Eternal Flame: The Check Engine Light that Never Dies

Part One: The Thermostat

The V8 Vantage comes with an awesome feature, one that lets you know when your engine is running: a yellow light on the gauge cluster. It’s right there in front of you and it never goes away. Mine has been so constant that I actually get worried when it turns off. This yellow indicator is a CEL. In any other car, CEL stands for check engine light.  In a V8 Vantage, it stands for check everything light.

Having to check everything is why this is the first in a series of blog posts rather than a fully-inclusive single post.  The good news is tha the CEL is caused by one thing the vast majority of the time – an emissions system fault. It’s easy to know when this is the case because the digital readout will actually tell you so.  The bad news is that everything causes an emissions fault.  The good news about the bad news is that it’s rarely something serious. The bad news about the bad news is that it’s a huge pain to diagnose because of the myriad of potential underlying causes, and labor costs for diagnostic work adds up quickly.

At best, the CEL is an annoyance. At worst, it can be expensive to diagnose and even prevent you from being able to register the car.  In some places, a CEL is an instant failure for vehicle inspections.  For Part One of the Eternal Flame series, I’ll be discussing a common problem that does, oddly, trigger an emissions system fault:  the thermostat.

Before we begin, I highly recommend getting an OBDII-compatible scan tool (AKA, code reader). Pulling fault codes should be the first step of any diagnosis, and having your own scan tool will make life so much easier.  Aston Martin’s proprietary one is called the Aston Martin Diagnostic System (AMDS). It’s the best out there for these cars, obviously, but they’re difficult to get and really expensive.  I use TOAD software and an ELM327 USB-connected OBDII plug in lieu of an AMDS. It’s a fraction of the cost and does much of what an AMDS can do, including clear CELs and fault codes!

A bad thermostat does in fact cause a CEL in the V8 Vantage.  The process goes something like this:

There is a rubber gasket on the thermostat that comes loose, preventing the thermostat from being able to close.

Since the thermostat can’t close, the coolant is unable to get to its proper operating temperature.

Since the coolant isn’t at its proper operating temperature, the engine isn’t running as efficiently as it should.

Since the engine isn’t running as efficiently as it should, it’s not burning fuel as cleanly as it’s supposed to.

Since the fuel isn’t being burned as cleanly as it should be, the exhaust contains extra contaminants.

Since there are extra contaminents in the exhaust, the emissions system is unable to clean the exhaust to regulated levels.

BOOM. A check engine light caused by an emissions system fault caused by a bad thermostat.

Here’s what my thermostat looked like when I pulled it out:

Faulty Aston Martin thermostat

Faulty Aston Martin thermostat

It isn’t hard to see that this could be a problem.

There are two ways to quickly figure out if your thermostat has gone bad. Well, specifically if the gasket has gone bad.  The first is that the coolant temperature doesn’t get up to the halfway point on the coolant temp gauge.  If it’s really hot out, it may just struggle to get there.  Regardless, you’ll notice that it hovers around the 1/3 point.  The second is to read the fault codes.  If the thermostat has a problem, you’ll see code P0128 Thermostat Range/Performance.

A dealership generally charges around $450 for a new thermostat and installation. Fixing the thermostat can be done as a DIY, and I’ll be putting together a comprehensive guide for it soon (UPDATE! Here it is). The job takes maybe two hours and is only moderately difficult.  I’ll try to remember to time the job when I do it on my buddy Justin’s car.  The really neat thing about the DIY is the savings. Rather than paying around $150 for a thermostat and about $300 for labor, you pay nothing for the labor and a fraction of the price for the thermostat.  How?  Use one from a Jaguar. I sell them in my online shop, so just jump over to the maintenance page and you'll find the part.

In the coming weeks I’ll be replacing the thermostat in Justin’s car and documenting the entire process to create a DIY. Stay tuned for the DIY and for the next installment in the epic Eternal Flame saga!

Part 2 of this series can be found here.