My sons '99 A4 1.8t 158,000 miles started throwing these codes randomly a few weeks ago.
Did some research and figured it must be coil packs.
Well I figured if I'm going to be in there I would replace the valve cover seals as well.
Pretty easy fix only took about an hour to replace the seals, new spark plugs and coil packs.
However after filling up with gas yesterday it threw the same codes again.
What am I missing? Anyone know what it could be?
I saw something about the engine speed sensor but why would that throw misfire codes?

 

I can't remember if 99 had an ICM or not, but if it does that could be your problem. It would be a small (not quite the size of a credit card) sensor with a heat sink on it attached to the top of your air box. There will be plugs on either end, one coming in from the wiring harness and one going out to the coil packs.

 

It would be best to find out what cylinder is misfiring then swap coils & plugs & see the fault follows the coil or plug.
Throwing parts at cars can be expensive , a little footwork into diagnosing can save you some $.
Remember , just because a part is new does not mean it can't be defective.
NEW=Never Ever Worked.

 

P0300 in random multiple misfires
P0303 is cylinder 3 specifically
The P1606 is pretty weird, but I don't think it relates to your misfires.

Is the car running rough, like it's misfiring, or did the CEL just come back on? Were the codes actually cleared? I'm thinking, if the car is running normal, the P1606 may be the current active code (I think it's ABS related, you will need VAG-COM to investigate that one better) and the misfires are old stored codes that your scanner is still picking up. Try clearing all codes and seeing which ones pop back up.

The 99 does have an ICM (about $150 part) but another possible cause for these codes is simply needing to clean the MAF (if you take the airbox apart, you'll find a cylinder with a "screen" on one end and a small metal sensor in it. A couple shots with MAF cleaner (easy to find at autozone or similar) and that might help. More often than not, if the ICM goes bad, it's cylinder 2 that goes dead (as I recently learned from experience) but that doesn't mean it can't be another cylinder.

Let us know what you find, and we will try to help more.

 

Upon further investigation (AKA, google) I think your P1606 means your ABS module has gone bad, is going bad, or simply blew a fuse, and it's sending the ECU a bad signal, thus the CEL. There is a place called module masters that will rebuild your ABS for a couple hundred, which is much cheaper than a new one.

 

All codes were previously cleared.
They came back today on his way home from school.
I cleared them again then swapped coilpacks 1 and 3 so we'll see if it moves.

I kinda thought that maybe the P1606 was due to the surging when the cel comes on, he says it runs real rough and then the light comes on.

 

Did you gap the spark plugs to .032 (if stock, .028 if chipped)? Sometimes "pre-gapped" spark plugs aren't right.

 

Ok, so my son was home sick Tuesday and Wednesday so the car never moved.
He drove it Thursday and no issues.
He came home from school today and said the CEL was on.
Checked codes and got P0300 and P0303 again.
P1606 was not in the fault codes.
This after switching coil packs #1 and #3.
He says it runs real rough when the cel comes on.
Pushes the pedal to the floor and the car has no power.

Will check spark plug gaps tomorrow afternoon and see what they are.

 

If the problem can definitely be localized to cylinder 3 and if it's not the plug, it could be an injector. You know air is getting there, and if the plug is good, then spark is there (since using a known good coil wasn't a fix), so fuel is all that's left...

 

Or the ICM...?

My my ICM started going bad, the same thing happened. It started running rough, I let it sit for a few days (it was a weekend and I was busy) and I bought a code reader. A few days later I went to scan it, and it started right up no problem. A few weeks later, it started running really rough again, I mean really bad, no power at all, scanned it and had a dead cylinder. I did the coil pack and spark plug swap and it was still cylinder 2. Turned out to be the ICM. Since you have the P0300 (Random Multi) I would lean more towards the ICM than an injector, but you never know. If you can check the injector for free, might as well try it out.

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