Fault codes
Pulse codes from the bay connector on early cars. OBD-II under the dash from 1996 (US) and on all Series III. Roof and PSE never speak fluent generic OBD.
The 16-pin diagnostic block sits in the engine bay, right side near the bulkhead / ECU, under a cap. Key ON, engine OFF. Bridge pin 1 (ground) to the pin for the system you want (HFM is commonly pin 4; the cap chart is gospel) for 2–4 seconds, then release. The Check Engine lamp flashes a two-digit code: long pauses between codes, short pauses between digits. 1-1 (or a single long flash on some systems) means no fault stored. Write them all down before you clear.
16-pin J1962 connector under the dash, driver’s side of the steering column. A generic $20 reader will pull P0 engine codes on ME 2.0 cars (Series III). On HFM cars (Series II V8/six) a generic reader is hit-and-miss — you still want the pulse method or a Mercedes-capable tool. Body, roof, PSE and ABS modules never speak fluent generic OBD.
Star Diagnosis / a modern MB-capable tool (iCarsoft, Autel with MB pack, DAS on an old laptop) talks to the roof module, PSE, ABS/ASR/ESP, climate, and SRS. If the complaint is ‘top, locks, windows, cluster’, do not waste the afternoon on P-codes.
The control unit has nothing stored. If the lamp is still on, you are reading the wrong module.
ECT signal implausible. Cold hard starts, rich smell, high idle.
IAT out of range. Usually baked into the HFM mass-air meter on these engines.
The HFM itself. Hesitation, black smoke, or a brick-like throttle.
Idle switch not agreeing with the potentiometer.
Idle too high, too low, or hunting.
ECU is not happy with the crank/TN pulse.
O2 slow, dead, or heater open. Fuel trim will be wrong.
On KE cars this is the electro-hydraulic actuator. On HFM it flags mixture adaptation limits.
ECU cannot talk to another control unit (EA/CC/ISC depending on year).
Misfire or coil current out of range.
Cam magnet or cam position not matching.
Mass-air circuit malfunction on ME 2.0 (M112 / M113).
ME has given up counting cylinders.
The classic hot no-start on late V6/V8. Car starts cold, dies hot, starts again after ice cream.
Mixture adaptation off the map.
Purge or tank-tightness. Rarely a breakdown, always an inspection fail.
Pneumatic control circuit short. Often after a door-boot repair gone wrong.
Second PSE control circuit. Same family as B1100.
Right-front window position unknown. The roof sequence will abort because it will not risk guillotining the glass.
Left-front window position unknown.
Same family as 098/099 — the module wants a full up/down learn.
ABS/ASR lamp on. Each wheel has a pin on X11/4.
From 1997 the passenger occupancy mat can disable the right bag. An SRS lamp after seat work is usually a connector under the seat, not a bag.