Electrical system and control unitsGeneric OBD-II codeUrgency: Garage soon
Code P0685: ECU power relay: open circuit
P0685 means: ECM/PCM power relay control circuit: open.[1][2] The engine ECU has detected that its own supply through the main relay has failed or that the voltage in that circuit is low. TroubleCodes.net specifies that the code only applies to cars in which a relay supplies battery power to the ECU.[2]
What code P0685 means
The engine ECU has detected that its own supply through the main relay has failed or that the voltage in that circuit is low. TroubleCodes.net specifies that the code only applies to cars in which a relay supplies battery power to the ECU.[2]
It is a generic OBD-II code (it starts with P0), so it means the same on any make.[1]
Original definition (SAE J2012): ECM/PCM Power Relay Control Circuit/Open[1][2]
Causes of P0685
From most to least likely, according to the sources.
Symptoms
- Warning light on, although in some cars it may not come on with this fault.[2][3]Engine warning light: faults that cause it
- Difficulty starting; in many cases the car does not even crank or start.[2][3]Hard to start: faults that cause it
How to diagnose P0685
In order: from the most likely and cheapest to check to the most expensive.
- 1Note down all the codes and the freeze-frame data, which are useful if the fault is intermittent.[2]
- 2Locate the ECU power relay using the manufacturer's diagram and visually check the wiring, connectors and fuses; do not replace a blown fuse without first looking for the short circuit.[2]
- 3Measure the voltage at the relay terminals and at the ECU connector; if voltage is missing with good fuses, suspect the ignition switch; check the grounds as well.[2]
- 4Test the relay itself according to the manual and, only if everything else is fine, suspect the programming or the ECU; that takes advanced equipment.[2]
Can I keep driving with P0685?
Garage soon
If the ECU loses its supply the car may not start or may cut out, and TroubleCodes.net warns that the absence of a warning light does not prove there is no fault. Get it checked soon.[2]
Parts related to code P0685
The parts that most often cause this code. The code points to the circuit, not always the part: diagnose before you buy.
Affiliate links: if you buy from these shops, MotorFiable may receive a commission at no cost to you. Check compatibility against the registration plate or VIN before ordering. More information
Documented faults related to P0685
Known faults of specific engines and cars, with their repair cost, the affected years and their sources.
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.
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.
12 V battery: the most serious faults by engine and car
- High severityFrequency: CommonHyundai-Kia E-GMP (Ioniq 5, Ioniq 6, EV6)Faulty ICCU: 12 V battery drains, charging fails and power is lostThe ICCU, which combines the on-board charger and the converter that powers the 12 V system, can fail on the Ioniq 5, Ioniq 6 and EV6: it cuts AC charging, drains the 12 V battery and, in the worst case, the car loses power and stops. Hyundai and Kia have recalled units built from 2021 to 2024.Repair: €2,000–€5,000
- Medium severityFrequency: Very frequent1.5 Hybrid12 V battery going flat while parkedYes: on the hybrid Yaris and Yaris Cross, the 12 V battery can go flat within a few days while parked because of the residual draw of modules such as eCall or keyless entry. It is fixed with a software update or, if the battery is already damaged, by replacing it.
- Medium severityFrequency: Very frequent1.8 HybridThe 12V battery drains on the C-HR and Corolla hybridsYes: on the first-generation C-HR hybrid, the 12V battery is one of the most frequently reported faults: it runs flat after a few days standing or on short trips, and the car will not start. The ADAC and Toyota itself point to it as a cause of faults on the C-HR and Corolla.
- Medium severityFrequency: Very frequent2.0 and 2.5 HybridThe 12 V battery goes flat if the car sits unused for several daysYes, it is a common complaint: the 12 V battery can go flat after just a few days without using the car, because of the standby drain from the electronics (multimedia, telematics, keyless entry). With the 12 V battery very low, the hybrid system can lock itself into a safe mode.
- High severityFrequency: OccasionalBMW Serie 6 E63/E64Battery drain and loose positive cableThe battery and alternator are frequently cited by Fiches-auto, and there is a recall for the battery's positive cable, which can work loose.
- High severityFrequency: OccasionalFord Galaxy IIIAcid leak from the 12 V battery (campaign 18P11)According to the fiches-auto.fr page, campaign 18P11 (August 2019) covers Galaxies built between February 2014 and February 2019 over a risk of battery acid leaking, which can worsen over time.
Frequently asked questions
Can I drive with code P0685?
If the car starts and runs, you can get to a garage, but the fault can leave it unable to start at any moment, so do not delay the check.[2][3]
Does P0685 mean the ECU is faulty?
No, that is rare. The usual culprits are the relay, the wiring, low voltage or the ignition switch; the ECU is suspected last.[2][3]
What is the difference from P0686 and P0687?
TroubleCodes.net lists them as related codes for the same relay: P0686 is control circuit low and P0687 control circuit high.[2]
Will I pass the MOT with code P0685?
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.[4]
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.
- P0685 DTCDTCSearch · Technical documentation · consulted 29 October 2026
- P0685 trouble codeTroubleCodes.net · Technical documentation · consulted 29 October 2026
- OBD Fehlercode P0685CarWiki · Technical documentation · consulted 29 October 2026 · in German
- 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