Electrical system and control unitsGeneric OBD-II codeUrgency: Garage soon
Code P0688: control unit relay: sense circuit open
P0688 means: control unit power relay sense circuit (ECM/PCM): open.[1][2][3] The engine control unit is powered through a relay and monitors the sense circuit of that relay. P0688 indicates that this sense circuit is open. PartsAvatar describes it as the circuit signal falling outside the manufacturer's values.[2][3]
What code P0688 means
The engine control unit is powered through a relay and monitors the sense circuit of that relay. P0688 indicates that this sense circuit is open. PartsAvatar describes it as the circuit signal falling outside the manufacturer's values.[2][3]
In the Volkswagen Group, Ross-Tech defines it as “control unit power relay (J271) charge circuit: open circuit” and mentions a striking symptom: the radiator fan runs at full speed with the ignition off and the engine cold, which can drain the battery.[1]
It is the sibling of P0685, the control circuit of the same relay (DTCSearch defines it as “ECM/PCM Power Relay Control Circuit/Open”). PartsAvatar also cites P0689 and P0690, which are the sense circuit with a low and a high signal.[2][4]
Original definition (SAE J2012): ECM/PCM Power Relay Sense Circuit/Open[1][2][3]
Causes of P0688
From most to least likely, according to the sources.
- 1Faulty control unit power relay. On VWs, Ross-Tech cites relay J271, including terminal 87 in poor condition or stuck.[1][2]
- 2Blown fuse: Ross-Tech mentions the fuses, and PartsAvatar the control unit fuse.[1][2]
- 3Damaged wiring between the control unit and the relay (on VWs, between J220 and J271), or a connector or electrical connection in poor condition.[1][2]
- 4Weak battery or low voltage on starting.[2]
- 5Faulty control unit: a rare cause, which PartsAvatar places last.[2]
Symptoms
- Engine fault warning light on.[2]Engine warning light: faults that cause it
- Difficulty starting; the car may not start at all.[2]Hard to start: faults that cause it
- The engine stalls.[2]Engine stalls: faults that cause it
- Sometimes there is no other symptom than the stored code.[2]
- On a VW, the radiator fan runs at full speed with the ignition off and the engine cold, and it can drain the battery.[1]
How to diagnose P0688
In order: from the most likely and cheapest to check to the most expensive.
- 1Confirm the code with a scanner and check whether there are other voltage or communication codes stored.[2]
- 2Check the battery: terminals and cables free of corrosion and play, full charge, a load test and a test of the starting and charging system.[2]
- 3Check all the fuses, including the system fuses.[1][2]
- 4Check or replace the control unit power relay (J271 on VWs). Disconnect its connector and check the voltage and earth on the associated circuits.[1][2]
- 5Check the wiring between the control unit and the relay (J220 and J271 on VWs) and the connectors for damage, corrosion or play. On some 2005–2007 North American Jetta 2.5s, Ross-Tech points to a damaged control unit power cable between 15 and 45 cm from the connector, which can cause a loss of communication and a failure to start.[1][2]
- 6Leave the control unit until last: PartsAvatar warns that replacing it requires reprogramming, so the other possible causes should be repaired first. After each repair, clear the codes and carry out a road test.[1][2]
Can I keep driving with P0688?
Garage soon
None of the sources consulted rate the severity of P0688 or say whether the car can be driven. They do cite the car not starting or stalling among the symptoms, and Ross-Tech records the fan running with the ignition off, which can drain the battery, so it is worth diagnosing soon, especially if the car is starting badly.[1][2]
Parts related to code P0688
The parts that most often cause this code. The code points to the circuit, not always the part: diagnose before you buy.
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Documented faults related to P0688
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
What does code P0688 mean?
That the control unit has detected an open sense circuit on the relay that supplies it with power. The main relay of the control unit, its fuses or the wiring between the two are the first suspects.[2][3]
What is the difference between P0688 and P0685?
P0685 refers to the control circuit of the control unit power relay, and P0688 to the sense circuit of the same relay. In both, the circuit appears as open.[2][3][4]
Can I drive with code P0688?
The sources do not rate it. Among the symptoms they cite the car not starting or stalling, and in some cases there is no other symptom than the code. It is worth diagnosing soon to avoid being left stranded.[2]
Does the control unit need replacing?
It is the last thing to consider. PartsAvatar points out that replacing it requires reprogramming, and that source and Ross-Tech both put the relay, the fuses and the wiring between the control unit and the relay first.[1][2]
Will I pass the MOT with code P0688?
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.[5]
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.
- 17072/P0688/001672 - ECM Power Relay Load Circuit: (J271): Open CircuitRoss-Tech Wiki · Technical documentation · consulted 31 October 2026
- P0688 OBD Error Code: ECM/PCM Power Relay Sense Circuit OpenPartsAvatar · Technical documentation · consulted 31 October 2026
- P0688 OBD Trouble CodeDTCSearch · Technical documentation · consulted 31 October 2026
- P0685 OBD Trouble CodeDTCSearch · Technical documentation · 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