This morning (sunny weather, ~8 °C) I parked my Smart ForFour 1.1 (first generation, made in 2007, ~43000 km), left it there for ~20 minutes, then, when I started it again, it had the “motor check” light on and the engine had clear power issues:

• when left idle, it stayed on the low side – ~900 RPM instead of its (IIRC) more usual ~1200;
• it didn’t respond well to the accelerator – left in neutral, even at full throttle it struggled to reach 4000 RPM, and it was as if the accelerator had an initial “dead zone” before actually doing something;
• when trying to move, it had really low torque – I had to engage the clutch really slowly to avoid stalling the engine.

(as soon as I noticed this I switched it off and on to make sure that it wasn’t some random electronic problem, but of course it didn’t do a thing)

Now, I had to drive for – say – 4 kilometers in this condition, then I parked the car to check if there wasn’t anything macroscopically wrong (refrigerant and motor oil in particular); after I started the car again, the “engine check” light was still on, but the power issues disappeared – now the engine feels exactly as usual.

Of course I’ll bring it to a mechanic ASAP, but I’d be interested in opinions about what could have happened and, in particular, why is it now apparently working fine but the engine check light is still on.

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Some extra details about the car: it’s a Smart ForFour 1.1 (first generation, 2005 model, gasoline, 47 kW) – it has actually the same engine of some Mitsubishi Colt. It was bought in 2007 by a relative of mine who didn’t use it much – I got it in 2015 when it had ~30000 km. I performed the regular 40000 km maintenance (oil change, air filter change) in June 2016; changed brake pads and passed bi-annual mandatory inspection in November. The only issue it had (two years ago) was some noise/slippage of the drive belt, which was replaced and later re-tensioned.

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Update

Using an ODB2 reader and ODB Car Doctor I got the error codes:

• P1961
  • Freeze DTC
  • Fuel system status (FSS): Open loop: engine load OR fuel cut due to deceleration
  • Calculated engine load value (EngineLoad): 31.37%
  • Engine coolant temperature (ECT): 63.0 °C
  • Short term fuel % trim Bank 1 (STFT1): 0.0%
  • Long term fuel % trim Bank 1 (LTFT1): 1.56%
  • Intake manifold absolute pressure (MAP): 32.0 kPa
  • Engine RPM (RPM): 832.0 rpm
  • Vehicle speed (Speed): km/h
  • Timing advance (TA): 9.0°
  • Intake air temperature (IAT): 34.0 °C
  • Throttle position (TP): 5.09%
  • Distance traveled with malfunction indicator lamp (MIL): 14.88 km
• P0122: Throttle/Pedal Position Sensor/Switch “A” Circuit Low

Reading around it does seem to be a very common problem related to the throttle body – it either needs cleaning or replacement; can someone confirm? How would I go to clean it up myself? I don’t really know even where it should be located…