P0742 on GMC: How to Diagnose & Fix
Torque Converter Clutch Circuit Stuck On
6 GMC models · 6 documented issues · 4 critical · $150-$4,500 repair
P0742 on GMC by model: symptoms, diagnosis and fix
The 8L90 8-speed and 10L80/10L90 10-speed automatic transmissions experience shuddering during light throttle acceleration, particularly at highway speeds between 35-60 mph. The issue is caused by the torque converter lock-up clutch slipping and chattering. Harsh shifts between gears, especially 1st to 2nd and 2nd to 1st, are also commonly reported.
Common Symptoms
- Vibration or shudder during light acceleration at 35-60 mph
- Harsh or jerky shifts between gears
- Transmission slipping sensation
- Shudder when torque converter locks up
- Intermittent transmission hesitation
How to Fix
Transmission fluid flush with updated Mobil 1 Synthetic LV ATF HP fluid (GM part number 19417577). Torque converter replacement. Transmission control module software update. Complete transmission replacement in severe cases.
Sources for this issue (1)
The Allison 1000 6-speed automatic transmission used behind the Duramax diesel in Sierra 2500HD/3500HD is widely regarded as extremely durable, but the Transmission Control Module (TCM) and solenoid pack do develop faults. Common issues include harsh 1-2 shifts, transmission sticking in one gear, slip codes, and the truck going into limp mode (3rd gear only). Using the wrong fluid or skipping fluid service is a common cause of premature solenoid wear. TCM software updates from GM address most calibration-related shift complaints.
Common Symptoms
- Harsh or clunky 1-2 upshift
- Transmission stuck in 3rd gear (limp mode)
- Delayed engagement from Park to Drive
- Slip or flare during 2-3 shift
- Check engine light with transmission codes (P0700, P0730, P0751)
- Transmission not shifting above 3rd gear under load
How to Fix
TCM software update at dealer. Fluid service (drain/fill every 30k miles, full exchange every 60k). Solenoid pack replacement (Allison 29539579). TCM replacement if software update fails. Use the GM-specified DEXRON-VI fluid (or an approved TES-295 fluid such as Allison TranSynd).
Sources for this issue (1)
The Allison 1000 automatic transmission paired with Duramax engines in the Sierra 3500HD exhibits torque converter clutch shudder during light throttle cruising between 40–65 MPH as the TCC applies and releases. This is caused by degraded transmission fluid, worn TCC friction material, or contamination in the hydraulic circuit.
Common Symptoms
- Vibration or shudder between 40–65 MPH at light throttle
- Shudder disappears under hard acceleration or deceleration
- Transmission hesitation or clunk when TCC engages
- Rough or inconsistent lockup feel during highway cruising
- Transmission fluid with burnt smell or dark discoloration
How to Fix
Drain and refill with Allison-approved TES-295 or TES-468 fluid (Transynd or equivalent); do not use generic ATF. If shudder persists after fluid service, a complete torque converter replacement (remanufactured unit $400–$800) and valve body cleaning is required. Complete transmission fluid flush recommended every 50,000 miles for prevention.
Sources for this issue (1)
The 8-speed (8L90) and 10-speed (10L80) automatic transmissions in the Yukon XL develop a torque converter clutch (TCC) shudder felt as a vibration or shake at light throttle between 25-80 mph. The shudder mimics driving over rumble strips. GM issued TSB 18-NA-355 (covering 2015-2017 Yukon models with the 8L90) and changed to a reformulated Mobil 1 Synthetic LV ATF HP fluid. GM attributes the shudder to moisture absorbed by the original, hygroscopic fluid.
Common Symptoms
- Vibration or shudder at 25-80 mph under light throttle
- Feels like driving over rumble strips
- Shudder disappears under heavy acceleration
- Vibration most noticeable in overdrive gears
- Intermittent — may come and go
How to Fix
First-line fix: complete transmission fluid exchange per bulletin 18-NA-355 with GM-approved Mobil 1 Synthetic LV ATF HP (blue label) — NOT the original black-label version. If shudder persists after the correct fluid exchange, further diagnosis per GM service information is needed and the torque converter may require replacement. Do NOT use aftermarket friction modifiers or additives — GM states intermixing other fluids or additives will not give satisfactory results.
Sources for this issue (2)
The heavy-duty 4L80E (4L85E behind the 8.1L and the diesel) fitted to Savana 2500 and 3500 vans develops a distinct failure set from the 1500's 4L60E. Owners report shuddering around 45 mph as the torque converter clutch tries to lock, hard or late shifts, no downshift on throttle demand, and in advanced cases the van reaching operating temperature and then behaving as though it is in neutral in every gear. Root causes documented on 3500 vans include a failed TCC PWM solenoid or its electrical circuit, worn valve body bores, and pressure loss from a worn pump. Commercial-fleet duty cycles - constant loaded stop-and-go, cutaway upfit weight at the top of the GVWR - accelerate all of it.
Common Symptoms
- Shudder at approximately 45 mph
- Harsh or late upshifts
- No downshift when throttle is applied
- Neutral-like condition in all gears when hot
- Overdrive inoperative
- Check engine light with transmission codes
How to Fix
No recall. GM bulletin 08-07-30-001 covers 4L80E harsh/delayed shifts, slipping and no forward/reverse. Diagnose electrically first: a TCC PWM solenoid or its wiring is a comparatively cheap repair ($350-$700) and P1860 points straight at it. If pressure tests show pump or valve body wear, or the van has lost gears, the fix is a rebuild or reman unit at roughly $2,500-$4,500 installed for a 3500. On any van used as a work truck, fit an auxiliary cooler and service the fluid every 50,000 miles rather than following the severe-service book interval.
Sources for this issue (3)
The 4L60E four-speed automatic in these trucks commonly fails in two ways. The first is a burnt 3-4 clutch pack. Transmission parts maker Sonnax traces most 3-4 slipping to low line pressure or to leaks in the third-gear oil circuit. Those leaks can come from input shaft seals, a leaking input drum, a clogged input housing checkball, a worn pump, a leaking 3rd accumulator checkball, worn accumulator pins, or worn valve body bores. The weak factory 3-4 backing plate also flexes when it applies, which builds heat. Heat can warp the 3-4 piston's rubber seal so oil pressure blows past it, and the clutches then slip and burn, so the truck slips in 3rd and 4th gear. The second failure is the sun (drive) shell. It can split in two, or the input gear can wear its internal splines smooth. Owners then report trouble shifting from 1st to 2nd or into reverse or overdrive, along with slipping and grinding noises.
Common Symptoms
- Slipping in 3rd or 4th gear
- No 3rd gear
- Harsh shifts
- Trouble shifting from 1st to 2nd or into reverse or overdrive, sometimes with grinding (sun shell)
- Burnt fluid
How to Fix
Start with the simple checks. Check the fluid level and condition, and scan for codes. Sonnax notes that faulty TP, MAP or MAF sensor readings, a bad EPC (pressure control) solenoid or voltage drops at the control module can all upset line pressure. On 1996 trucks, Sonnax says to verify the ground wire update in GM bulletin 00-07-30-026. To fix a burnt 3-4 pack or a broken sun shell, the transmission has to come out and be taken apart, which SlashGear describes as essentially a rebuild. During the rebuild, find the leak that burnt the clutches as well as replacing them. That means air-checking the loaded input drum, air-checking the input housing-to-shaft seal area, vacuum-testing the input housing checkball, and checking the pump passages, the 3rd accumulator checkball and the valve body bores. Replace both the 3-4 clutch pack and the 3-4 piston, fit a heavy-duty 3-4 backing plate (for example Sonnax 74140-01K), and replace the sun shell if it is cracked or its splines are worn. Owners report quotes of $1,700-$1,900 for a complete rebuild, and SlashGear says to expect at least $1,000-$2,000 for a 3-4 clutch and piston repair.
What you need to diagnose it
The repair instructions explicitly call for reading stored codes. Use a scanner that can read the named code families before choosing a repair.The recommended scanner supports these code families; confirm your exact year, model and module coverage before buying.
ANCEL AD310 Classic OBD-II Scanner
Read & clear engine (P) codes
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Reading and clearing the code is DIY work. Module replacement or reprogramming is not — most modules are VIN-programmed, so a new one does nothing until a shop codes it to your car. Check for an open recall or service bulletin before buying anything.
Sources for this issue (2)
P0742 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0742 mean on a GMC?
P0742 stands for "Torque Converter Clutch Circuit Stuck On." P0742 means the torque converter clutch (TCC) circuit is stuck on, meaning the lockup clutch that mechanically links the engine to the transmission stays engaged when it should release. The computer commands the clutch on and off, and when it detects the converter clutch remaining applied, it sets this code. The classic symptom is the engine stalling when coming to a stop, similar to driving a manual without pushing the clutch in, along with possible harsh shifts or poor fuel economy. It points to a hydraulic or solenoid problem inside the transmission rather than the engine. On GMC it is documented across 6 models: Sierra 1500, Sierra 2500HD, Sierra 3500HD, Yukon XL, Savana, C/K 1500.
What are the most common causes of P0742?
The most common causes of P0742 are: Stuck or failed torque converter clutch solenoid, Stuck valve in the valve body (TCC apply valve), Contaminated or degraded transmission fluid, Internal transmission hydraulic leak holding the clutch on, Wiring or connector fault to the TCC solenoid, Faulty torque converter, Dirty or clogged transmission filter. The specific cause varies by vehicle.
How much does it cost to fix P0742 on a GMC?
Published repair estimates on GMC range from $150 to $4,500 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.