These Are The 5.9 Cummins Years To Avoid (According To Owners)

Among engine manufacturers, Cummins is well-known around the globe, with sales that reached $34.1 billion in 2023. The company has come a long way from the first Cummins engine built, and now supports a variety of industries with engines, portable and stationary generators, and all the parts to keep them running. Cummins also developed a partnership with Ram trucks, the brand that was spun off from Dodge's truck division in 2009.

The Cummins 5.9-liter inline-six diesel engine first appeared under the hood of the 1989 Dodge Ram. The engine started with 12 valves, but later generations saw improvements such as the addition of an intercooler, a switch to a 24-valve configuration, and finally common-rail fuel injection. The 5.9 was produced until 2007, when it was replaced with Cummins' larger 6.7-liter engine. With a production lifespan of 18 years, you can expect there were some common problems with the 5.9 Cummins, and some iterations owners would definitely not recommend. Many of these issues can be remedied if you have mechanical aptitude, a good set of tools, and a clean, dry place to work, but you'll still want to know what years of the 5.9-liter Cummins you should steer clear of.

The transmission is a critical component that can greatly impact the quality of your ride, even leaving you stranded. The first generation of 5.9-liter Cummins engines ran from 1989 through 1993, and many owners of these early trucks report slow upshifting and gears slipping. This behavior can be problematic when trying to pass slower traffic on the road. One driver posted to Diesel Truck Resource, writing "I went to accelerate to get out in-front of a car and there was a pop and I could not go." Another owner reported to Diesel Bombers, "[I] put it in drive and it slipped really bad in first gear." Many owners blame the weak torque converters, which bear all the engine's load as it is transferred to the gearbox. 

Another issue common to many early 5.9-liter Cummins owners are problems with the transmission switching back and forth from overdrive. Overdrive in an automatic transmission vehicle is a feature that helps save fuel at cruising speed, but only if it functions properly. One owner posted to Cummins Forum, writing "Every time I hit a bump on the highway it kicks out of OD." A popular theory is that the throttle position sensor might be to blame, as it's what facilitates the shift into overdrive.

While the mid-year 1998 refresh to the Dodge Ram saw updates to the 5.9-liter Cummins like an electronically controlled fuel-injection system and a 24-valve configuration, its Engine Control Module (ECM) received some complaints. The ECM is a computer that communicates with the truck's sensors, manages the fuel injection system, and reports issues in the form of on-board diagnostics codes. When it's working properly, the ECM can also fine-tune ignition timing for optimal performance. However, when an ECM acts up, it wreaks havoc on the performance of an engine.

One user posted to Cummins Forum detailing the 20-plus codes thrown by their 2002 Cummins-powered truck. After a mechanic cleared those messages, they were back within less than a mile of driving. Replacing the ECM under warranty didn't clear up the problem. Another owner echoed that they had a similar experience with their ECM failing, only in this case, the truck couldn't even be driven. The commenter wrote, "The one code I never got was the P0606 for a failed ECM. I only got that code with the first rebuild I received."

You can replace the ECM in a second-generation Cummins fairly easily. However, some owners cautioned against choosing cheaper third-party parts. One owner relayed how they were on their third non-OEM ECM replacement and still experiencing problems. The consensus from several owners was to opt for the more expensive genuine Cummins ECM, which runs north of $1,100.