Why the right-side sensor kept failing, why the fuse kept blowing, and how a shorted intake actuator was hiding behind the whole thing

If you've landed here because your Mercedes GL350 diesel has thrown fault codes 100D00 and 100E00 on the right-side hot-film air mass sensor (B2/7), and the check engine light simply will not go away no matter how many times you clear it — you're in the right place. This is a real diagnostic case from our workshop, not a theory pulled off a forum. We're writing it out step by step because this exact combination of faults trips up a lot of technicians, and the actual root cause is easy to miss if you only chase the sensor.

The Complaint

A Mercedes GL350 diesel (OM642 platform) arrived with the check engine light on. Two codes came up on the scan:

• 100D00 – Lower limit reached on component "B2/7 (right-side hot-film air mass sensor)," currently active
• 100E00 – Same component, same lower limit condition, currently active

There was also a fault stored for the intake air temperature sensor. On top of that, the codes wouldn't clear at all, even with the engine off — which is usually a sign that whatever is causing the fault is still electrically present, not just a one-time glitch sitting in memory.

Reading Between the Lines: What "Lower Limit Reached" Actually Means

On the OM642 diesel engines used in the GL350, the hot-film mass air flow sensors (there are two — one per bank, referred to as B2/6 on the left and B2/7 on the right) send a signal voltage back to the engine control unit that represents the amount of air passing through. When the ECU logs a "lower limit reached" fault, it means the signal it's reading from that sensor has dropped below the minimum value the ECU expects to see under any normal operating condition.

That's an important distinction. A drifting or dirty sensor usually gives you an inaccurate but plausible reading — the ECU sees something odd but not impossible. A "lower limit" fault, especially on both associated codes at once, points toward something more basic: no signal, or no reference voltage, reaching the sensor at all.

Step One: Check the Sensor's Power Supply

Before pulling the sensor itself, we went straight to the wiring diagram. On this circuit, pin 1 of the air mass sensor connector is the power supply feed. With the connector in place and the key on, we measured voltage at that pin.

The reading came back at just 1.3 volts — nowhere near what a healthy 5-volt reference supply should show. That confirmed it: the sensor wasn't getting proper power. Whatever the sensor itself was doing, it physically could not produce a valid signal without a proper supply voltage feeding it in the first place.

Step Two: Trace It Back to the Fuse

With a supply voltage problem confirmed at the connector, the next logical step was to trace it back through the fuse box. Following the circuit back to the F58 fuse box, we checked fuse F215, which feeds this circuit.

Sure enough — it was blown.

This is a good reminder for anyone diagnosing similar cases: when a sensor circuit shows a partial or missing voltage instead of a clean zero or a clean five volts, don't assume the sensor is bad. Check the fuse and the supply line first. A blown fuse upstream, combined with resistance or bleed-through elsewhere in the circuit, can easily produce a "half voltage" reading like 1.3V that looks confusing until you trace it to its source.

Step Three: The Fuse That Wouldn't Stay Fixed

We replaced fuse F215, and the system came back to life — for a moment. The vehicle ran fine right after the fix. But on a later attempt to start it, turning the ignition on sometimes blew the same fuse again.

This is the part of the case that separates a real fix from a temporary patch. A fuse that keeps blowing after replacement isn't a fuse problem — it's a downstream short problem. Replacing the fuse over and over without finding what's pulling too much current is a losing game, and on a modern diesel ECU it risks damaging other components sharing that circuit.

So we kept digging.

Step Four: Finding the Actual Short

After starting the engine again with a fresh fuse, a new fault appeared: the intake manifold shut-off actuator motor was showing a short to ground. That was the missing piece. We checked the actuator directly and confirmed it — the motor itself was internally shorted.

That explained everything:

• The shorted actuator motor was pulling excessive current on the shared circuit
• That excess current is what kept taking out fuse F215
• With the fuse blown, the air mass sensors lost their proper supply voltage
• Low or unstable supply voltage on the sensor circuit is exactly what triggers a "lower limit reached" fault like 100D00 and 100E00
• Because the short was still physically present, the fault condition never actually went away — which is why the codes wouldn't clear even with the engine off

One component — a shorted intake shut-off actuator motor — was quietly triggering fuse failures, sensor supply voltage drops, two active air mass sensor codes, an intake temperature sensor fault, and a check engine light that refused to reset.

The Fix

We replaced the intake manifold shut-off actuator motor. Once the shorted component was out of the circuit, the new fuse held under load, sensor supply voltage returned to a normal, stable reading, and the air mass sensor codes cleared and stayed cleared. No more random fuse failures on startup, no more logged faults on B2/6 or B2/7.

The Real Lesson Here

This case is a textbook example of why chasing the fault code by name can send you in the wrong direction. The code says "air mass sensor." A technician working only off the code description might order two new sensors, install them, and still end up with the exact same fault fifteen minutes later — because the sensors were never actually broken. They were starved of power by a fuse that was being sacrificed to protect the circuit from a completely different, shorted component elsewhere in the intake system.

The pattern worth remembering for future diagnostics on this platform:

• A "lower limit" or "signal too low" code on a sensor circuit is often a wiring or supply issue, not a sensor failure. Check voltage at the connector before you condemn the part.
• A fuse that blows once might just be age. A fuse that blows repeatedly is telling you there's a short somewhere downstream — go find it before you reach for another fuse.
• Related codes on nearby components (in this case, the intake air temperature sensor) are a clue, not a coincidence. When multiple faults share a power source or a fuse, look at what they have in common electrically before treating them as separate problems.
• A code that won't clear even with the ignition off usually means the fault condition is still physically present, not that the ECU needs a harder reset.

Frequently Asked Questions

What does fault code 100D00 mean on a Mercedes GL350? It indicates the ECU has detected that the signal from the right-side hot-film air mass sensor (component B2/7) has dropped below the minimum value it expects, known as the "lower limit." This is usually caused by a supply voltage or wiring problem rather than a failed sensor.

Why won't my Mercedes air mass sensor fault code clear? If the underlying electrical fault is still present — such as a blown fuse, a short circuit, or missing supply voltage — the ECU will keep re-detecting the same condition, so the code will not clear permanently even after multiple reset attempts.

Can a blown fuse cause an air mass sensor fault code? Yes. The air mass sensors need a stable reference voltage to produce a valid signal. If the fuse supplying that circuit blows, the sensor's voltage drops, and the ECU logs it as a sensor fault even though the sensor itself may be completely fine.

Why does the fuse keep blowing after I replace it? A fuse that blows again shortly after replacement almost always means there's a short circuit elsewhere on that same circuit pulling too much current. On this platform, a shorted intake manifold shut-off actuator motor was the actual cause.

Is the intake shut-off actuator related to the air mass sensor circuit? On this vehicle, yes — they share the same fuse (F215) on the F58 fuse box. A short in the actuator motor overloaded the shared fuse, which in turn cut power to the air mass sensors and triggered the sensor fault codes.