Chasing The Elusive Check Engine Light

Volkswagen Jetta check engine light

The good news is that Massachusetts has a fifteen year rolling exemption on emissions testing. The bad news is our 2003 VW Jetta Ute donor is fourteen years old, just shy of that exemption. This is the worst case scenario – too new to be exempt, but so old that a check engine light is virtually inevitable – especially because it’s a Volkswagen.

How do you find a VW meet?

Naturally, ours was on, so I needed to get that light shut off to get that all important inspection sticker. Yanking out the bulb for the Check Engine light isn’t good enough. They plug a computer into the OBD2 port and pull the codes for themselves. It could be worse. At least now it’s just an OBD2 check instead of the old dyno tests that were supposed to simulate real world conditions. Everybody hated those – especially the inspectors, according to the guy who inspects my cars. Also, if he had a nickel for every time he was asked “Can you measure my horsepower too?” he could’ve retired a long time ago. The answer, by the way, is no. Years ago I couldn’t get my Honda Civic wagon to pass that test, as diesel trucks rolled by spewing massive clouds of thick black smoke into the air. So I did the only reasonable thing I could at the time. I moved from Massachusetts to an area of Maine without emissions testing and kept on driving.

Unfortunately such a move isn’t reasonable for me now, so I was faced with the seemingly impossible task of getting an older VW to pass emissions. This means addressing each of the trouble codes the OBD2 system records after running its internal tests while you drive. Fortunately, my wireless OBD2 interface and DashCommand let me read these codes rather than having to take it to a shop like in the old days. I already mentioned the “P2181 Cooling System Performance” error that existed when I got the car, as well as “P0507 Idle Air Control System RPM Higher Than Expected” and various misfires. I recorded, then reset these codes to start over with a clean slate. Who knows how many of these codes were left over from the previous owner? He hadn’t had the car inspected since a year and a half ago.

Too Cool For School

As I drove the car a bit – and solved my battery drain issue – the engine ran more smoothly, and the misfire codes didn’t come back, nor the idle air control system error. I’m guessing those were caused by low battery power, which is no longer a problem. But I did get a “P3081 Engine Temperature Too Low.” The engine temperature gauge confirmed this. It would warm up to 190 if I let it idle long enough, but as soon as I started driving the temperature dropped, getting colder the faster I went. There was definitely a temperature problem. But was it the cooling system itself, or just the coolant temperature sensor?

VW 1.8T coolant temp sensor
Photo credit: Pelican Parts

Since it’s conveniently located on top of the engine, easy to replace, and less than $10, I started by replacing the sensor. That was easy. Unfortunately, when I took a drive, the problem didn’t go away. But at least I knew I could trust the information the sensor gave me, which was that the engine was running too cold when in motion. This would be caused by a thermostat that was stuck open, allowing coolant to flow through the radiator when the engine was so cold that it shouldn’t.

A replacement thermostat and gasket also cost around $10. The problem, however, was the location. In their infinite wisdom, Volkswagen put the thermostat in the side of the engine block, then buried it behind the alternator. Despite efforts with angled, U-joint, and long reach extensions, I couldn’t remove the two bolts that hold the cover and radiator hose on. So I had to remove the alternator. This also meant removing the throttle body, which prevented me from pulling the alternator out, and the serpentine belt tensioner, which blocked the lower alternator bolt. I felt like I might as well start removing pistons and rods while I was at it just to get to the thermostat. At least I got the chance to inspect the serpentine belt when it fell out. It’s not new, but it’s still in good condition, so I reused it. I also gave the dirty throttle body a good cleaning while it was out of the car and I could easily reach both sides of the plate, for once.

Mk4 Jetta 1.8T thermostat location
It’s under all this crap somewhere.

Finally I got the access I needed to those two bolts. Out with the old, in with the new, and installation really was the opposite of removal. It was much easier, in fact, because I was simply replacing what I’d already spent a great deal of time figuring out how to remove. Everything worked on the first try. I took a drive and found the temperature shot up quickly to 190, and stayed there like it should, even at highway speeds. One problem down, one to go.

Oxygen’s For Losers

The other code was the dreaded “P0420 Catalyst System Efficiency Below Threshold (Bank 1).” In English, that means that the ECU doesn’t think the catalytic converter is cleaning the crud out of the exhaust well enough. I had hoped that this was related to my other issue of the engine running too cold. It’s entirely plausible that if the engine’s cold, the cat would be cold, not work as efficiently, and trigger the error. After replacing the thermostat I reset the codes again, then drove the Jetta to work to let it run its tests again. No dice – the light came back on, and the code once again was P0420.

I felt like I needed a 4:20 myself to calm down. (Don’t judge me – it’s legal in Massachusetts.) There’s no cheap way to fix this properly. Either the downstream O2 sensor had gone bad, or the catalytic converter itself was bad. Both are expensive to replace. Online research led me toward some less expensive ways to cure the code, if not actually repair the problem. The most common method was to add a spacer between the O2 sensor and the exhaust pipe. This way, the sensor doesn’t sniff air directly in the exhaust path, giving it a better chance of sending its seal of approval to the ECU. The cheapest method is to modify a pair of spark plug arrestors, but I bought a $16 spacer for this purpose. It includes a right angle and various bits and pieces to alter the configuration to fit your particular car.

Some of you may be questioning my life choices here. This is a trick commonly used to trick an ECU into thinking a catalytic converter is there when it’s not, and it’s illegal to remove one from a street car. Technically, it’s cheating. But my cat is still there, intact, and due to the car’s age this is the last emissions test it will ever need to pass in Massachusetts. As long as the code is gone, it’ll pass.

Downstream O2 sensor location
Photo credit: VgTr6 on VW Vortex

The O2 sensors in the Jetta are located most inconveniently on top of the exhaust pipe, with little room between the sensor and heat shield, which is why I chose the right angle adapter. Unfortunately, the entire area is so cramped that there’s no room for much of anything in there. The angled piece was too long to clear the cat when I tried screwing it in, so I ended up just using one of the two short spacers intended to extend or shrink the main piece. Even that didn’t go in without a fight – one that broke the wires off my oxygen sensor. Needless to say, I was cranky. Unlike the coolant temp sensor, oxygen sensors aren’t cheap – especially for this car, since it’s a wideband sensor.

A trip to the parts store and $110 later, I was ready to try again. This time I got it right. There wasn’t even room for the special O2 sensor socket with the spacer in there, so I used an open ended 7/8″ (22mm) wrench, turning the sensor a fraction of a turn at a time. Slowly, but surely, the sensor threaded into the spacer, and then screwed the spacer into the cat. Finally, after a great deal of work, time, and muscle cramps, it was in. I put it all back together, then went out for another test drive.

It’s Not Just Flipping a Switch

OBD2 tests in DashCommandOne mistake a lot of people make when using their own OBD2 scanner to troubleshoot and clear codes before inspection is thinking that as long as the Check Engine light is off, they’ll pass. That’s not true. After a reset, the ECU has a number of internal tests it needs to run before determining if there’s a problem worth turning the Check Engine light on for or not. There’s a manufacturer prescribed test cycle of idling, driving, and such, but normal street driving is usually enough to tick off all the boxes. If these internal tests aren’t complete, the inspector will know that when he plugs his computer into your OBD2 port, and punt you because of it.

This put me in an awkward Catch-22. I was long past my seven day window after registering the Jetta to get it inspected thanks to the extra repairs that became necessary after I started driving it. But I also couldn’t get inspected until I drove it a bit to run through these tests, either. So I took one of my favorite loops up through some New Hampshire back roads to flee my home state, in hopes that the Live Free or Die state wouldn’t care about an uninspected car from another state on its roads. I had no issues.

When I got home I checked the diagnostics in DashCommand, and all but two tests – the Evaporative System, and the Secondary Air System – had passed. The Check Engine light was not on, and no trouble codes had been recorded. According to my research, you’re allowed to have one incomplete test when you take the car in for inspection. I had two incomplete, so I had to figure out how to get one of the remaining tests to finish. The answer was shockingly simple – a cold start and warm-up at idle. This made the Evaporative System check pass. The remaining incomplete test was OK – I was now ready to face the music at the inspection station.

Did I Pass?

Emissions, yes! All of my research and hard work had paid off and saved me hundreds of dollars in having a shop make it pass for one last time.

Volkswagen Jetta Type R

Safety? No – for one reason. One stupid reason. All of my other repairs were definitely worth it, but it failed because one of the two tailpipes coming out of the muffler was missing. That’s it. If this was my only car, I’d be screwed. That inspection sticker with a red R doesn’t mean that it’s a VW Jetta Type R – it means the car is illegal to drive. ANYWHERE. Technically it’s even illegal to drive home afterward, though any reasonable cop could compare the current date and time to the date and time of the inspection and let me get my dangerous menace to civilized society off the road. But despite passing emissions, the Jetta is now grounded, all because of a missing tailpipe.

Stupid problems deserve stupid solutions, and I have one up my sleeve. But that’s a story for another time.

Follow @justinhughes54

  1. I traded in my silver 2002 1.8T Jetta wagon in with the engine light on. And it was pretty new.

    1. There were a lot of problems with bad sensors early on, including the two I just replaced. Later designs improved on them and reduced the errors, but yeah. That V Dub lyfe yo.

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