OK, my temp sensor (four prong) was indeed bad as the codes said it was. Now the only code left to eliminate is #11 (air injection system, open or short circuit).

What does this mean?
What do I have to fix to stop this code?

Thanks,

AL

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03 CLK430 Cabrio 211K Pweter Silver/ Oyster Leather
90 300SEL 214k Pearl Black/ Alto Grey/Black Leather

Prior: 85 190E, 88 300TE, 89 300TE, 90 300E 2.6, 90 300TE, 92 300E 2.6, 91 and 93 300SL, 87 Ferrari Mondial Cabrio

I don't know for sure on this system, but typically this describes the electrical portion of the vacuum solenoid or air pump clutch. ( the computer monitors current, some systems look for an inductive kick once power to the coil is dropped. )

There is an ever so slight chance it monitors the oxygen sensor and is looking for an increase in exhaust oxygen but given it is OBD I I doubt it.

It turns on at warm up (I can see the clutch engage and hear a minor hissing sound) and then turns off once the engine is been running for a couple of minutes. It does turn on again from time to time even afterwards though.

Do this mean that it's time to replace the air injection pump?

Thanks.

__________________
03 CLK430 Cabrio 211K Pweter Silver/ Oyster Leather
90 300SEL 214k Pearl Black/ Alto Grey/Black Leather

Prior: 85 190E, 88 300TE, 89 300TE, 90 300E 2.6, 90 300TE, 92 300E 2.6, 91 and 93 300SL, 87 Ferrari Mondial Cabrio

Try cleaning the electrical connectors at the pump and going from the pump through the engine compartment. Maybe an electrical connector along the way?

A code like that means there is a poor or marginal electrical connection or that there is an electrical "leak" between the wires or to ground (metal in the engine compartment). Oil or gunk on an electrical connector can conduct a little bit of electricity from the connector prong to metal in the engine causing a "short" [to ground].

In addition to the air pump you should have a vacuum solenoid that controls a diverter valve. Follow the large air pump hose to a device ( diverter valve ) that will have a small vacuum hose, follow that vacuum hose to a solenoid valve, check the electrics here.

Also, pull the vacuum hose from the diverter valve and install a vacuum gauge, if the air pump runs the solenoid should trigger some of the time and you should see vacuum.