Mercedes A200 CDI: Under tyre pressure?

  • Our A-Class throws up a concerning warning
  • Low tyre pressures mean a trip to the garage
  • All is not always what it seems...

I jumped into my Merc the other day to find the dash looked odd. A more careful inspection showed that it was displaying a poetically romantic message: “Check tyre pressures then restart Run Flat indicator”.

Since it asked so nicely, I decided to pay attention. The first thing I did was a visual check to see if any of the tyres were flat. They weren’t. None looked remotely deflated, but that’s no guarantee there isn’t a slow puncture which could lead to a blow-out on the motorway – not something you want to be dealing with at any time.

My next course of action was to get myself down to the local filling station and find out for sure which one of the tyres was causing the problem.

Turns out our local garage’s air had run out, so it was on to the next closest one.

Since you’ve got to pay for the air you put into your tyres at such establishments, a little bit of strategy is required here.

Basically, make sure you’ve removed all four of your dust caps before applying the nozzle to your tyre’s valve.

The second thing to ensure is that the machine is set to the correct pressure. You’ll be able to find this in your car’s manual, and in my case, inside the fuel filler cap too. My car had two readings – one for hot tyres and one for cold. This is because the pressure inside the tyre will be higher when it’s warm, which means you don’t want to put as much in.

My filler cap informed me that for my 18-inch wheels I needed 28 bar of pressure once I’d compensated for the heat of my tyres. I set the machine thus, and began my tour around the four corners of the A-Class.

When you initially place the nozzle onto the tyre valve it tells you the pressure already in your tyre, and then it beeps when it has pumped the tyre up to your selected pressure.

In my case, the tyres were perfect on all four wheels. This was puzzling. Why was it telling me to check pressures when they were all perfect?

Further investigation revealed that if you bump over a pothole, kerb or speed bump a little too hard then the air in the tyre will move. This means the pressure sensor will occasionally read low pressure for a fraction of a second, and this can trip the car into suggesting a check of the pressures.

Since where I live is completely impossible to get to without the navigation of at least 10 speed bumps, the A200 has lowered sports suspension and also has low-profile tyres, this could well be the cause.

To celebrate not having to buy a brand new tyre, I took a picture of the car next to an arch near Silverstone:

Mileage: 7,979

Fuel economy: 49.5