Shop Talk: Check Transmission Light

Categories: Shop Talk, Transmissions

Craig has been working on transmissions for decades. He started in 1975 working for Detroit Diesel Dealer and Distributorship. He’s always liked engines but he was always curious about what makes engines move. By 1989, he has been a full-time mechanic working on Allison Transmissions as a mechanic or service manager. He is fascinated by the diagnostics and troubleshooting element. When Craig calls the factory, Allison knows they’re up for a challenge because Craig’s already done extensive troubleshooting.


This is a Q&A session where we discuss a Check Transmission light on a 2012 Freightliner with an Allison transmission.



We often hear from customers who experience common issues, so this interview is intended to help others who may find themselves in similar situations.

Catalyst Powertrain: So you’re driving down the road on the way to the next job site when the dreaded ‘Check Transmission’ light comes on.

You may be thinking, ‘Oh, no. This is going to be a pretty big job. The transmission is not shifting properly and I’m in a world of hurt because this is a big job and I need to get to it.’

So Craig, what should the driver do in that situation?

Craig: The first thing I’m going to ask any customer is if they are off the road in a safe place before we do anything else. That’s our big concern – safety. Once safety is addressed, I’ll ask for the vehicle type and year.

In this situation, let’s assume it’s a 2012 Freightliner. The driver sees a Check Transmission light.

Checking the Codes Off the Shift Pad

If the driver doesn’t know how to check the codes off the shift pad, it’s simple and I’ll walk them through that right on the call.

  1. Simply turn the key on
  2. Push the Up/Down arrow keys 1 through 5 times

This process enables the driver to access the diagnostic part of the shift pad. These simple steps will prompt the system to provide a diagnostic code.

In the 2012 Freightliner, the code is P-0721. When we have the code, we have a better idea of the situation and can give the driver a better idea of what the driver can do.

That particular code (P-0721) happens to deal with the output speed sensor.

Common Reasons for Code P-0721

We can advise the driver that there are one of three likely issues going on. Most commonly, this is the result of a:

  1. Bad Sensor
  2. Bad Connector
  3. Broken Wire

To clear the code, the driver should shut the vehicle off and let it sit for a few minutes.

Quick Driver Troubleshooting

Craig: Drivers who are mechanically inclined can go underneath the vehicle and check that connector. In this case, they will be looking for the sensor on the rear section of the transmission. Go ahead and unplug the connector and check for moisture or corrosion. Clean the connector if necessary and plug back in.

Then, fire it back up and see if it resets itself. If the check transmission light goes out the code is now inactive. We recommend you still bring the vehicle in to verify the sensor and systems are working properly. The driver can then get the truck into the shop — hopefully ours — so we can address the issue.

Catalyst Powertrain: When a code pops up is that something that needs to get looked at right away or can a driver continue the rest of the day, finishing up what they are doing?

Craig: Most diagnostic codes are not immediately detrimental to the transmission if you’re still able to drive the vehicle. Drivers may experience trouble shifting, such as being able to properly upshift. Another result may be that the transmission will only engage in one gear or enter a failsafe “limp” mode. Fuel consumption may drastically increase. The engine may sound like it’s misfiring or the speedometer may display erratic readings.

It’s always best to bring the vehicle into the shop and get it taken care of as soon as possible to avoid further and more costly damage.

A New Truck, 10,000 Miles

Catalyst Powertrain: A customer has a newer truck with 10,000 miles on it. Suddenly the ‘Check Transmission’ light comes on. They’ve dealt with US Transmissions in the past and we’ve taught them how to check their own codes. This time, a code P088B comes up. What is that code and what do they need to do after they’ve called US Transmissions about it?

Craig: The P088B code is an early sign that you’re going to need an oil filter and change. But, you only have 10,000 miles on that truck, so we know that’s probably not the issue.

Allison Transmission has released a Field Action notice about issues with the internal valve bodies. That means we need to get that truck to us. We can get the serial number off to Allison so they can give us the OK to get the driver back on the road with an updated internal valve body, springs, and gaskets. Typically, the customer will be on the road in a day.

Catalyst Powertrain: A typical code that will pop up while driving without the engine or transmission blowing up — is that a typical one day turnaround time or is it a case by case basis with codes popping up?

Craig: It could be case by case. The truck systems are so sophisticated these days that the engines, the transmissions, the brakes – they’re all talking to each other. We need to eliminate who’s causing the problem. Even though we have a Check Transmission light coming on, it still could be something from the engine causing the issue. All the systems are communicating over what’s called a J-1939 backbone, a high-speed information line. So, even though the Check Transmission light comes on, the engine thinks it’s more important.

So most of the time it’s a fairly quick turnaround but some of these codes where the transmission is slipping, and it can’t make that next shift, may be happening because the clutch is worn out, so that requires a longer time because the transmission has to be removed, rebuilt, or exchanged.

We have exchange transmissions available to help get folks on the road and back to work.

Catalyst Powertrain: What about this situation?

I only have one truck in my fleet. I’m a one-man crew, so a day down with my truck down and I miss out on my job so they’ll call someone else.

This truck is relatively new, I’ve only had it for about 18 months. What do I need to do to it to make sure codes don’t pop up on me and I end up having to call you at 2:00pm trying to be on the job site at 3:00pm trying to get work done before the end of the day?

Craig: Do your daily fluid checks in the morning, do your normal walk-around the truck, check for leaks or signs of oil where the truck has been sitting. Just do a normal daily check routine before heading out to the job site.

MD-3000 Servicing

Catalyst Powertrain: I’m considering purchasing a Freightliner that has an MD-3000s. It’s a couple of years old and has about 75,000 miles on it. Would that be a good truck to buy, and if so, how often does it need to be serviced for oil and filter to keep it in tip-top condition for my business?

Craig: Well, the MD-3000 Series transmission has been around since the 1990s and they’re a good transmission. Beginning in 2006, when Allison was building transmissions, the MD-3000 Series left the factory with what is called Transynd synthetic oil. With that particular oil, you can run extended intervals on it. On the filters, you can run as far as 75,000 miles, 3,000 hours, or 36 months and on the oil, you can go 300,000 miles, 6,000 hours, or 48 months for general use.

A note about Severe Duty: This is used when the vehicle has a high start/stop ratio or is operated in a dirty environment. For severe use, oil changes should happen at 150,000 miles, 6,000 hours, or 48 months.

If you’re not sure of the service on it, and you’ve got 50,000-60,000 miles on it, I recommend going ahead and changing the oil, putting a new set of filters on it, filling it back up with the Transynd synthetic oil, and resetting the computer.

Vehicles today are pretty smart. They can tell when they need to be serviced.

Catalyst Powertrain: Here’s another question we sometimes hear:

I’ve got an auto parts store in my neighborhood. Can I just pop down there and buy fluids and filters then service my truck?

Craig: We strongly recommend you stay with genuine Allison extra-high capacity filters and the Allison TES2-95 Synthetic oil. The reason for this is all the specs and computer information is based on the Allison filters and the Allison TES2-95 Synthetic oil.

I understand that your neighborhood auto parts store may be convenient, but non-Allison products have different micron ratings. The computer within the transmission may misinterpret the data which in turn means it won’t be telling you the correct information. At the auto parts store, the filters may look exactly the same on the outside, but on the inside, they may have a little bit different media in them or they may have a different micron rating or they may have a bit different design so the flow doesn’t happen properly. We have seen premature transmission failure by not using Allison transmission filters. They can plug and cause major internal failures. Not only am I recommending the Allison filters, I know they work.

They’ve performed years and years of testing. Not only will you get the value from extended oil change intervals, but you’ll experience smoother shift quality and longevity in the transmission itself. It’s the same with the Allison filters. They have the correct micron rating so the computer can be accurate in calculating when to service them.

In fact, we’re running a special right now on all Allison Filters and the Allison TES2-95 synthetic oil, so we can make you a deal on it and get your truck serviced fast. You’ll be on the road, confident because you know you’re using quality parts and oil meant exactly for your transmission.

Catalyst Powertrain: Thanks, Craig. Any last thoughts to share today?

Craig: I would just like to say that if you’re driving down the road and the ‘Check Transmission’ light comes on, don’t panic. Just pull over safely to the side of the road.

Once you’re safe, give us a call at 800-872-6691 and we can walk you through the self-diagnostic part of troubleshooting right over the phone.