Mercedes GLA and GLC: OBD diagnostics on the compact SUVs

Despite sitting next to each other in the range, these two share very little. The GLA is a raised A-Class on a transverse front-drive platform; the GLC is a C-Class underneath, longitudinal and rear-biased. Their fault lists differ accordingly.

Shared reference: Mercedes-Benz common DTC codes, Mercedes C and E-Class OBD diagnostics and Mercedes-Benz engine diagnostics.

GLA: the A-Class inheritance

Dual-clutch gearbox

The 7G-DCT dual-clutch is the defining component. It needs a fluid and filter service on a real interval, and skipping it is the main cause of complaints.

Symptoms of a tired unit: hesitation pulling away, jerky low-speed shifts, and a shudder in traffic. Some low-speed hesitation is normal dual-clutch behaviour rather than a fault — the clutch has to physically engage.

P0700 is a pointer only; read the transmission module. Then P0730, transmission shudder at low speed and ATF diagnostics.

Petrol engines and carbon

Direct injection, so intake valve deposits accumulate. Misfires that survive new coils and plugs point here rather than at ignition: GDI carbon buildup and P0301 to P0304.

Timing chain

Chain and tensioner wear on some engines in this family. Cold-start rattle is the warning: P0016 and timing chain vs belt DTC codes.

GLC: the C-Class inheritance

Air suspension where fitted

Optional on the GLC, and where fitted it is the biggest financial risk. Air springs perish, the compressor works harder, then fails too.

The warning sign: the car sits low after standing overnight and rises when started. Fixing the leak before the compressor burns out is dramatically cheaper.

Outside generic OBD-II — scan tool limits and suspension modification electronics.

9G-Tronic automatic

Generally robust. Complaints usually trace to fluid condition or an adaptation reset needed after work. Harsh engagement immediately after a service is very often the wrong fluid specification.

Clunk when shifting into drive and P0740 and P0741.

Diesel emissions stack

• EGR carbon — P0404.
• DPF — DPF light on and P2002 and P2463.
• AdBlue and SCR — mandatory on these and a common source of expensive faults: AdBlue warning countdown and AdBlue system diagnostics.
• Injectors — common rail injector diagnostics.

Oil dilution from repeated interrupted regenerations raises the dipstick level. Above maximum means an oil change.

Shared: electrical and module count

Both carry a substantial module count, and both are sensitive to battery condition. A weak battery produces communication codes across systems that have no relationship to each other.

Load-test the battery before diagnosing anything else. U0100, U0140 and U0155 and when the scanner lies about ghost codes.

Many of these cars also have a second, smaller battery for the electrical system or stop-start. A failing auxiliary battery produces symptoms that look nothing like a battery problem, and people replace the main one twice before finding it.

Battery replacement and registration and auto start-stop diagnostics.

Water ingress

Blocked plenum and sunroof drains put water onto modules mounted low in the car. Check the carpets on any used example: damp carpet and electrical faults and sunroof drain blockage.

Driver assistance

Both carry extensive assistance systems that require calibration after windscreen, bumper or suspension work. A car that has had a screen replaced without calibration may have systems that appear to work and do not aim where they think they do.

ADAS calibration.

What generic OBD-II reaches

Codes, live data, freeze frame and readiness read normally. Mercedes-specific data, coding, body and comfort modules need enhanced access: manufacturer coding for VAG, BMW and Mercedes. Newer models move onto Ethernet diagnostics: DoIP explained.

Buying used

1. Load-test the battery, and ask about the auxiliary one.
2. Cold start it yourself and listen for chain rattle on petrol models.
3. Check soot load and AdBlue system status on diesels.
4. Leave it overnight and check ride height on air-sprung GLCs.
5. Read every module, and record before clearing.
6. Readiness monitors — all “not ready” means recent clearing: OBD readiness monitors explained.

Full routine: used car pre-purchase OBD check.

Try it on the free tier of the app — pair an ELM327 adapter and check resting voltage after the car has stood. On a modern Mercedes that number explains a large share of the warnings owners find inexplicable.

Read next

• Mercedes W204 and W205 C-Class: the fault list by generation