This One Simple Modification Turned My Perfectly Reliable Toyota Into A Brick

A year ago, my wife and I made perhaps our wisest decision in a while when we replaced a beaten and rusty BMW with a reliable Toyota. My wife’s Scion iQ has done everything she’s ever asked of it, including driving nearly 40,000 miles in a single year. It’s been bulletproof, too, aside from a really stupid failure caused by one small modification, which left the car stranded once and otherwise perpetually stuck in limp mode. The car effectively became as useful as a transportation device as a brick. Who knew wanting to go a single speed would be such a headache?

As many of our readers know, I own pretty much nothing but terrible, unreliable cars. I work from home, so nothing I own needs to run for any longer than a few hours at a time. I’ll happily buy the worst junkers in automotive history because they rarely need to go further than to the beach and back. My wife, Sheryl, has the exact opposite requirement from her cars. She drives around 40,000 miles a year for work.

Unfortunately, as Sheryl has learned the hard way, there aren’t a lot of cars that enjoy being driven hard and put away wet. The BMW E39 that we bought from the Bishop eventually began misfiring and sputtering. My poor wife also had a reliable Toyota Prius, but our friend managed to cause $14,000 in damage hitting both a curb and a deer at apparent warp speed.

Heck, my wife is so unlucky that she once had a car powered by the legendarily reliable Buick 3800 V6, yet a local shop was so incompetent in fixing it that the engine inhaled its own coolant until it windowed its block. Poor thing. Her car before that, a Chevy HHR, fried its body control module during a road trip. Even one of my Smarts tried failing on her.

Understandably, Sheryl is tired of cars dying on her. So, this time, she thought of herself as making a boring choice and getting a Toyota. But she didn’t want to just get something like an Avalon or whatever. She wanted it to be tiny and weird like my Smarts. In other words, a Scion iQ.

I tried to be brutally honest with my wife. There are about 15,700 Scion iQs in America, Scion itself is dead, and it’s unlikely we’ll find many mechanics who have even seen these cars before. Some parts might be rare and parts that are specific to the iQ might be expensive. That’s the life I live with my Smarts and my Japanese imports.

Sheryl chose the iQ, anyway. I’ve been impressed with this little car. In the past year, we’ve driven this car as far west as the Grand Canyon and as far east as North Carolina. We’ve taken the little car down historic Route 66 and it has survived giant truck tire “gators,” a minor traffic collision, and of course, nearly 40,000 miles of punishment. Toyota never engineered this car to be a road warrior, but the iQ is doing it every day without letting my wife down.

The Modification

It was during our Route 66 trip back in February that we discovered the biggest problem with using an iQ as a highway car. These cars didn’t come with cruise control, not even as an option. Yeah, my wife didn’t like the idea of 40,000 miles a year without cruise.

Thankfully, installing cruise control into countless modern cars is so easy. Many aftermarket cruise control systems tie into the car’s drive-by-wire by using a plug-and-play wiring harness that plugs into the accelerator pedal’s harness. There are cruise control systems out there that are so easy to install that they do not require any cutting or soldering, but basic tools and 15 minutes of your time.

That’s the case for aftermarket cruise control units for a variety of Toyota models, but the Scion iQ isn’t one of them. Installation of a cruise control into a Scion iQ involves the use of a plug-and-play harness, but also wiring that needs to be tapped into an existing harness. You’ll also need to drill out your steering column plastic.

Admittedly, I’ve been super busy this year, so I couldn’t give Sheryl an exact date on when I’d be able to install a cruise control unit. She really didn’t want to wait, either. Hey, I get it! So, she paid our local Toyota dealer $800 for the aftermarket cruise control system and to install it. About $315 of that cost was for the device itself.

When she got the car back she couldn’t stop talking about how the cruise control was a game-changer. I later drove the car myself and I had to agree. It’s such a simple modification, if you can even call it that, yet it makes the car infinitely more pleasurable to drive. Remember, the iQ doesn’t have a lot of ponies in its stable, so you have to give it constant accelerator pressure to keep with the Indy 500-like speeds of Illinois highways. It’s silly how much nicer it is not to have to do that.

It’s hard to say how many miles Sheryl has driven since the installation, but if I had to wager a guess, it has been maybe 15,000 to 20,000 miles. But something started getting weird.

The Dumb Failure

Three months ago, Sheryl started reporting a rather bizarre problem.

Every once in a while, the car randomly went into limp mode after misfiring. The check engine light flashed, the car lost power, and slowed down to a crawl. Thankfully, restarting the vehicle restored all functions. The issue didn’t happen again until October. Then, out of nowhere, the issue came back and it appeared both frequently and seemingly entirely at random.

I pulled the codes and found myself baffled. The vehicle recorded a misfire on every cylinder but also a P2121 “Throttle/Pedal Position Sensor/Switch Circuit Range/Performance” code. The vehicle has fresh, name-brand plugs and excellent compression on every cylinder. I couldn’t find any obvious reason for a supposed total engine misfire. Even the car’s battery tested healthy. Eventually, the reported misfires disappeared, so I’m not entirely sure what’s going on there.

What didn’t disappear was the flashing check engine light and limp mode condition. It seemed as if at least once a week Sheryl would report the car going into limp mode with the P2121 code being thrown during the event. Then, this hit a fever pitch.

One day, Sheryl left a southern Illinois courthouse to find someone actively slashing her tires. She scared off the guy before the tool went through the tire.

But then something truly confusing happened during her five-hour drive home that made us think the vandal did something else. A few minutes after she entered the highway, the engine’s revs shot to redline. She didn’t change her pressure on the pedal. Instead, it seemed the car was trying to accelerate entirely on its own. Thankfully, the car entered into a safe mode where it wouldn’t travel faster than about 10 mph while the engine was seemingly running away.

Over the phone, I instructed Sheryl to shut down the vehicle, let the computers power down, and then bring the car back online. When she did that, the car came back to life, but now the gas pedal did nothing and the car wouldn’t move. Eventually, she coaxed the car into moving, but once again it was doing redline just to go 10 mph. The car’s CVT also didn’t appear to be fond of any of this.

Something clicked in my head. I knew exactly where I’d heard of this failure before. Some Smart owners have reported similar erratic engine behavior. Usually, they’d hit the gas at a green light and the car would either sit there at idle speed and doing nothing or the engine would rev to the moon but the car would still not move. The cause was almost always traced to the installation of an aftermarket cruise control unit. At least in those Smarts, the bad solders in the control boards of those cruise control units would fail, sending all kinds of wild signals through the drive-by-wire system.

What’s interesting about these failures is that the drivers weren’t using cruise control at the time, just like Sheryl wasn’t during the above failure. However, as I said earlier, the cruise control unit is tied in line with the throttle pedal’s wiring, so if the cruise control dies or shorts out, it can interfere with the pedal.

The solution? Disconnect the cruise control harness from the pedal harness. In my experience, this solves the erratic throttle problem and limp mode issue about 95 percent of the time. Then, you either just live without cruise control or replace the faulty unit. But, usually, those Smart pedal failures didn’t happen until years after the cruise was installed. We’re talking about only months here.

Unfortunately, Sheryl was still hundreds of miles from home, far further than even AAA would have towed her. I recommended letting the car sit turned off for a while, then trying to make a run for home. Letting the car sit for a half hour or so seemed to do the trick, and Sheryl limped the car home taking backroads in case the issue happened again.

Once home, the issue hit with a vengeance, with the car starting in limp mode and having an ineffective gas pedal. The car’s check engine light and traction control off light glowed proudly. Every green light was met with the engine either trying to run away or the car doing its best impression of a brick in traffic.

Yanking the cruise control from the accelerator pedal had the immediate change of turning off the check engine light and turning traction control back on. The limp mode also went away. However, that pesky P2121 consistently returned no matter how many times we cleared the code. Part of the cruise control was still hardwired into the car, so that’s where I ended my troubleshooting. At this point, I had Sheryl use her warranty on the cruise control unit to have the dealer figure out what was going on.

I described to the dealership what the car was doing, but not what I thought the problem was. I was curious if the dealer would come to the same conclusion I did. A couple of techs at the local Toyota dealer worked through the weekend to replicate and troubleshoot the problem. The limp mode issue never showed up for them.

It’s Supposed To Do That, Right?

However, the tech who installed the cruise control found a bunch of fault codes stored in the cruise control unit itself plus that persistent P2121 from the car. Their diagnosis? The cruise control is having a failure of some kind, which sends bad signals to the accelerator pedal sensor. As for the P2121, it was found that the pedal sensor was also failing. The consensus is that the cruise control failure damaged the accelerator pedal sensor.

I did some digging around and this seems to be what other Scion iQ owners have experienced over the years, with the fix varying based on the intensity of the issue.

Unfortunately, Toyota doesn’t sell this part by itself, so you have to buy the whole gas pedal assembly to the tune of over $200 plus the labor to install it. Sheryl went ahead and paid for the work, though I’m pretty sure we could have handled it ourselves and saved on the labor costs.

At any rate, Sheryl got her car back and it’s running great, as if the pedal problem never happened. From here, the dealership says we’ll wait a week to see if the problem comes back. If it doesn’t, they’ll investigate why the cruise control failed and replace it. Thankfully that is under warranty so it’s not an extra charge. Hopefully, the replacement unit doesn’t blow up.

If this was a story that someone told me in a bar I wouldn’t have believed them. However, I’ve now seen enough aftermarket cruise controls fail in front of my own eyes and I’m not even surprised that this happened. This is just the first time I’ve seen one fail so hard it also supposedly killed the pedal it was attached to. It figures that my wife’s Toyota was bulletproof until we modified one thing on it.

So, if you happen to own a car with an aftermarket cruise control and suspiciously erratic throttle pedal behavior. I would first disconnect the cruise before throwing parts at it. Maybe your cruise control is trying to star in the next Speed movie.

(Images: Author)