Why does my Mercedes GLB 250 run rough with a check engine light?

Short answer: Owners say rough running with a check engine light on a GLB 250 is often the M260 cylinder head problem, worn exhaust valve seats and guides. Mercedes extended the warranty on that part, according to letters owners posted: 15 years/241,000 km in Canada, and an exhaust valve assembly extension of 15 years/150,000 miles in the US. Don't ignore a flashing or repeat check engine light. Have the dealer diagnose it and ask whether your VIN is covered.

Symptom: The check engine light comes on (sometimes once and goes away), the engine idles or runs rough, may misfire, or makes a ticking or rattling sound. Some owners noticed harder starts after sitting overnight.

What to check first

1. Get the codes read. Misfire codes start with P030.
2. Ask the dealer whether your VIN has the M260 cylinder head / exhaust valve warranty extension.
3. Ask for a compression or leak-down test to confirm before parts are replaced.
4. Keep records of every check engine light, even ones that go away.
5. Expect waits: owners waited weeks for back-ordered heads.
6. Run your VIN at nhtsa.gov/recalls.

Likely causes

1. Exhaust valve seat and guide wear in the cylinder head. The cylinder head is the top part of the engine that holds the valves. Mercedes told Canadian owners in a letter that on some M260 four-cylinder engines (GLB 250 and others), exhaust valve seat rings and valve guides can wear, so the valves don't seal, the engine runs rough and the check engine light comes on.
2. Misfire from another cause. Spark plugs, ignition coils or injectors can also cause a misfire. A shop should confirm with a compression or leak-down test before replacing a head.

What it usually costs

Owners had heads replaced under the warranty extension at no cost. Repair time ran from days to weeks waiting for parts. No out-of-warranty prices were posted.

Is it safe to keep driving?

Mercedes' letter said the defect itself has no safety impact, but a flashing check engine light means a bad misfire that can damage the catalytic converter. Drive gently and get it in soon. Stop if it overheats.

DIY or shop?

Not a DIY job. Cylinder head replacement and confirming the diagnosis are dealer work, and the extension only pays at a dealer.

Recalls, bulletins and complaints in plain words

• Recalls and warranty extensions: NHTSA lists 17 recalls for the 2020-2023 GLB 250, and none are a cylinder head recall. The cylinder head and exhaust valve coverage is a warranty extension (covered repair only after the part fails), not a recall, according to the letters owners posted. Check your VIN at nhtsa.gov/recalls. If your car is part of a recall, the dealer fixes it for free.
• Plain words: The complaint count below shows how often owners reported problems that mention these words to NHTSA. A count is not proof of a defect.
• NHTSA complaints: 22 keyword-matched complaints for Mercedes-Benz GLB 2020–2023 (of 91 unique complaints queried, NHTSA complaints API by model year, queried 2026-11-02; keywords: check engine, misfire, rough, cylinder head, valve, shak, engine light, loss of power) (NHTSA returned no data for 2 model-year queries). This counts complaints that mention these words. It is not a defect finding.

What owners report

• In Warranty Repair - Cylinder Head! on MBWorld.org Forums, a GLB 250 owner in Canada posted a Mercedes-Benz Canada letter extending the warranty on the M260 cylinder head to 15 years or 241,000 km for some 2019-2022 A-Class, CLA, GLA and GLB models, because exhaust valve seat and guide wear can cause rough running and a check engine light. The owner had had one check engine light and a 'snare drum' sound. Later in the thread, US owners said they got an MBUSA warranty addendum extending the exhaust valve assembly to 15 years or 150,000 miles. Others noted that an extended warranty only covers the part once it fails, unlike a recall.
• In How reliable is the replacement head for the GLB 250? on MBWorld.org Forums, a newly bought GLB 250 was in the shop waiting on a back-ordered replacement head. An owner whose M260 head failed and turned on the check engine light at 27,000 miles said it was fine 16,000 miles later. Another said a C250 showed nothing but the check engine light. A 2020 owner with 30,500 miles asked whether hard starting after sitting overnight was an early sign.
• In GLB: worst vehicle I've ever bought! on MBWorld.org Forums, an owner's GLB 250 needed a cylinder head replacement at about three years and 20,000 miles, with 2-3 weeks waiting on a back-ordered head. They then had an overheating warning and later a '4MATIC inoperable' message that ended with a new rear differential.

Quick tip

If you own an M260 GLB, keep a paper trail. An extended warranty only helps once the part fails, so report every check engine light.

Vehicle focus: Mercedes-Benz GLB 250 (M260 2.0L turbo), 2020-2023. The letters also listed A-Class, CLA and GLA.

Safety / mechanic note

Overheating or a flashing check engine light means stop and get it towed. Verify with a mechanic.