What does it mean if the EPC warning light comes on in a Volkswagen?

What does it mean if the EPC warning light comes on in a Volkswagen?

Since the inception of the motor vehicle, several manufactures have claimed that their brand of car, has sold the most units globally. But since there is no definitive comparative yardstick to measure the exact amount of units sold,  Volkswagen claimed that their Volkswagen Beetle is the "bestselling car in history", considering they had sold more than 21 million units. To be more exact, The New York Times, date July 31, 2003 stated that the very "Last Volkswagen Beetle" manufactured was in fact number 21,529,464. 

Ford on the other hand manufactured their 1 millionth Model T's during the last month of 1915, with total sales of 15 million units produced by 1927 when they ceased production. In 1972 Volkswagen broke Ford's record when their 15,007,034th Beetle came off the production line in Puebla, Mexico. 



Be that as it may, Toyota boasts that in reality, their Toyota Corolla is the "bestselling automobile in the world", after reaching their 40 millionth unit in July 2013. But regardless of their claim, Volkswagen is undoubtedly still the "bestselling automobile in the world" seeing that the Volkswagen Beetle didn't change its shape or form since 1945 up to and including 2003, whereas Toyota used their Corolla label for eleven generations of Toyota cars, built since 1966 and this very Corolla label is still in common use. Non-the-less, statistically Volkswagen has been the best selling car globally until late 1998; meaning the Volkswagen Beetle remains the "bestselling vehicle ever". 

Enter the Volkswagen Kombi, Volkwagen City Golf, Volkswagen Fox, Vokswagen Jetta, Volkswagen Golf, Volkswagen Polo, Volkswagen Passat, Volkswagen Kombi, Volkswagen Sharan, Volkswagen  Toureg, Volkswagen Amrok, Volkswagen Transporter and the list goes on. However this entire Volkswagen fleet doesn't come without their fair share of  problems. The most common of which is probably the EPC light accompanied by limp-mode home.

What is the VW EPC Warning Light, 

The Electronic Power Control warning light, is commonly known as the EPC warning light. It's a dashboard warning indicator that suggests that there may be a problem with your Volkswagen’s throttle system. The throttle system consists of the accelerator pedal, the throttle body, traction control, cruise control, and the ECU, which is collectively called the drive-by-wire system. 
The drive-by-wire system is also sometimes referred to as an Electronic Throttle Control (ETC), roughly described as a technology that electronically "connects" the accelerator pedal to the throttle, instead of a mechanical linkage. 

In a drive-by-wire system, an electric stepper motor determines the angle of the throttle valve. How wide it opens or closes, is calculated by a —closed-loop control algorithm— software controlled embedded system built into the ECU. The ECU makes decisions based on various input data from several sensors, among which are, the accelerator pedal position sensors, engine speed sensor,  vehicle speed sensor, and the cruise control switch. The Electronic Control Unit (ECU) can also be described as a computerized ignition / engine management system  that operates in conjunction with the stability and cruise control systems. When the EPC light comes on, it means the problem is not emission related, whereas the Check Engine Light (CEL)indicates emission related problems but when it lights up, can be accompanied by the EPC light. If the EPC warning light comes on in your vehicle, it is important to have it seen to as soon as possible, because it you don't invariable it's going to trigger again when you least expect it.

EPC Car Warning Light

When the EPC light comes on in your Volkswagen, its throttle may be limited in power, yet have sufficient power to drive the car to a dealership or mechanical workshop, but it shouldn't be driven elsewhere. This is called limp-mode-home. Limp mode home is  inherent in its the software design in order to to protect the car from run-away and  its passengers from possible injury. Volkswagen cars equipped with a Electronic Stability Program (ESP) program, will display its ESP light instead of the EPC light and may automatically trigger the car into limp-home mode. 

Vehicles fitted with Traction Power System (TPS) are also prone to enter into 'limp-home-mode'  which effectively prevents software glitches from initiating or activating unintended acceleration.  In 2011 the U.S. National Highway Traffic Safety Administration (NHTSA) commissioned a team of NASA software engineers to evaluate software of late model year Toyota and Lexus vehicles, alleged to be the cause of  unintended acceleration collision, which caused the death of a passenger in a September 2007. 

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The Definitive EPC Guide 2

The Definitive EPC Guide (Continued)

For part ONE Click Here!

Having said that, in my humble opinion, the  EPC circuit with its drive-by-wire circuit and its "Limp-mode home" mode, is a value-add for modern car design; and though frowned upon by many, is a very valuable safety feature. Also bearing in mind that when the communication network is momentarily unavailable and an application that depends on it is unable to communicate, it may trigger the ECU into "Limp-mode home" and set a DTC. Exception errors do occur in  synchronous or asynchronous distributed system, where software entities act as clients or servers or both. So when where 1...n clients are requesting services via a specific protocol from typically one server; the server services the client  but  temporarily blocks its service request and data flow control, whilst other client expects a response from the server.  

If 1 client to n server communication can be established, it would certainly solve the problem but it’s currently not supported. Also remembering that the CAN bus is a serial communication network through which all modules communicate with the ECU via a Central Gateway, some uses FlexRay frames and others Lin bus or CAN, each with a differing topology, some synchronous and others asynchronous. The Central Gateway is responsible for frame or signal mapping function between two communication systems, like from LIN/MOST/CAN/FlexRay to Ethernet transport systems. However ECU gateways often have two or more internal Gateways like the Service Data Unit (SDU) gateway, Layer 3 Tunnelling Protocol (L3TP) gateway and the Signal Gateway needed for frame or signal mapping function between two disparate communication systems.

The best analogy with which to explain the above would be your cell phone. Cell phones are native Global System for Mobiles (GSM) devices with a microprocessor and its support chips, much like the automotive ECU. Cell phones support the Hypertext Transfer Protocol (HTTP) and the Wireless Application Protocol (WAP) for Wi-Fi as well as the Bluetooth communications protocol through a "Gateway" which translates it to a common data stream native to the Cell phone operating system. 

So, while you’re downloading an email and the phone rings and you answer; the email download temporarily stops but will resume when the call is terminated. The same applies to a Bluetooth file transfer which will be interrupted even possibly terminated when a phone call is answered. However, not all services are prone to such interruptions because some services are inherent to the design. For example, when you’re listening to music on your cell phone and you answer an incoming call, the music is temporarily suspended until the call is terminated then the music will resume. 

This function is purposely part of the design whereas exception errors are not and are more prone to happen with asynchronous communication, caused by propagation delays. This is what also happens when the ECU inter communicates with other modules and when the messages are not delivered within a prescribed period of time, can cause the car to go into "Limp-mode home" . So if for some reason the radio that is a non-essential service is hogging the network bus, and you step on the brake which the ECU will undoubtedly detect but the brake light didn't illuminate within the expected response time, which the ECU will also detect, then the car may go into "Limp-mode home" . 

Or you could hit a speed bump in the road at high speed and bounce back onto the road; the thump if hard enough or loud enough to trip a knock sensor would cause the car to go into "Limp-mode home" . Or you could have just done some high rev spirited driving to cause the car to go into "Limp-mode home" . One consolation is that, when the ignition is initially turned on; right before the car is started the ECU does a self-test to verify that all’s well, and if that is the case, the car will start and drive normally. 

However, if the self-test fails, one of the caution lights may remain on. It could be either the EPC light or the Check Engine light or perhaps both, or one or both could be blinking, which is a sure sign that you may have a problem. Once again, a diagnostic scanned would be the most informative way of establishing what the problem is. But I’ve encountered car owner who took their vehicles for diagnostic tests and no errors were flagged, so in some cases may be a futile exercise. 

Having said that, if you are stranded along the road side, the best would be to reset the ECU by removing the battery earth terminal for a minute or two, but make absolutely certain that you have access to your radio / DVD code before  you disconnect. Whatever errors occurred prior to this will be safely stored in the ECU non-volatile memory and may be checked when a diagnostic scanner is available but all stored values will be lost.  The downside of doing this, is that an additional error would be flagged because the battery (+30V) supply was interrupted, and now the ECU has to relearn your driving style.

ECUs are fairly robust and are more-likely-than-not the cause of your EPC problems, though unlikely but not impossible.  The most likely component in my humble opinion that would cause constant and annoying EPC problems would be the wiring harness. Not the actual wires, unless they are frayed and damaged by rubbing against the chassis but rather its edge connectors. 

The ECUs and associated modules each have multi-pin connector that’s tin plated and can become intermittent due to the constant vibration of the car. Copper contacts also oxidizes fairly quickly, so to prevent this from happening, they are tin plated which provides for a low resistance, good contact surface. However when water enters this connection, especially in the presence of a voltage or current flow, the contact normally turns black or corrodes green, impeding continuity hence connectivity. 

A poor vehicle ground connection is another culprit that can intermittently trigger the EPC light without leaving a DTC.  A blown tail light / brake light bulb can also cause the EPC light to turn on without leaving a DTC. Always make sure that the replacement is a 12V 21/5W bulb because the wrong bulb will also cause the EPC to trigger also without leaving a DTC.  A faulty brake-light switch (DPST) is also commonly known for triggering the EPC light and sometimes causes the 10A fuse to blow. 

So, when the brake is applied and the switch throws, its contacts reverse, meaning the normally-open contact closes and the normally-closed contact, open. If there is a delay in the switching time, even as short as .5 of a second, the EPC light will trigger, without leaving a DTC.  A tell-tale sign that the brake-light switch is faulty or that the fuse is blown; is that the Cruise Control appears to be faulty. Since they are both interconnected, the Cruise Control is reliant on a voltage provided through the brake switch. 

DTC can often be intermittent or sporadic so it’s best to ignore those, in preference of repairing those that are permanent. In so doing, they may have interrelations and the sporadic ones may disappear after the permanent errors are fixed. So, it’s best to perform a diagnostic scan, preferably an autoscan with VCDS first, then delete all DTC in the process. Thereafter save this scan and printout a hardcopy to store with the vehicles registration papers. 

The importance of this printout is that it’s a reference to all the modules coding, and if something is inadvertently changed, there would be an unalterable record to refer to. Once done, and as mentioned above, disconnect the battery's negative terminal then disconnect the positive pole and place the battery on charge for about two hour.  Replace the battery by first connecting the positive pole then lastly the negative pole.

 By following this sequence you will prevent damage to the electronics modules strew throughout the car.  At this point, all pre-learnt and all stored values would have been erased from the ECU. Essentially all the control modules would have been completely reset, restarting with a clean slate.  After starting the engine and driving the car for a short distance, check to see if any DTC have been flagged.

Resetting an ECU occasionally is a good thing, because any and all electronic equipment with a microprocessor can experience an unrecoverable error or an internal parity error or fail a cyclic redundancy check and requires a "cold" system restart. Computer  memory (RAM) also experience memory read/write errors, interrupt or address errors and general protection errors; so  when the ECU is reset/cleared, along with its non-volatile memory, the memory (RAM) is also reset.  AUOTSAR and its core partners are trying their utmost to rectify such problems in automotive ECU, but until they do, we will have to contend with EPC problems.

To read the first part... click here.

The definitive EPC guide

The Definitive EPC Guide

The following is the definitive guide to the Electronic Power Control (EPC), but before we continue, let me expound on what this guide is and what it's not. The very purpose of a guide is to provide information; to aid, give direction, to grant assistance, to facilitate, to support and to help you towards a solution or provide you with an explanation or an answer. However, even though this guide may furnish you with several answers, not every question can be satisfied by the same answers, though it could possibly steer you in the right direction. Hopefully you'll find this EPC guide definitive enough to provide you with the necessary knowledge to fix your Volkswagen, Audi, Seat or Skoda's EPC problem. Or at the very least, steer you in the right direction or share the lingo with which to discuss your EPC problem with your mechanic. 

But before I continue, let me first explain what EPC is. EPC stands for Electronic Power control and is a subsection of the OBDII. OBDII sometimes writen as OBD2 stands for On-Board diagnostics 2, which is composed of mandatory equipment installed in every motor vehicle manufactured since 1996. The purpose of this real time OBDII system is to control harmful exhaust emissions, by rigorously controlling the moment of combustion as close to the Stoichiometric  ideal as possible. Stoichiometric combustion is therefore controlled by an ECU (Electronic Computer Unit) in conjunction with the mass air flow components and the lambda oxygen sensors through constant feed-back in order to increases or decrease the short term fuel trim.  

To define an ECU instance, one could say that it consists of one microcontroller with its peripheral chips and its configured  software application.  This also implies that if more than one microcontroller in packaged in the same ECU housing, each microcontroller requires its own description of said instance. As such, each ECU forms an integral part of the OBD2 system, with strong interaction between hardware sensors and actuators; constantly monitoring and controlling several other engine components to provide the best engine performance at the most efficient fuel use , as well as provide the driver with the best driving experience. Another part of the OBD2 system is its network bus which interconnects several distributed computerized modules, ECUs, sensors and actuators, collectively geared towards better functionality and passenger safety. But before I get carried away, let me return to EPC.

The EPC sub circuit when triggered turns on the EPC light or LED which displays the letters EPC quite brightly in the display console. Many VAG car owners have asked “What's the meaning of EPC warning light when it comes on in a Volkswagen"? Short answer; it’s just a caution light like any other warning light, drawing your attention to a possible problem but with one exception. This exception is that the EPC light is an amber/yellow light and not red. 

Much like road signs that have different colour boards for danger/warning than for  prohibitory/restrictive signs  or than for  service/information  purposes, the automotive sector uses, red lights to signify danger/warning which warrants immediate attention and amber/yellow lights for informative/attention which also requires your attention, though not immediate. When the ECU detects that the engine oil is low for example, it turns on a red light – red oil-can image – which implies that it's dangerous to drive the car in that condition because its engine could seize and therefore begs you to respond and remedy this situation immediately. 

Likewise when your handbrake in engaged, a red light – red disk with a P or exclamation mark inside –  is displayed, implying that it's dangerous to drive with the handbrake engaged, expecting you to release it immediately. Disregarding this red light and driving the car with the handbrake engaged could cause the brake pads to overheat and fail to function when needed but that’s to say if the car would even move from its stationery position with the brake engaged. Headlights on the other hand are indicated by a green dashboard light or a blue dashboard light when the Brights are switched on, neither implying danger nor attention. So in that sense, the yellow EPC light is just an attention light, much like the yellow safety-belt-light provides information that you haven’t bucked up or the yellow reserve-fuel tank-light informing you that you would have to add fuel shortly.  

Very importantly, a yellow light doesn't mean disregard, because disregarding a fuel tank yellow light could leave you stranded without fuel along the roadside. Or disregarding the yellow seat belt light could result in a traffic fine for non compliance to traffic laws. Therefore do not disregard the yellow EPC light and have the possible problem seen to at your earliest convenience. I hear you asking “What can I do when the Yellow EPC light comes on"? Normally when the EPC light comes on, it is accompanied by "Limp-mode home" and sets a DTC in the ECU non-volatile memory. So you need to have your car scanned with a diagnostic tester to determine and analyse the DTC error. 

I hear you asking "What is limp mode". Short answer, when the ECU detects a problem in the toque circuit it prevents the engine from revving above 2000rpm. In effect, it limits the electronically controlled throttle valve – Drive-by-wire – from opening wider.  “So what’s this Drive-by-wire thing”? you may ask.  Well the drive-by-wire circuit is a sub circuit of the EPC circuit and it simulates an accelerator cable, where the depression of the accelerator pedal is monitored by the ECU which in turns drives the throttle body valve open, in unison with the pedal action without any physical cable connection. Essentially it's an electronic version of an accelerator cable /acceleration process. Its electronic circuit is composed of dual sensors constantly monitored by the ECU, and any interruption in its data, will more-likely-than-not trigger "Limp-mode home", turn on the EPC light and set a DTC.

The ECU also constantly monitors the dual sensors in the throttle body as well as  the motor that drives the throttle body butterfly valve. Any interruption in its data will more-likely-than-not trigger "Limp-mode home", turn on the EPC light and set a DTC. This is a safety feature built into OBD2 to prevent the car from going into runaway mode with the possibility risk of injury. Image what would happen when a frayed accelerator cable jams inside its armoured sleeve, unable to decelerate the engine whilst travelling at a speed in excess of 72kph. Considering more than 50% of all accidents in rural areas happens at speeds between 72kmph and 80kmph whereas 70% of all fatal crashes on the freeway occurs at speed of 100kmph or higher.

Continue to part TWO.... Here!