P0172 System Too Rich on Smart Fortwo 451 / 453
P0172 means the engine is running rich — the ECU is pulling fuel back because the air-fuel ratio is reading too heavy. On a Smart Fortwo this code is less common than the lean codes. The usual suspects are a leaking injector, a stuck-open fuel pressure regulator, or a MAF sensor reading low.
Typical Symptoms
- Check engine light with code P0172
- Long-term fuel trim consistently below -10%
- Black smoke from the exhaust under load
- Sooty exhaust tip
- Fuel smell at start-up
- Rough idle and noticeably worse fuel economy
What it means
P0172 is the OBD-II code for "System Too Rich (Bank 1)." It is the opposite of P0171: the ECU is seeing too much fuel for the air the engine is taking in, and the long-term fuel trim has dropped well below zero as the computer pulls fuel back to compensate.
On a Smart Fortwo this code shows up less often than the lean codes, but when it does, the cheap-first list is short. A leaking injector dripping fuel into a cylinder, a fuel pressure regulator that is not bleeding off correctly, or a MAF sensor reading low so the ECU thinks more fuel is needed — those three cover most P0172 cases. Worn spark plugs that are not igniting cleanly can also leave unburned fuel in the exhaust and trip the code, though that is less common as a sole cause.
Likely causes, cheapest first
- Spark plugs worn and fouling. If the plugs are over 30,000 miles, replace all three regardless. Cheap insurance and a common cause of mild rich-running.
- MAF sensor contaminated. A dirty MAF reads low, the ECU thinks airflow is low, and it adjusts fuel up. Clean with proper MAF cleaner before replacing.
- Fuel pressure regulator leaking or stuck. A regulator that does not bleed off properly leaves fuel pressure too high, so injectors deliver more fuel than intended.
- Injector leaking at one or more cylinders. Confirmed by a balance test or by pulling the plugs and looking for one that is much wetter than the others.
- Coolant temperature sensor reading cold. If the ECU thinks the engine is colder than it really is, it runs a richer warm-up mixture continuously.
DIY check steps
- Read live fuel trims with a Bluetooth OBD-II dongle. A long-term fuel trim below -10% confirms a real rich condition rather than a one-off code.
- Pull the spark plugs. Soot-black plugs across the board are a textbook rich symptom. Replace as a set if any are worn.
- Clean the MAF sensor with proper MAF cleaner. Let it dry fully before reinstalling. Re-test after a short drive.
- Inspect the coolant temperature sensor connector for corrosion. Compare the live reading to the actual engine temperature — if it reads much colder, the sensor or connector is the issue.
- Listen for fuel pressure regulator leaks by sniffing the vacuum hose at the regulator. Fuel smell on the vacuum line is a regulator that is leaking internally.
When to call a shop
If new plugs, a clean MAF, and a clean coolant temperature reading still leave the fuel trims pulled deep negative, the next step is a fuel pressure test and an injector balance test. Most independent Smart-experienced shops can run both. If an injector is leaking, replacement is the fix; the shop will also clean the affected cylinder before reassembly.
A persistent rich condition will eventually load up the catalytic converter and contaminate the oxygen sensors, so this code is worth chasing rather than living with — different from P0171, which can sometimes be tolerated short-term.
Related parts & typical prices
| Part | Typical price | Search |
|---|---|---|
| Fuel injector | $80-180 | Search Google |
| Fuel pressure regulator | $50-130 | Search Google |
| Mass Air Flow (MAF) sensor | $80-220 | Search Google |
| Spark plugs (set of 3) | $15-35 | Search Google |
Prices are rough community-reported ranges, not quotes. Aftermarket vs. genuine Mercedes parts swing the spread; call a Smart-experienced shop for an actual quote.
Manual references
- Service manual on Manualslib — external mirror (we don't host this specific document).
- Browse Smart manuals on smartcarmanuals.com — pick your chassis code section on the home page if a specific manual isn't listed above.
Community references
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