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Old 08-28-2017, 08:40 PM
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Join Date: Jul 2016
Posts: 1
W124 Air Injection Code


I had my car's air pump replaced in Nov last year due to bearing noise. I believe a month after that I was getting the Air injection code (4 LED blinks).

I just reset it. It stays off for about 3 to 7 days. If it comes on, it will always happen after starting the engine and always first thing in the morning.

My daily driving pattern is:

- between 10 to 15 engine starts per day
- twice I need to drive for 18 miles round trip (school for my boy)
- the rest are .2 to .5 miles drive

I am using the following for reference:

And also found the following in some forum (couldn't remember):


AIR system :
There are 4 components to check on the system:
1-Pump w/clutch:
You know this is electrically OK cuz it comes on. [clutch engagement]
2-Switch Over Valve:
This is an electrically energized valve that allows engine intake vac to reach the AIR shut off valve. You know this is recieving an electical signal cuz it is wired in parallel with the pump clutch
[ which has power , so same for valve].
This SOV is located under the front cover at front of engine. It has a clear plastic vac. feed line from the intake. Check this for breaks, as they are common cuz the plastic hose dries up and cracks from eng. heat.
3- AIR shut off valve:
This valve is a gate between the pump output line and the eng. head. It is energized by the small vac. line coming from the SOV.
So, if you have vac at this line from the SOV when the pump comes on, the valve is suppose to open to let the pumped air continue on to the eng.
4- Check valve:
This is the last valve between the shut off valve and the engine. It is there to prevent pressure from engine going back to the system. This is a one way valve and is self operating according to pressure.
One should be able to blow through it in one direction only.

Any of these can cause a fault code if they are not operating.
[ clogged/no vac/bad valves/etc.
I doubt you have an electrical failure as the pump is coming on.
Could also be bad pump.
As one can see [ and I listed these components in order of air flow], the pump comes on---blows air to shut off valve[ which is opened by vac line from SOV]- ---air goes through shut off valve and the pressure forces check valve open --- air reaches engine to heat exhaust/lower emmissions.
Thats the general operation and will give you an idea of what to check.
Once you see it , this will become a lot less complicated than it sounds reading it...
My first check would be all vac lines , second would be to see if
you have vac at shut off valve when pump comes on [ a vac gauge is nice here]

PS Note*
For anyone reading this far, it is noted that the plastic vac line mentioned above is also the feed for the EGR SOV stacked together with the AIR SOV under the front cover .. so EGR faults can also be this same plastic line...

Can I rule out the pump (and clutch ) ?

Is there anything obvious from the symptoms that I have described ?



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