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991.1 Coolant Fault - Change Over Valve Diagnosis


PCGB Member
Porsche 991.1 Carrera 'Coolant Fault' (Change Over Valve) Diagnosis


Back in July, whilst bedding in my new Girodisc brakes (operating temp: water 90C, oil 103C, in Sport mode, repeated heavy stops), my car suddenly flagged up a 'Coolant Fault'. This was the first time it had ever missed a beat in my ownership and unfortunately was just prior to me attending another high performance driving day, at PEC Silverstone (Arghh).


I checked the code with my Foxwell NT530 scanner and it showed as P1433 - (implausible signal) Vacuum Leaks: Basically a leak in the car's vacuum system. This system uses electrical solenoids called COV's (Change Over Valves), to control a range of components, using vacuum actuation. Some are related to thermal management, some to items as diverse as the exhaust flaps for PSE (Porsche Sports Exhaust) - so even though it shows as a coolant fault, it might not be coolant related at all!

My Porsche ( 991.1 C4S Exclusive) has 8 x COVs within the vacuum system - see schematic and list attached.


The Power of Google: Having done some research, it seemed unlikely to be an electrical failure of a COV solenoid - more likely a vacuum leak inside a COV, one of the connecting pipes, or even a disconnected pipe - apparently the one for the exhaust flaps can sometimes come loose. My good friend Peter (who helps with any physical work, now my bone cancer prevents me doing much), crawled under the car and checked the exhaust flaps but the pipework underneath was fine.

Active Testing with the Foxwell NT530: Covering all bases, I decided to rule out any chance the solenoids were faulty and used the 'Active Test' function of my Foxwell scanner, to isolate each COV solenoid in turn and test it but each one returned a satisfactory result. So, with the day at Silverstone looming, I cleared the code and took the car for a test drive, travelling the same roads I'd been on when the fault flagged (including a section of very rough tarmac on a country B-road) and after several passes, 'ping' the fault flashed up again.

Given the circumstances, I decided the fault wasn't constant, cleared the code again and a few days later, was off to PEC Silverstone. I had a great time, putting the car through its paces and once up to temperature, it got a thorough shake-down (in Sport & Sport Plus) but all temperatures remained fine, everything worked, no codes flagged and it never skipped a beat.

Testing COVs and Pipework: I don't like leaving things to chance, so once back home, the next step was to remove the rear bumper, active spoiler, air-box etc and start testing. Thanks again to Peter for his invaluable help!

Note: Any DIY competent home mechanic, can remove the rear lights, bumper and active spoiler - if you don't have the knowledge or a manual, there are guides on how to do this on YouTube. Just get someone to help lift things off and don't switch the ignition on whilst the spoiler is off, or you'll have to re-calibrate the spoiler end positions. Any codes that are stored when removing the rear bumper, such as rear lights, can be cleared with an OBD scanner, once the work is completed.



Part of the process is removing the air-feed (fresh air) blue pipes, electrical connectors and air-box - so you can more easily access the COVs, at the top of the engine.

Each COV has 3 connections - 1) an air inlet 2) a vacuum outlet - to the controlled component and 3) a common vacuum inlet.

Using my vacuum pump, we tested the vacuum inlet/outlet of each COV in turn (working from the back of the car forwards). All the COVs checked out fine. The only two we couldn't check (due to access), were the ones underneath on the PDK (Heat Exchanger for Gear Oil / Clutch Fluid Heat Exchanger), as you need a lift for those. Each COV held circa -25 to -27 inHg, for 3 minutes - proving their integrity (in a static situation).



Close Up of Exhaust Flap COV and Coolant Shut-Off Engine Bypass

When dismantling the car, we'd noticed that one of the plastic vacuum pipes (the one that will most often be disconnected when removing the air-box) didn't seem to be very solidly located into the rubber T-piece and also the one that connects between the two COVs on the air-box (controlling the Acoustic Simulator and Air Cleaner Flap), wasn't fully home - you can see a black ring around the plastic pipe and this should be seated, all the way into the rubber connector.


We then checked the vacuum integrity of the accessible connecting pipework, to make sure there were no obvious leaks. Again all checked out fine (at the time of testing), with the pipework consistently holding over -27 inHg, for 3 minutes.

Attaching a smoke machine can help identify leaking pipework but few people have one at home and it isn't an absolute must.

Conclusion & Solution: Given that the fault occurred on my car, on an exceptionally rough stretch of tarmac, on a rural road I rarely travel, I wondered whether a combination of extreme vibration and a couple of pipes that weren't seated 100%, created a temporary fault? It only takes a momentary loss of vacuum to create this fault, so that would seem plausible and as after clearing the fault code, I drove more than 200 miles to Silverstone & back and hard on-track in Sport/Plus without incident, that also pointed to something very specific.

As a consequence, I purchased some new OEM rubber fittings, to replace any that looked tired (the car is 10 years old after all), seated the plastic pipes fully and then zip tied them, for added security.


I've driven the car around 200 miles since, on A roads, B roads and Motorway and whilst I haven't been back to that specific section of rough tarmac (yet), so far everything's been fine.

There have been numerous iterations of these COV solenoids over the years, with some owners having several versions fitted to their car, during their ownership: One point worthy of note, in my investigations I noticed there was a Porsche Technical Bulletin from 2015 (referenced at the top of an excellent Blog, by a user called Plenum), that mentioned any replacement of COVs, should now use a specific part number for each installed location - my understanding is that the original system used the same part, irrespective of installed location (though I'm happy to be corrected).

IMPORTANT: It's important to note, that there isn't just one cause for this fault and therefore there isn't just one solution. One answer is no more 'right' than another - only the one that fixes that particular car. The work I've undertaken, seems to have fixed the problem on my car but this post isn't to suggest, that what I found is the answer - in fact my vibration related loss of vacuum, may be an outlier! I've given an overview of the 991 Carrera vacuum system, to help give fellow owners some insight but not attempted to cover all the possible causes, which include but are not limited to: Faulty COV units, rusty COVs (internal condensation), leaking vacuum pipework (as in my case), leaking coolant caps - which sometimes warp, coolant contamination of the vacuum system and other system related components (different codes), such as the Map Controlled Thermostat - which can all give rise to the same 'Coolant Fault' warning.

Anyway, I hope some of that's been useful and am keeping my finger's crossed that my work on securing the pipework, bears fruit long-term.

Mark Davis (AKA: 911Time)

Note: A million thanks to my good friend Peter, for his hard work and enabling me, to vicariously continue tinkering with cars

COV Schematic/Image: Courtesy of Plenum's very useful page!
Many thanks for your very interesting and detailed post. I too had a similar alarm show up, and with no obvious effect on coolant or oil temps. As my 2012/ 2013MY 991.1 is under the Porsche extended warranty, my local OPC carried out the repairs FOC. From a dull memory they replaced 3 COV`s the one that caused the fault one with a cracked casing and another with suspect electrical connections.

As you described it does indeed appear that there have been a few versions of COV`s, and my OPC reported that only one has not been replaced over time, and that is the one for the rear wing lift, which they said was a simple task to replace.
Thanks to you both for two really interesting and informative posts.

While I have not suffered a similar problem the comments on "COV's" is extremely interesting particularly with reference to the one for the rear wing lift.

I have a 911.1 Turbo S and the rear spoiler has failed for some time to deploy automatically on attaining 50mph.

Our local OPC has run the so called "diagnostics" several times while under warranty and insist that there is no fault. I live on an Island with a very low speed limit and therefore cannot demonstrate this to the technician.

However when in Europe I have had friends following me on many occasions who have confirmed this to be the case although I can deploy it manually.

The car is now out of warranty but it would be helpful if I could suggest that the COV is causing the problem.


Hi fireblade, on my 991 C4S there is a button with which you can deploy the wing....I suspect you may have the same..? That being the case then I presume that your issue is speed related triggering, as opposed to the ability of the wing to deploy and park..?
Some 992 owners including myself have had problems with the spoiler/wing not deploying or if it did deploy it got stuck in the deployed position. The fix is to put the car in Sport+ mode and run the car for maybe a quarter mile and the spoiler/wing software will then reset itself and all is well. I’m not sure if this fix applies to a 991 but maybe worth a try. It’s cost free too.
Thanks both.

I can deploy the spoiler manually but it does not deploy automatically when stationary when Sport or Sport+ modes are activated either which it used to do so I discounted a problem with speed.

As suggested I will give it a run with Sport + and see what happens but I will not be able to get to 50mph.

Thank you all for your comments - pleasing to know that the time taken has been worthwhile :)

Can I just clarify, those suffering with rear spoiler deployment issues - what specific model are we talking about? FWIW, the 991.1 Carrera 2/4 Non S/S models all have electric motors to operate the rear spoiler deployment (manually or speed related) and are not controlled via a COV (Change Over Valve).
Mark (911 time) Many thanks for the clarification.
Just that I thought the assistant service manager at my OPC suggested that there was a COV involved in the spoiler deployment, as he suggested it was the easiest to access.... I must have picked that up incorrectly... Sorry for the confusion.
I am less confused than before these posts!!

Just disappointed that these so called "diagnostics" cannot pick up a fault let alone suggest rectification.
I could strip down and rebuilt an MGB engine (shows my age) but modern cars are a mystery to me.

Despite "fireblade" as your chosen forum I.D. it seems we may be from a similar I too was very comfortable working on pre 80`s technology..(-: As soon as things went digital I gave in.
I have restored and maintained my air cooled Porsches over the years, up till my last air cooled which was an 82 SC. I leave maintenance of my 991 to my OPC. I am also more than grateful that Mark is kind enough to share his knowledge of the 991 here.
Gents, I can readily identify with your comments: Despite having a spanner in hand from a young age and training as an engineer with Audi (early 80's), an awful lot's changed since then! I left the automotive industry 20+ years ago, so my knowledge base is routed in the days when you could diagnose problems the old-fashioned way (without access to OBD) and rebuild an engine/'box, tune an engine etc at home, without very much in the way of specialist kit! The engineering of the later 911 models, is very interesting (to me) and there's some very clever work been done but it does make working on them all the more interesting/challenging (depending on your viewpoint). I've a copy of the official 991 manual and it's over 5,000 pages! Try that with a Haynes manual :)

I should mention that the overview given in this post, is just an insight into the operation of the vacuum system and the problem I faced with mine - one (of several) potential causes for failure, which include faulty COV's, rusty COVs (internal condensation), leaking vacuum pipework, leaking coolant caps (which sometimes warp), coolant contamination of the vacuum system and other system related components (different codes), such as the Map Controlled Thermostat - which can all flag the same warning.

Unfortunately, the vacuum system can't self diagnose where the problem originates (it's a dumb system), which sometimes leads to both OPCs and Indies failing to apply old-school diagnosis, or getting to the root cause, without significant investment of time, or replacing parts willy-nilly....I know of several Indies that just replace all COVs (well over £1,000), in the hope that will cure the problem. Understanding the basic system and following a logical path to diagnosis, is just as important as a scanner.

As these cars get older, we need greater dissemination of information, to help anyone willing to give it a go, a fighting chance :)


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