L5P Duramax Repair for Derates, Regens and Shift Complaints
Most L5P Duramax repair work starts in the systems around the engine: DEF and SCR, the DPF, the EGR, the Denso fuel pump and the Allison. When one fails, the truck often cuts power, so we find the part that started it before anything gets replaced.
What GM changed when the L5P replaced the LML
The L5P took over from the LML for 2017, and it was largely new to the pickups, though the design was already in use in military applications. The change owners notice most is the fuel side: GM dropped the Bosch CP4.2 for a Denso high-pressure pump. The emissions package stayed familiar: EGR, a DPF, and SCR fed by DEF.
If you are not sure which transmission you have, the model year tells you: 2017 to 2019 trucks have the six-speed, and 2020 and newer have the ten-speed.
- Emissions faults and derates
- Regens that never finish
- Shift and heat complaints when towing
The trouble on an L5P tends to start around the engine, not inside it. An emissions part, a sensor or the transmission can put the truck in reduced power, often with a trailer behind.
What the L5P is telling you, and where we look first
The dash message is a starting point. The fault is often one step upstream.
| What you notice | What it often points to | Where we start |
|---|---|---|
| Speed limit countdown or reduced engine power | DEF quality, a DEF heater or pump, a NOx sensor or the SCR | Codes, DEF quality, then sensor data |
| Exhaust filter message, frequent regens | Soot coming in faster than the DPF can burn it off | Soot load, then what is making the soot |
| Coolant dropping with no puddle | An internal leak, often the EGR cooler | A cooling system pressure test |
| Hard start, stalling, low power under load | Fuel supply, water or rail pressure | Filters, fuel quality, rail pressure |
| Harsh or flared shifts, hot transmission when towing | Fluid condition, a sensor, or internal wear in the Allison | Transmission codes, fluid and a shift check |
| Temperature climbing on a long pull | Airflow, fan, thermostat or a cooling leak | Cooling pressure test, fan and airflow check |
If the gauge keeps climbing on a grade, pull over and let it cool. If it still rises, shut it off. Not on the list? Our diesel diagnostics start from what the truck is doing.
Where the L5P gives trouble, one system at a time
Pick a system to see how it fails.
The system most likely to slow you down
The SCR uses DEF to clean up NOx, and NOx sensors check the result. Bad fluid, a failed heater or pump, a crusted doser or a lazy sensor can all start a countdown. DEF freezes near 12 degrees Fahrenheit, so heater faults show up with the first hard cold snap. See DEF system repair.
What you notice
- DEF quality or service DEF messages
- A countdown to limited speed
- NOx sensor codes after a cold night
A full DPF usually has an upstream cause
The DPF traps soot and burns it off during regens. Idling, short trips or an engine making extra soot fill it faster than it cleans. Before we clean or replace one, we find why it loaded up. See DPF repair.
What you notice
- Exhaust filter full messages
- Regens that keep restarting
- Fuel mileage falling off
Soot and coolant in the wrong places
The EGR sends some exhaust back into the intake through a valve and a coolant-filled cooler. A sticking valve sets flow codes and makes soot. A cooler leaking inside sends coolant out the tailpipe. See EGR cooler replacement.
What you notice
- EGR flow or position codes
- Sweet-smelling white smoke
- Coolant level dropping
A different pump than the LML
The L5P uses a Denso high-pressure pump instead of the Bosch CP4.2. It wants clean, dry fuel and good filters. Water, air or bad fuel shows up as hard starts, stalling and rail pressure codes. See diesel fuel system repair.
What you notice
- Long crank before it fires
- Stalling or surging under load
- Water in fuel warnings
Six-speed and ten-speed Allison problems
Both Allisons can slip, shift hard or run hot when worked hard, and both get checked for codes, fluid and electrical faults before anyone talks about internal repair. See Allison transmission repair.
What you notice
- Flares or slips between gears
- Hard downshifts when slowing
- Transmission temperature high on a pull
The L5P work that rolls through our bays
DEF and SCR faults
Fluid testing, heaters, pumps, dosers and NOx sensors.
DPF and regen problems
Sensors, forced regens, cleaning or replacement.
EGR valve and cooler
Flow testing, leak testing and cooler replacement.
Fuel system and injectors
Supply, filters, rail pressure and injector testing.
Allison transmission work
Shift complaints, heat and fluid service on both units.
Sensors and software
Sensor faults, and factory updates when the fix calls for one.
We do not do delete work. When an L5P leaves here, the EGR, DPF and SCR are still on the truck and doing their job. See our diesel emissions repair page.
How mountain towing and cold mornings find the L5P's weak spots
A heavy trailer on a long climb heats the coolant and the Allison at once, and halfway up a grade is the worst place to find a weak sensor. Job-site idling and short trips load the DPF. Running hot? See truck overheating repair.
- Watch both gauges on loaded grades
- Have DEF and NOx sensor codes checked before winter
- A steady highway run lets regens finish
Have the coolant, transmission fluid and any warning lights checked first.
How an L5P job moves from drop-off to the phone call
Get the history
What the dash said, what you were towing, what was already replaced, and how the truck has been behaving.
Scan and read the data
OEM factory scan tools and live sensor data.
Test what set it
Prove the failed part, then check what feeds it.
Call before work starts
What needs attention now, what can wait, and where jobs share labor. Nothing starts without your approval.
Trucks that work for a living come back here
319 reviews
- Customer
- Brian Rothlisberger
- Business
- Bear Creek Towing LLC
Best place in the Denver metro area for Diesel Truck Repairs. Five Stars everytime! Recently had a bunch of work done and I'm very satisfied with the results. Warranty work is also top notch! Thanks Jeremiah, Mel and the rest of the crew. Appreciate all you do. Brian Rothlisberger, Bear Creek Towing LLC.5 STARS
What moves the price of an L5P repair
We price L5P work after diagnosis, because one derate message can trace back to a sensor or to a part deep in the exhaust.
All L5P work is drop-off. Diagnosis typically takes 24 to 72 hours depending on workload, and we call with findings and options before any repair starts.
| Cost factor | Why it matters on an L5P |
|---|---|
| Which system failed | A sensor and an exhaust component are very different jobs |
| What caused it | A loaded DPF needs its cause fixed too |
| Model year | Six-speed or ten-speed Allison |
| Parts choice | New or remanufactured |
| Shared labor | Jobs in the same area cost less done together |
Tell us what your L5P Duramax is doing
Send the year, the engine and what you are seeing, and we'll get back to you as quickly as we can.
- Year, make and engine
- What it is doing, and when it started
- Any codes or recent repairs
Rather talk it through? Call (303) 808-1563
L5P questions owners ask us
The ones that come up on the phone most.
Most often it is the emissions system. Clearing the code buys time, but the derate comes back until the cause is fixed. We start with the codes and sensor data, then test the part that set them.
No. The LML before it ran the Bosch CP4.2. The L5P uses a Denso high-pressure pump. It still needs clean fuel and regular filter changes, and a fuel complaint gets the same supply and pressure checks as any L5P Duramax repair.
2017 to 2019 trucks use the Allison six-speed. 2020 and newer trucks use the Allison ten-speed. We work on both, and check codes, fluid and wiring before recommending internal work.
No. We do not do delete work. We keep the EGR, DPF and SCR in place and fix what made them fail.
How long an L5P Duramax repair takes depends on what we find and on parts. We call with the diagnosis and a timeline before work starts.
L5P counting down or stuck in regen?
Call with the year, the mileage and the exact dash message, or use Request Service to send the details. Or drop us a line right here from the website and we'll get back to you as quickly as we can.