Certain Santa Fe (TMA), Santa Fe Sport (AN), Sonata (LFA), and Tucson (TL) vehicles equipped with Theta II engines may exhibit elevated engine-out emissions. This bulletin provides instructions for updating the Engine Control Module (ECM) to address this condition.
full bulletin at NHTSA ↗2018 Hyundai Sonata engine problems
moderate 135 complaints filed with NHTSA · avg repair $3,100 · see engine across all vehicles →
Failure mileage
Complaints
135
Recalls
0
Avg fix
$3,100
1crash
4fires
When does it fail?
Of the 135 engine complaints filed for the 2018 Hyundai Sonata, here's the actual mileage breakdown — failures cluster heaviest at 100,000-125,000 mi.
0-25k
25-50k
50-75k
75-100k
100-125k
125-150k
150k+
Each bar shows the share of total complaints filed at that mileage range. Peak failure window highlighted. Some owners report problems earlier; some make it well past 150,000 miles symptom-free. Maintenance habits and driving conditions shift the curve as much as mileage alone.
What stands out
Engine accounts for 38% of every owner complaint on file for this vehicle — the dominant problem area across 12 categories tracked.
Owners have filed 135 engine complaints with NHTSA against this vehicle, but no formal recall covers the issue — the federal record reflects what manufacturers have admitted, not everything owners are reporting.
Is there a fix? Manufacturer service bulletins
The manufacturer has issued service bulletins covering engine on this vehicle — documented repair instructions, service campaigns, or warranty extensions sent to dealers. A TSB isn't a recall (it's not a free safety remedy), but it's the manufacturer acknowledging the issue and how to fix it.
Certain Santa Fe (TMA), Santa Fe Sport (AN), Sonata (LFA), and Tucson (TL) vehicles equipped with Theta II engines may exhibit elevated engine-out emissions. The California Air Resources Board has determined that these vehicles may be releasing air pollutants which exceed Federal and California standards.
full bulletin at NHTSA ↗Following completion of Service Campaign 9C2 (Theta II Emissions Improvement), certain vehicles may exhibit a brief hesitation or rough shifting condition during cold start operation. If present, this condition is expected to occur only within approximately the first 1-3 minutes after engine start, while the catalyst is reaching its normal operating temperature.
full bulletin at NHTSA ↗Following completion of Service Campaign 9C2 (Theta II Emissions Improvement), certain vehicles may exhibit a brief hesitation or rough shifting condition during cold start operation. If present, this condition is expected to occur only within approximately the first 1-3 minutes after engine start, while the catalyst is reaching its normal operating temperature.
full bulletin at NHTSA ↗Some vehicles may exhibit difficulty starting in cold weather. This bulletin provides the procedure to perform an inspection on the starter magnetic switch for potential freezing and, if necessary, replace the starter magnetic switch (solenoid) assembly.
full bulletin at NHTSA ↗Source: NHTSA manufacturer communications. Bring the bulletin number to your dealer or shop.
The failure pattern owners describe
The 2018 Sonata's 2.4L Theta II engine exhibits rampant oil consumption—1 to 4 quarts every 1,000 miles in many cases—that forces owners to add oil weekly or every two weeks. The issue typically surfaces between 33,000 and 120,000 miles and appears to be baked into the design, not a leak. Spark plugs foul with oil within weeks of replacement, the engine knocks loudly, exhaust smells of burning oil, and in some cases the motor simply seizes mid-highway with no prior warning.
Dealers confirm the problem through oil consumption tests, yet Hyundai refuses warranty repair, claiming owners skipped maintenance—despite Carfax records and receipts showing regular oil changes. Hyundai also penalizes second owners with a reduced 5-year/60,000-mile warranty, even though the defect is identical. Temporary fixes (pan replacement, combustion cleaning, software updates) fail within weeks. One owner's engine caught fire while parked; another lost all power on the highway. Hyundai settled a class-action suit in 2021 over this exact engine and defect but continues denying claims on technical grounds (time, mileage, ownership, documentation). Owners report dealerships openly acknowledging they see this problem daily and that Hyundai knows about it but refuses a recall.
Same Hyundai Sonata engine reports on nearby years: 2015 · 2016 · 2017 · 2019 · 2021
Failure modes owners describe
Excessive Oil Consumption (Theta II 2.4L Engine)
Uncontrolled oil loss between service intervals, requiring drivers to add oil every 1–2 weeks or every 700–1,500 miles. Oil consumption rates reported range from 0.75 to 4–5 quarts per 1,000 miles. Owners describe oil running completely dry or critically low despite regular maintenance and short intervals between oil changes. No visible external leaks in most cases.
When: Typically begins between 33,000 and 120,000 miles; one report at 65,000 miles, several in the 80k–110k range. Issue progressive and ongoing.
Symptoms owners cite: Oil dipstick completely dry or near empty between scheduled service; Low oil warning light illuminating intermittently or suddenly; Oil light flashing briefly while driving; Engine smell of burning oil; blue or gray smoke from exhaust; Spark plugs fouled with oil residue, requiring replacement every 2–4 weeks; Engine knocking, tapping, or rough idle noises; Oil turning black abnormally quickly (within 500–1,000 miles); Evidence of oil burning visible in exhaust
Codes mentioned: P0300 (General misfire), Check Engine Light codes related to misfires in specific cylinders
Repairs/costs cited: Owners cite partial fixes: oil pan replacement (cracked or defective pans), crush washer or drain plug gasket replacement, combustion chamber cleaning ($350–$700), PCV valve replacement, valve cover gasket and oil pan gasket replacement. None of these are permanent solutions. Hyundai approved an 'induction cleaning' or 'top engine cleaning' in some cases, which temporarily reduced consumption but did not resolve the underlying issue. Some dealers recommended engine replacement as the only permanent fix.
Recalls/TSBs owners mention: Hyundai settled a 2021 class-action lawsuit regarding Theta II engine oil consumption. Owners cite TSB 23-EM-008H regarding excessive oil consumption but no formal recall issued. Hyundai denies warranty coverage claiming: (1) vehicle is out of warranty (time or mileage), (2) owner neglected maintenance despite documentary evidence of regular oil changes, (3) consumption is within manufacturer's 'standard' despite exceeding owner's manual guidelines, (4) recommending drivers top off oil at every gas fill-up rather than repair the engine. Extended warranty claims denied after denying initial claims. Many owners report Hyundai refusing engine replacement even when dealerships confirm faulty engines. One owner reports Hyundai told dealer to tell customer to keep driving until engine failed, then seek replacement.
Knock Sensor Malfunction and Limp Mode Activation
Knock sensor triggering unexpectedly or incorrectly, causing the engine control module (ECM) to enter limp/protection mode and restrict vehicle speed to 40–50 mph or lower, creating sudden loss of acceleration and power while driving.
When: Reported at various mileages; multiple incidents in 2022–2024 period. Some cases occur shortly after software updates (ECM Campaign 953).
Symptoms owners cite: Check Engine Light illuminates with knock sensor codes; Vehicle suddenly enters limp mode during normal driving; Engine speed restricted to 40–50 mph without warning; Sudden loss of acceleration or inability to merge on highway; Vehicle stalling or failing to restart; Engine control system failure warning lights; Rough idling or hesitation upon acceleration; Knocking/tapping sounds from engine prior to limp mode activation
Codes mentioned: Knock sensor failure/code, Engine control system failure codes, Check Engine Light
Repairs/costs cited: Dealers replaced knock sensors, updated knock sensor detection (KDS) software (Campaign 953 ECM update), and replaced wiring harnesses. Some owners report the software update was previously completed in 2019, yet reapplied in later incidents. Wiring harness replacement cited as the 'fix' in several cases, but one owner reports the sensor was reset multiple times only for the issue to recur. One case reports the vehicle went into limp mode immediately after leaving the dealership following a software update, nearly causing an accident with four occupants.
Recalls/TSBs owners mention: Campaign 953 is an ECM and cluster update for engine monitoring logic product improvement. Owners report dealers claiming they had never encountered the issue before, despite multiple complaints. Hyundai and dealers emphasize that the limp mode is a 'protection mode' to prevent engine damage, but owners note the unsafe driving conditions it creates on highways. Some owners report Hyundai refusing to investigate further once dealership diagnostics are complete.
Engine Misfire and Cylinder Failure
Engine misfiring, particularly in individual cylinders (cylinder #4 cited), with loss of power and rough running. Spark plugs frequently fouled with oil, requiring replacement as a temporary 'bandaid' fix.
When: Reported at various mileages from 14,000 to 138,000 miles; multiple cases around 80k–120k miles.
Symptoms owners cite: Check Engine Light illuminated with misfire codes; Rough, hesitant acceleration; Violent knocking noises or 'near-stalling' events; Loss of power during acceleration or on highway merges; Engine bucking or sputtering; Vehicle hesitating and failing to accelerate as needed; RPM abnormally elevated; Spark plugs fouled with oil residue
Codes mentioned: P0300 (General misfire), P0301–P0308 (Cylinder-specific misfire codes)
Repairs/costs cited: Spark plug replacement is the repeated temporary fix (every 2–4 weeks in one case). Owners cite ignition coil replacement, PCV valve replacement, and combustion chamber cleaning as attempted solutions. One case mentions diagnostic imaging revealing 'scarring' inside a cylinder, suggesting internal damage from fluid intrusion. Engine replacement identified as necessary in multiple cases but denied warranty coverage.
Recalls/TSBs owners mention: Dealers initially diagnosed as ignition coil, spark plug, or sensor issues. In one case, manufacturing instructed dealer to run leak tests after discovering cylinder scarring. Hyundai has not provided permanent solutions; owners report being told to continue driving and monitor.
Engine Seizure and Complete Failure
Total engine failure with catastrophic stall, refusal to restart, or sudden loss of motive power while driving. Some owners report complete loss of oil preceding failure.
When: Reported at 55,000–138,000 miles; several cases in the 100k–120k range despite warranty coverage.
Symptoms owners cite: Loud knocking or grinding noises prior to stall; Engine stall with no restart capability; Complete loss of motive power on highway; Engine oil completely depleted or critically low; Check Engine Light and oil pressure warning lights; Vehicle becoming immobile; requiring tow
Codes mentioned: Check Engine Light (specific codes not always documented), Oil pressure warning lights
Repairs/costs cited: Engine replacement is the only remedy, but Hyundai has refused warranty coverage in multiple cases despite vehicles being within stated powertrain warranty mileage. Owners cite: (1) denial based on alleged maintenance neglect despite documentary evidence; (2) denial based on ownership status (non-original owners subjected to reduced warranty terms); (3) denial based on vehicle being outside time/mileage limits despite meeting one criterion or the other. One owner reports being told by dealership to keep driving until engine failed, at which point replacement would be covered—but warranty was then denied. Another reports Hyundai stating 'once a claim is denied, that is the final decision.' Cost to replace engine estimates not provided by Hyundai; private shop estimates suggest $5,000+.
Recalls/TSBs owners mention: Hyundai extended powertrain warranty to 150,000 miles for vehicles subject to the class-action settlement, but owners report warranty denial on technical grounds (time, mileage, maintenance records, ownership status). Some owners report class-action settlement eligibility but inability to prove it or obtain benefits. Hyundai has not issued a recall; instead, owners report being informed the vehicle is 'available for inspection' but no further action taken.
Engine Fire
Vehicle catching fire while driving or parked, with smoke originating from under the hood or through cabin AC vents. Hyundai Theta II engines implicated in fire risk recalls.
When: Reported at low mileage (3,700–4,000 miles in one case) and at higher mileages (unknown in one case). Incidents in 2019, 2023, and 2024.
Symptoms owners cite: Burning odor detected inside cabin or engine compartment; Smoke visible under hood or through AC vents; Flames visible under hood; Vehicle completely engulfed in flames; No prior warning lights in some cases
Codes mentioned: Not typically documented pre-fire; some mention oil consumption or knock sensor codes prior
Repairs/costs cited: One vehicle was destroyed by fire while parked. One fire was extinguished by bystanders using fire extinguishers. One fire destroyed the vehicle; insurance company determined total loss. Hyundai's stated cause in one case was a 'missing oil cap,' but owner alleges manufacturing defect.
Recalls/TSBs owners mention: Hyundai acknowledged Campaign 953 (ECM and cluster update) and promised 48-hour guidance in one case, but no details provided. Hyundai cited the missing oil cap as the cause in another case, absolving itself of responsibility. No recall issued; vehicles destroyed and not repaired by manufacturer.
Connecting Rod Bearing Failure
Internal engine bearing wear and failure, typically detected during diagnostics after misfire or loss of power events. Metal debris observed inside engine.
When: Detected at 67,000 miles in documented case; likely present at higher mileages in other unreported incidents.
Symptoms owners cite: Check Engine Light; Loss of motive power or inability to accelerate; Vehicle unable to restart after stalling; Knock sensor activation prior to bearing failure
Codes mentioned: Check Engine Light (specific bearing codes not detailed), Knock sensor codes preceding bearing diagnosis
Repairs/costs cited: Engine replacement required. Metal debris observed during internal inspection.
Recalls/TSBs owners mention: Manufacturer notified; case opened and contact referred to NHTSA hotline. Hyundai Motor America has not yet provided assistance or warranty coverage details. Vehicle not repaired.
Oil Pan Defects (Cracks and Leaks)
Cracked, defective, or failing oil pans resulting in rapid oil loss. In some cases, Midas overtightened the pan during service, cracking it; in others, panels appear defective from factory.
When: Detected at 40,000–99,000 miles; one case at 47,432 miles, another at 99,000 miles.
Symptoms owners cite: Engine oil running completely dry; Low oil warning light; No visible oil spots on driveway or garage floor (hairline cracks); Sucking noise from engine indicating near-empty condition
Codes mentioned: None; purely mechanical
Repairs/costs cited: Oil pan replacement performed at dealership. However, in one case, the issue recurred despite replacement, suggesting an underlying internal consumption problem, not just a leak. One owner reports paying $564 for combustion chamber cleaning after pan replacement did not resolve the issue.
Recalls/TSBs owners mention: Dealers attempted to deny warranty for repair when Midas performed the replacement, claiming Midas overtightened it. This shifted liability to the independent shop rather than acknowledging a defective part. In cases where Hyundai replaced the pan, the underlying consumption problem persisted, indicating the pan crack was a symptom, not the root cause.
Synthesized from 135 NHTSA owner complaints — unverified consumer allegations, summarized for patterns. The verbatim filings appear below.
What owners are reporting 7 most recent
- Component or system that failed: engine (cylinder #4 misfire) - Safety was put at risk twice: once when knock sensor went off on interstate (October) and engine went into limp mode (aka protection mode). Required a tow from the, which was tricky with a vehicle full of passengers. Safety was put at risk gain when vehicle began to stutter/sputter on interstate (December). Drove 4-cylinder vehicle…
The car buck going threw lights it had no power. It would bog down when you lease expect it. This doesn't always happen. But once happen at 1:30am. The light been on for awhile. I go threw a green light the car went lip and the semi laid on his horn at 1:30am. Because he didn't know I been having this problem for months.the next morning I went to the dealer just as they got there no appointment…
The contact owns a 2018 Hyundai Sonata. The contact stated that while receiving an oil change, the technician advised that the oil level was very low. The contact stated that the low oil warning was intermittently illuminated. The contact was advised to take the vehicle to a dealer to be diagnosed. The contact had taken the vehicle to a local dealer, where an oil consumption test was performed.…
The contact owns a 2018 Hyundai Sonata. The contact stated while driving approximately 35 MPH, the vehicle jerked and failed to accelerate as designed. The contact stated that the check engine warning light was illuminated. The contact drove the vehicle to the residence and then contacted the dealer. The vehicle was diagnosed but was not repaired due to the cost. The manufacturer was made aware…
Oil pan crack. Leaking oil can get on engine or underbody components causing fire or failure of breaking systems causing an accident resulting in death. Problem was identified by dealer and in fact the dealer rep said this was common. It has not been inspected by police or insurance reps. There is no warning systems in place for a cracked pan.
I own a 2018 Hyundai Sonata SEL. I purchased the car in 2020 with ~17,000 miles on it. At ~50,000 miles the engine begin burning oil. It has progressively gotten worse. I can’t bring myself to have anyone drive my car because of this issue. It is all to likely that the result of the car burning so much oil in a short period of time. That it can pose a serious risk to anyone driving the car if the…
The contact owns a 2018 Hyundai Sonata. The contact stated while driving at an undisclosed speed, the check engine warning light illuminated. The vehicle was taken to the dealer where it was diagnosed that the engine had failed and needed to be replaced. The manufacturer was contacted and approved the engine replacement. The vehicle was repaired; however, the contact stated that the check engine…
Had engine trouble with your 2018 Hyundai Sonata?
File a complaint with NHTSA → It's free, official, and how every report above got here — owner filings are the federal safety record this page is built on.
Common questions
How serious is the engine problem on the 2018 Hyundai Sonata?
It's a documented issue but not catastrophic. 135 complaints have been filed. Repairs average $3,100 and most owners catch it before it causes a breakdown.
At what mileage does the engine typically fail?
Across the 40 complaints that reported odometer mileage, most engine failures cluster between 14,000 and 99,000 miles, with the median around 40,000. A quarter of owners report trouble before 14,000; a quarter make it past 99,000. Maintenance history matters more than the odometer alone — this is the reported failure window, not a guarantee.
What does it cost to fix?
Independent shops typically charge around $3,100 for engine repairs on this vehicle. Dealer pricing tends to run 20-40% higher. The exact figure depends on the specific failure mode, parts availability, and your local labor rates. If you're outside factory warranty, an extended service contract often covers this category.
Are there any recalls related to engine?
No active recalls currently cover engine issues on this vehicle. The complaints filed represent owner-reported failures that haven't risen to the level of a manufacturer-issued recall — but they're still worth knowing about before you buy or budget for repairs.
Related
Complaint and recall data sourced from the National Highway Traffic Safety Administration (NHTSA)
public records database. Verify the raw federal record at
nhtsa.gov/vehicle/2018/Hyundai/Sonata.
Severity ratings are derived from reported crashes, fires, injuries, and fatalities. Repair cost estimates are
independent-shop national averages and may differ in your area. Some links on this page are affiliate links.