFrequency: Very frequent2.0 HDiAdBlue tank crystallisation and failures (BlueHDi)On the 2.0 BlueHDi with SCR and AdBlue (from 2013), the system's tank, pump or sensors can fail or crystallise: a warning light appears, sometimes a false one, and if ignored the car may end up not starting. The OCU has reported it in Spain and Stellantis compensates some of those affected.Repair (garage): €800–€1,200
- Medium severityFrequency: Very frequentOM642Emissions defeat device (AdBlue/EGR) and software update campaignGermany's KBA found indications of emissions manipulation in cars with the OM642 engine (alongside the OM651): AdBlue dosing and the EGR changed between the test bench and the road. Mercedes-Benz denied it, but in 2018 it recalled hundreds of thousands of diesels in Europe.
Wiring, connectors and short circuits: the most serious faults by engine and car
- Critical severityFrequency: OccasionalBMW N62 / N63Additional turbo coolant pump with a risk of short circuit and fireIn 2011 BMW recalled 5, 6 and 7 Series, X5 and X6 models built between 2007 and 2011 because of the additional turbo coolant pump: moisture, a short circuit and, in extreme cases, a fire. In 2012 there was another campaign for a crack in the pump, with a more robust version.
- Critical severityFrequency: Occasional1.6 GDi híbrido (Kappa)Main relay of the power relay assembly (overheating and fire risk)Yes, on the first units: Hyundai and Kia recalled 2016–2018 Ioniq and Niro hybrids and plug-in hybrids because the main relay that connects the high-voltage battery could come loose, overheat and, in the worst case, cause a fire.
- Critical severityFrequency: Occasional1.6 GDi híbrido (Kappa)Clutch hydraulic actuator (HCA): leak or moisture with risk of short circuit and fireYes, this is the most documented hybrid system fault: the hydraulic actuator that engages the combustion engine with the hybrid assembly has had two recalls (2018 for a leak on the Niro and 2025 for moisture in its electronic board on the Niro, Ceed and XCeed PHEV), with a risk of short circuit and fire.
- High severityFrequency: Occasional1.3 TCeBlown fuel pump fuseOn the Renault Captur II and Clio V with the 1.3 TCe engine, a 20-amp fuse can blow and leave the car without an engine. According to L'argus, in April 2021 this fault still had no factory fix; the official service centre's diagnosis includes checking, and sometimes replacing, the catalytic converter.
- Critical severityFrequency: CommonRover 75Seat SRS connector fault with airbag and seatbelt warning light onAuto Express warns that the Rover 75's seat SRS connector can cause the airbags to fail, and dealers were fixing it with revised connectors. The Dutch APK inspection puts the airbag or seatbelt fault warning among the most frequent defects across almost all series.
- Critical severityFrequency: OccasionalAlfa Romeo GiuliettaShort circuit from badly routed wiring that can prevent gear shifting (KBA campaign, 2010-2015)Yes: according to Germany's KBA, on Alfa Romeo Giulietta cars built between 2010 and 2015 inadequate cable routing could cause a short circuit and, as a result, gear-shift failure.
Control units and comfort electronics (BSI, SAM...): the most serious faults by engine and car
- High severityFrequency: CommonPeugeot 408Electronic faults with the doors, boot and screen (suspected BSI control unit)In 2025 Peugeot recalled around 60,000 2023-2024 Peugeot 308 and 408 cars (code MUV) over a software fault in the door control unit that can cause unintended opening or closing and disable the anti-trap protection on the windows; 408 owners also describe a power boot lid that will not open or close and a screen that goes black.
- High severityFrequency: CommonRenault Laguna IIMultiplex electrical system and hands-free card faultsThe Laguna II had a reputation problem because of faults in its multiplexed electrical network: the hands-free card, the card reader and the ABS, ESP and handbrake warning lights came on erratically, especially in the early years of production (2001-2003).
- Critical severityFrequency: OccasionalMazda MX-5 IVDC/DC converter control unit fault with risk of short circuit and fireA 2018 Safety Gate alert covers MX-5 ND models built between August 2014 and April 2017: the DC converter control unit could fail, cause a short circuit and damage the converter, with a risk of fire. It is a precautionary recall; check the chassis number at the official service centre.
- Medium severityFrequency: Very frequentFiat StiloElectrical and electronic faults in the multiplex systemThe Stilo has had a reputation for widespread electrical faults since its launch: poor contacts in the multiplex wiring, an instrument cluster that switches off and sensors that disconnect, sometimes while the car is running.
P20B9 by make
Documented faults that can trigger P20B9 in each make's engines and cars.
Frequently asked questions
What is the AdBlue heater for?
AdBlue can freeze in cold weather, and the tank includes a heating element to prevent it. CarParts explains that the heater stops the AdBlue from freezing or crystallising in cold weather, because that liquid is injected into the exhaust to reduce NOx.[2][5]
Can I keep driving with P20B9?
The sources do not agree: CarParts rates it as severe and advises not driving with the code, while PartsAvatar only describes the warning light and the inability to regenerate the particulate filter. RACE warns that a fault in the SCR can end up preventing a restart. It is worth taking the car to a workshop soon.[2][4][5]
Does the AdBlue tank need replacing?
Only if the heater is part of the tank module and is the part that fails. First the fuse, the wiring and the connectors, the heater and the temperature sensor are checked. RACE notes that when the electrical fault is in the tank module, the usual remedy is to replace the whole assembly.[1][4][5]
What does the “A” in the name mean?
It identifies the heater when the car has more than one. CarParts warns that the definition may vary by manufacturer, so it is worth confirming in the car's documentation which heater the code refers to.[2]
Will I pass the MOT with code P20B9?
If the code has switched on the engine warning light, no: with that light on, the inspection records a serious defect, and on Euro 5 and Euro 6 cars the ITV (Spain's roadworthiness inspection, similar to the MOT) also reads the OBD diagnostic system. Repair the fault before you go; clearing the code just before the inspection does not help either, because it counts as a serious defect.[7]
Not sure what is wrong with your car? Describe what you notice in the symptom diagnosis or see all fault codes.
Sources
Every fact on this page links to its source. If you find an error, let us know.
- OBD DTC Fehlercode P20B9 | Reduktionsmittel-Heizungssteuerung A – offener StromkreisCarWiki.de · Technical documentation · consulted 31 October 2026 · in German
- P20B9 Code: Reductant Heater "A" Control Circuit / OpenCarParts.com · Technical documentation · published 19 December 2024 · consulted 31 October 2026
- p20b9 Generic DTC: Reductant Heater "A" Control Circuit/OpenDTCSearch.com (OBD Solutions) · Technical documentation · consulted 31 October 2026
- P20B9 OBD Error Code: Problem & SolutionsPartsAvatar · Technical documentation · published 2 June 2022 · consulted 31 October 2026
- Qué es un catalizador SCR: funcionamiento, AdBlue y mantenimientoRACE · Motoring press · consulted 31 October 2026 · in Spanish
- On-board diagnosticsWikipedia · Other source · consulted 31 October 2026
- Desmontando mitos: ¿puede pasar mi coche la ITV con el testigo de «avería motor» encendido?Diariomotor · Motoring press · published 8 March 2021 · consulted 2 October 2026 · in Spanish