Duramax Limp mode; what is the most common cause.

Chevy GMC LB7 Duramax parts

Low rail pressure is one of the most common cause of a limp mode in the 2001 – 2010 6.6 Duramax. DTC codes P0087, P0093 or P1093 may set due to low rail pressure.

Low rail pressure could be caused by a restricted fuel filter, air in the fuel, a bad pressure relief valve, excessive return fuel from the injectors or a worn CP3 pump. The least likely cause would be a fuel rail pressure sensor.

Fuel Supply and Fuel Filter

Check the fuel supply side first. Connect a gauge to the test port and check for excessive restriction. The “Filter Life Indicator” may falsely set if the rail pressure is too low. We often see this set within 500 to 2000 miles of installing a new filter. Vigorously shake the fuel filter into a clean container and look for contamination. If the filter is not restricted, or severely contaminated, then you likely have low rail pressure, but it may not be low enough to set a DTC. Or the filter life indicator may set with a low rail pressure code, leading you to incorrectly assume the filter is the cause.

For more information on checking the fuel supply and fuel filter, see our diagnostics information. 2001 – 2007 or 2007.5 – 2010

Rail Pressure Relief Valve

The pressure relief valve, or limit valve, if bad, will usually leak at higher rail pressures. The relief valve is in the system to prevent excessive rail pressure. Excess rail pressure could cause an injector to fail and dump the rail pressure into one cylinder, which could cause a hydro lock and engine damage.

Connect a clear line to the relief valve return, ramp up rail pressure and watch for a leak. If there is any fuel leaking from the relief valve, then it will need to be replaced.

Even if you replaced it with a block off plug, you should still check for a leak. We have seen several block off plugs leaking into the return, because they did not seat correctly into the rail.

Buy Duramax Rail Pressure Relief Valve

High Pressure CP3 Injection Pump

On vehicles with more than 200,000 miles on the high-pressure pump, we commonly see the CP3 pump cause low rail pressure. It fails to maintain enough rail pressure when the vehicle is pulling a load up a hill, and your Duramax may go into a limp mode. If the actual rail pressure is between 2176 psi or 2900 psi (depending on model year) below desired pressure, this would cause a P0087 or P0093 or P1093 code to set.

Depending on year of vehicle: P0087 will also set if the rail pressure drops below 3263 psi. P1093 will set if the difference between commanded rail pressure and actual rail pressure is more than 2900 psi.

Worn CP3 pumps that are not producing enough rail pressure under load usually do not cause hard starting. Buy Duramax CP3 Pump


Excessive return fuel from the injectors can cause low rail pressure. If the injectors are worn enough to cause excessive return and low rail pressure, they will often cause a hard start. Perform the “Enhanced Injector Return Flow Test” to verify excessive return. Buy LB7 InjectorsBuy LLY InjectorsBuy LBZ InjectorsBuy LMM Injectors


James Pryor

Information is great. Very detailed with clear guidance to thoroughly help explore possibilities of issues

Christopher Larson

My 06” duramax has less than 60,000 miles on it, issue started with excessive cranking, but after I learned about priming the pump it’s not had a hard start. I’ve replaced fuel filters & done periodic maintenance, now about 1-2 times a week it’ll go into a limp mode, the problem happens sometimes b-4 I go somewhere & also after she’s run fine! This problem will correct itself if I pull over & shut it down & fast turn my key?? So I thought it was mapping or electronic! As I said no issues starting it, & these events come & go within minutes after pumping the key, it’s not a consistent problem but a very aggravating problem, I’ve read the article above but if anyone has a more direct diagnosis after reading my post please let me know if there’s a direct link to a direct issue I’ve stated! Thanks


Hello Christopher,
The first thing to check is what codes are setting. When you cycle the key, it will reset some codes until it happens again. Once you know what code(s) set, then you can perform the diagnostics related to that code.
Thanks, Mark

Tim del Villar

Hi Mark. I appreciate your informative article. I have a ’04 2500HD with about 126K on it. I’m the third owner and I’ve had it about two years. It’s run great until now. I came a stop at an intersection the yesterday morning, and when I pulled out into traffic I suddenly had no power. It seemed the power dropped more as I pressed the accelerator further, then the power slowly returned as I let off the pedal. It happened again twice on that same drive, though less severely, and both times while ascending hills. It hasn’t been driven since. I picked up a code reader this morning and checked the OBD2. It threw codes of P1093, U0106, & P0198. I’m 100 miles away from home (in McMinnville) and must drive back home (Florence) tomorrow for work. Any suggestions would help relieve my state of panic. I think…


Hello Tim,
It does sound like the pressure relief valve is loosing rail pressure. Another possibility would be the cascade overflow valve however, I would check for return off the relief valve, or replace it. I assume you can not increase rail with your scan tool to verify leakage at higher pressures.
Give us a call when we open at 7:30, if you need to.

Dakota Shaw

Hey Mark, I have a 2004 GMC 2500 HD Duramax. It has about 260k miles on it. I am having some trouble with my truck. When I first get in my truck and drive it, it drives fine until it gets warmed up. When it gets warmed up, as I am going down the road, all of a sudden it will start acting crazy. My engine will start making a bad noise and my truck will start jerking from it accelerating on its own. My RPMs will jump up without me even pressing the peddle. My fuel pressure will also fluctuate from 0 psi to 10,000 when it does act up. I have tried switching out my injector clips and it hasn’t helped. Please help. Thank you!


Hello Dakota,
Do you have any codes, not just a check engine light, but codes stored in the ECM? Cam or Crank sensors can cause the rpm to fluctuate erratically. We have also seen instances of the tone wheel coming loose. If you have a crank sensor code, you may need to look through the crank sensor hole and verify the tone ring is not wobbling. You may need to monitor, with a scan tool, the actual versus desired rail pressure while driving.
Give us a call after you have checked these possible issues

Eli Sulmasy

hey I have a 2006 LBZ with 295K on it and i just changed the fuel filter about a month ago (800-2000 miles is per month) i was pulling my landscape trailer up hill no problems and as soon as i crest over the hill CEL comes on, limp mode comes on and i pulled over plugged in my code reader and got a p0087 code. same code for the fuel filter that i replaced. i called a buddy who was a gm tech said to clear the code to get me home. got home and the CEL is back on. I have some thought to think it’s the relief valve, but its not leaking, other is the pressure sensor by the filter, I’m kinda lost could use some advice thanks!! also don’t have a lift pump don’t know if that helps or not.


If the pressure relief valve is not leaking, and the high pressure pump is original, it is most likely the pump causing the issue. Excessive injector return can also cause low rail pressure, and you can check that here https://oregonfuelinjection.com/services-repair/diesel-diagnostics-repair/chevy-gmc-diesel-diagnostics/#01-injectors However, we see a lot of pump issues over 250k miles – see notes here https://oregonfuelinjection.com/services-repair/diesel-diagnostics-repair/chevy-gmc-diesel-diagnostics/#01-high-pressure-cp3-pump

Pump and install kit are here https://oregonfuelinjection.com/shop/chevy-gmc-duramax/2006-2007-6-6-l-lbz-duramax/cp3-fuel-injection-pump-6-6-lbz-lmm-06-10-duramax/ They do last longer with a FASS pump feeding them.


I have a 06lbz with 230k just went on a 50 minute drive, 50 minutes there and back and it went in fuel limp thought it wasn’t anything to worry about but it kept happening I just replaced my filters with cat filters one week ago


Hi Kaden,
What codes do you have setting? https://oregonfuelinjection.com/services-repair/diesel-diagnostics-repair/chevy-gmc-diesel-diagnostics/#01-dtc-codes
Did you check the pressure relief valve for leakage?
Do you have a way to monitor rail pressure? https://oregonfuelinjection.com/diy-repairs-and-obd2-diagnostic-phone-apps/

The most likely cause would be the pressure relief valve, then the CP3, then injectors. Follow the troubleshooting information above to try to pinpoint the cause.


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