P0174 on GMC: How to Diagnose & Fix
System Too Lean (Bank 2)
5 GMC models · 5 documented issues · 3 critical · $300-$3,500 repair
P0174 on GMC by model: symptoms, diagnosis and fix
The LML Duramax EGR (exhaust gas recirculation) cooler is prone to coolant leaks internally, allowing coolant to enter the intake manifold and combustion chambers. Coolant in the intake causes white smoke, rough running, and potential hydrolocking of cylinders. The EGR valve also sticks open or closed from carbon buildup, causing rough idle, black smoke, and check engine lights. The EGR cooler failure can occur as early as 60,000 miles on trucks used in stop-and-go driving. GM bulletin 08-06-04-003D provides the EN-48974 adapter set for pressure-testing the EGR cooler for coolant leaks into the combustion chambers.
Common Symptoms
- White smoke from exhaust especially on startup
- Coolant loss with no visible external leak
- Rough idle or engine misfire
- Check engine light with EGR codes (P0401, P0404)
- Coolant smell from exhaust
- Overheating or coolant in oil (severe cases)
- Black smoke and poor fuel economy (EGR valve stuck open)
How to Fix
Replace EGR cooler with updated GM design or aftermarket unit (e.g. BulletProof Diesel LML EGR cooler). Clean or replace EGR valve. Flush cooling system and inspect for coolant contamination in intake. Consider EGR delete kit for off-road/race use only (illegal for street use). Prevent premature failure by maintaining proper coolant pH and avoiding extended idle periods.
Sources for this issue (2)
Yukon XL V8 engines (notably 5.3L and 6.2L) can develop broken exhaust manifold bolts and/or warped manifolds, leading to an exhaust leak that is loudest on cold start. Owners describe a ticking sound that diminishes as the engine warms and the manifold expands. This is a well-documented GM truck/SUV issue in owner reports and service discussions for these engines.
Common Symptoms
- Ticking noise on cold start
- Exhaust smell in engine bay
- Soot marks near manifold flange
- Noise decreases as engine warms
How to Fix
Replace broken manifold bolts/studs and install updated hardware; resurface or replace the exhaust manifold if warped. Inspect for missing bolt heads and use proper extraction methods; replace manifold gaskets if applicable for the engine/year.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Exhaust Manifold Gasket SetCross-refMS19936
Fitment confirmed against the ShowMeTheParts catalog for this vehicle's year and engine.
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Sources for this issue (2)
A common GMT800 Sierra problem is failure of the in-tank fuel pump module, often from worn pump motors or failed level-sender/module electronics. Owners report long crank, hard starting, loss of power under load, or sudden stall/no-start conditions, especially as mileage climbs. This issue is widely documented in owner forums and complaint databases across 4.3L, 4.8L, 5.3L, and 6.0L trucks.
Common Symptoms
- long crank before starting
- intermittent no-start
- engine stalls while driving
- loss of power on acceleration
- fuel gauge may act erratically if sender/module is failing
- whining noise from fuel tank
How to Fix
Diagnosis usually starts with checking fuel pressure at the rail and verifying pump prime at key-on. Low or unstable pressure, especially after sitting, points to a weak pump or failing module. Repair is replacement of the complete fuel pump module and, ideally, the fuel filter on applicable years; corroded electrical connectors at the tank should also be inspected and replaced if heat-damaged.
Sources for this issue (2)
On the 5.7L Vortec V8, the central sequential fuel injection (CSFI) "spider" assembly's poppet nozzles clog and its fuel lines crack.
Common Symptoms
- Hard starting
- Rough idle
- Fuel smell
- Poor fuel economy
How to Fix
Confirm the failure by checking fuel pressure and leak-down, then remove the upper intake plenum and inspect the CSFI "spider" for fuel staining, cracked nylon lines, or sticking poppet nozzles at the intake ports. The permanent repair is to replace the original central sequential fuel injection assembly with the updated GM MFI spider conversion kit (commonly GM/ACDelco 12568332 or superseding part), along with the upper plenum gasket and, if needed, the fuel pressure regulator. Parts typically run about $250-$450, and labor is usually 2-4 hours, bringing total repair cost to roughly $500-$900 depending on shop rates.
Sources for this issue (2)
On the 4.3L Vortec V6, the lower intake manifold gaskets seal the coolant passages between the intake manifold and the cylinder heads. The original gaskets are made of a plastic (composite) material. Heat cycling, plus the coolant flowing past them, wears away or tears the gaskets at their coolant ports, and coolant starts to leak out. The Chevrolet S-10 uses the same 4.3L engine and gasket, and S-10 owners report the same leak. One owner said the plastic deformed only where coolant touched it, and another owner's mechanic called the failure a very common problem on the S-10. One Sonoma owner found the intake gasket worn away at the front left corner at 75,000 miles. S-10 owners report the leak at about 31,000 to 80,000 miles. One S-10 owner specifies that the leak was external, not into the crankcase or cylinders. Owners usually notice a coolant leak or residue around the intake, a coolant level that keeps dropping, or overheating once enough coolant is lost.
Common Symptoms
- Coolant seeping or crusted coolant residue where the intake manifold meets the cylinder heads
- Coolant level that keeps dropping with no other visible leak
- Overheating once enough coolant has been lost
How to Fix
What to check first: with the engine cold, look for wet coolant or dried, crusty residue where the intake manifold meets the cylinder heads, including the front corners. Pressure-test the cooling system to confirm the leak. Check the oil for a milky look in case coolant is getting inside the engine. Repair: remove the upper plenum and the lower intake manifold, then clean the sealing surfaces. Install an updated lower intake gasket set that has a metal core with rubber seals around the ports instead of the original all-plastic type. One repair guide lists the Fel-Pro MS 98002 T for the 4.3L V6. Confirm it fits your year and engine before you order. Tighten the lower intake bolts in the factory sequence with an inch-pound torque wrench. The same guide gives three passes: 27 in-lb, then 106 in-lb, then 133 in-lb. Overtightened bolts can crush the gaskets, and loose bolts can cause vacuum and coolant leaks. Replace the bolts if your service manual calls for it. Then refill the cooling system and bleed out the air. If you found coolant in the oil, change the oil and filter right after the repair. Coverage: no safety recall covers this. One S-10 owner says the dealer had a technical service bulletin on the gaskets but ignored the complaint while the truck was under warranty. We could not confirm a bulletin number, so treat that as an owner report.
Sources for this issue (5)
- CarComplaints - 2000 GMC Sonoma intake gasket leaking (4.3L V6)owner-reports
- GM-Trucks forum - Head gasket or intake manifold gasket, 4.3L Vortecforum
- BITOG - Had to replace another GM 4.3 lower intake gasketforum
- Coolant Leaking From Intake Gaskets (GM 4.3L, 5.0L, 5.7L)repair-guide
- 2002 Chevrolet S-10 plastic intake gaskets leak (CarComplaints)owner-reports
P0174 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 P0174 mean on a GMC?
P0174 stands for "System Too Lean (Bank 2)." P0174 means the engine computer detected that the air-fuel mixture on Bank 2 (the side of the engine that does not contain cylinder 1) is running too lean — too much air relative to fuel. The computer monitors this through the oxygen/air-fuel sensors and fuel trims; when it has to add a large amount of extra fuel to compensate and still can't reach the target, it flags the mixture as too lean. A lean condition can cause rough idle, hesitation, or misfires, and over time can raise combustion temperatures. Because it's Bank 2 specific, an unmetered air leak on that bank or a fuel delivery shortfall is typical. On GMC it is documented across 5 models: Sierra 2500HD, Yukon XL, Sierra 1500, C/K 1500, Sonoma.
What are the most common causes of P0174?
The most common causes of P0174 are: Vacuum or intake air leak (hoses, gaskets, PCV), Dirty or failing mass airflow (MAF) sensor, Weak fuel pump or restricted fuel filter (low fuel pressure), Clogged or underperforming fuel injectors, Leaking intake manifold or throttle body gasket, Faulty or contaminated oxygen / air-fuel sensor, Exhaust leak upstream of the oxygen sensor, Stuck-open EGR valve. The specific cause varies by vehicle.
How much does it cost to fix P0174 on a GMC?
Published repair estimates on GMC range from $300 to $3,500 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.