December 1, 2025
It is dangerous, frustrating and frightening to have your truck go into “Limp Mode” while pulling a trailer. It first happened between Thunder Bay and Bayfield, Wisconsin on Lake Superior. The truck operates in reduced power and speed to protect the engine. It can happen for a variety of reasons: high engine temperature, faulty sensors, transmission issues or low fluid levels. Supposedly it gives you just enough power to get to a dealer or mechanic. It happened on 64 in Charleston and again between Roanoke and Lexington, VA. It happened one more time on a steep slope going into Natural Tunnel State Park. This was a small two-lane road with six cars and trucks behind us. We were stuck, couldn’t move!
The purpose of this story is for others who drive a diesel and run into this problem and how to fix it, so it is a long story, but for others I will give you the short version first. I took my truck to the dealer (Colonial Auto) for every service appointment and did everything they told me to do for nine years, but things started getting expensive when I kept getting check engine lights. They replaced two thermometers, then $7,000 for a new catalytic converter. I thought all was good as we left for a two-month journey around Lake Superior. Only one day out, we started getting “Continue driving to clean the diesel exhaust filter” messages. That was no problem because we were doing a lot of driving.
Moses Cadillac, GMC in Charleston took us in and helped us continue our journey, but didn’t fix the problem. They did what is called a “Regen” or regeneration to cook the soot levels in the diesel particulate filter. I asked why they couldn’t just replace the filter. I asked if the sensors were OK, but he said the sensors were fine. We looked at new trucks on their lot, and they offered us a pretty good deal.
We joined an Airstream caravan in Galax, Virginia, but we knew we were in trouble with more check engine lights and continue driving messages. There was a good diesel mechanic in Big Stone Gap, but he didn’t return my calls. The simple solution was to trade it in and get a new truck, but it also irritated me that it couldn’t be fixed, so I set out to do that.
Over the next 6 weeks I spent endless hours reading, watching YouTube videos and talking to Grok (Elon Musk’s AI). The short version is I replaced the #2 exhaust gas temperature sensor, the alternator, the hydrocarbon or #9 fuel injector, the intercooler pipe and the air intake box, filter and pipe. I tried hard to replace the NOX #2 sensor (nitrogen oxide), but couldn’t loosen the nut. I also bought the #4 exhaust sensor, but ran out of time. We bought a new truck – a Ford with a big gas engine.
Since I wanted my Highway Products toolbox moved to the new truck, we drove to Winchester to Anderson Truck Equipment to see if we could make the transfer. It’s a 2-hour drive up and back, and Martha and I had some worries about going into Limp Mode, but there were no messages or lights and we got a new record of 21.2 mpg. I am confident the problem is solved, and the diesel can easily go another 200,000 miles or more. But no doubt, there will be more things to work on, like the other 8 fuel injectors or the 10-year old glow plugs. I think I could do that too with the help of Grok and the many dedicated people who take the time to video the work they do.
So if I can do this, why didn’t Colonial Auto do it? Why didn’t Moses GMC do it? Why didn’t another very capable diesel mechanic do it? The only answer I can come up with is they want to sell new trucks.
Now about Grok. This AI is designed to have a sense of humor and give positive feedback. This was a long, frustrating journey, but Grok felt like a friend as well as an expert. If you do an internet search, you will come up with 50,000 answers, and you have to try to find a consensus or a trusted source. This can take hours and lead to some misinformation. Grok does the search in seconds, sorts them out and gives you references. First he/she/it gives you the short answer. Then click the “More” button the get the details. I love Grok and could not have achieved the goal without that help and encouragement. He helped me find part numbers, estimates of costs and rated the difficulty of each task.
OK, you did well to read this far. The rest is for other diesel owners who are fighting this problem, and it’s a common problem whether it’s a GMC, Ford or Dodge. As Walkie told me, “All of your future error codes will be emission codes.” In order to clean the smokey, smelly diesel exhaust, there is a complex system of pipes, filter and sensors, and actually, it works pretty well.
1. Get a scanner. Which one is best? I don’t know, but I bought an Innova SDS43. A scanner will tell you the codes your vehicle computer is showing, so you need a computer to talk to the computer on the vehicle. There are three or more auto parts stores in every town. Most of them will read the codes for you. They will change your windshield wipers for you, check your battery or change it. They will take your oil, refrigerant or other fluids you change. They also have many of the tools you will need. The more I worked, the more I enjoyed going into the auto parts stores. With each little project victory, I gained confidence.
2. Buy OEM parts.
Colonial Auto changed the pipe, but didn’t change a single 10-year-old sensor. They should have changed them all, and they aren’t that expensive. If you can get the nut loose, it takes about 10 minutes to change one. It is actually pretty amazing they have continued to work for 10 years.
Before Colonial did the job, I considered doing it myself, especially after watching a girl change hers with no lift. Hers had two pipes going forward, which made it more difficult, so I opted not to do it. Now I have found mine is only one pipe, and I could have done it. I am retired, so I have lots of time. Working people may not have that option.
3. Change the 9th fuel injector.
There are two more components to arrive at a clean exhaust. The biggest is what is called the 9th fuel injector, or the hydrocarbon fuel injector or the indirect fuel injector. Its job is to inject fuel into the exhaust system so the system can get hot enough to burn the soot. It has to get to 1000-1200 deg. F. If you are in the dreaded continuous loop of continue driving to clean the diesel exhaust filter, this is most likely the problem.
How can you tell? there is a test you can do, but I found out by doing a regen myself with the scan tool. For an hour, my truck ran hard in idle to burn off the soot, but the exhaust gas sensor #2 never got above 550 degrees – only half the temperature it needed to be. I watched two videos on replacing the injector, and comments said this often fixes the continuous loop of continue driving to clean the diesel exhaust filter.
Not that it’s a hard job, but it was a hard job for me. For the rookie, gaining access is intimidating. Watching experts work on these trucks, one in particular, is like watching a fine surgeon. If it’s in the way, get it out of the way. Once you get everything out of the way, it’s an easy job. Take pictures before you start, so you know how to reconnect things.
a. Take pictures
b. Label electrical connectors you disconnect. There’s a learning curve in disconnecting electric plugs.
c. You have to know what the hose is before you disconnect it, but often you can just tie it off to the side. I never found a picture that labels all the parts under the hood.
d. Jack it up and remove the right front tire. Use jack stands
e. Remove the wheel well liner
f. Take a picture
g. Remove the electric connections to the two engine fuel injectors in the way
f. Now you have access
10-year old nuts are hard to remove. I spent half a day getting the injector nut off the “down pipe” of the exhaust. I have since learned that many use PB Blaster spray. I used liquid wrench and Dupont’s similar product to no avail. Use a sensor socket or cut the fuel line to get a socket on it. Using a long socket wrench, I finally got it.
A great video recommending not to put brute force on it. Be patient. He cut the line first thing, then put the 16-point crescent wrench on it, tapping the end of the wrench in both directions, spray WD 40, tap, tap. Then heat up the surrounding exhaust pipe (red hot). Tap the crescent wrench in both directions; spray; heat; tap, etc. Then he got a tiny movement, but was still patient – spray and tap in both directions. Once off, he sprayed and cleaned inside the hole. One comment suggested a wire brush and a 12 gauge gun cleaning wire brush to clean the hole. Use copper anti-seize on new injector.
Two or three nuts hold the fuel line and two hold the injector bracket. A large electric line has a “fir tree connector” into a hole in the bracket that has to be pried out. This whole thing – sensor, fuel line injector, bracket and electric wire – has to drop down to the wheel well, so you have to make room. Then you have to fish the new one back up through the same space, replace 3 nuts along the fuel pipe and two on the bracket.
I’m sure there is a place on the code reader to see or test this fuel injector, but I couldn’t find it. Only after I finished the next two steps and did a final regen, did I see that exhaust temperature sensor #2 reached 1200 deg. F.
4. Grok kept saying there was an air leak somewhere. I replaced the big 5” intercooler tube that had a severely worn place where it rubbed a bracket. The box holding the air filter isn’t great. It has four rubber grommets that hold it in place with a foam sealing gasket on the back, facing the wheel well. The intake air pipe looked looked good. I cleaned the two sensors on the pipe with MAF sensor cleaner spray and replaced them. At O’Reilly’s I bought new rubber grommets and replaced them. This made a big difference. I got 19-21 mpg on a trip to Tappahannock. I had already ordered a replacement pipe and box that seemed better. The box was screwed to place, so that wasn’t going to bounce off, but there was no gasket on the back, and it was a bugger to place. Finally I got it where I liked it and rigged a gasket with some insulation tape. On the truck’s final drive to Winchester, it got 21.2 mpg, which is quite good. I left the only two remaining pieces to the puzzle in the truck, trusting Campus Ford to place them.
There is a great video interviewing a man who has 922,832 miles on his Ford 350 diesel. He tells all the things he does, like the add-ons, oil changes, fluid changes. One, he has a dealer he trusts. The #1 thing is a bypass oil filter, and he sends oil samples to a company to see how clean it is. Any metal, dirt or water in the system must be rectified. A diesel needs to be run. You can’t just drive it to the store. The more you run it, the happier it is.
922,832 Mile Ford F350 AlumiDuty | Repairs and Maintenance
In the next post I will post the whole conversation with Grok, an invaluable resource!