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R34 25GT Engine Light 21

CF.Panda
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CF.Panda Apprentice

CF.Panda

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12 hours ago, GTSBoy said:

Yeah - I mean, go the other way. Smaller range. Not larger.

AHHHH gotchaa, I'll do that once I am home again. I tried doing the harness with the multimeter but it seems the car needed a jump, there was no power when it was in the "ON" position. Not sure if I should use car battery jump starter or if its because the stuff that has been disconnect the car just does send power.

joshuaho96 Veteran

joshuaho96

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20 hours ago, CF.Panda said:

I attached the image of the only thing I found, nothing to do with ohms in the manual only torque values etc. However, being they are so close to each other I am almost certain they are good. Tomorrow I will test the spark plugs while I am there. and look at the ECU connections.

image.png

Screenshot2025-08-25at6_24_12PM.thumb.png.76d7d373ceaed50e97a81299d1541de3.png

Diagnostic flow for a code 21 says check signal at the ECU pins, on a voltmeter of some kind it should read 0.1V at idle, 0.12V cranking, 0.15-0.25V at 2000 rpm. Check each coil such that +1, -3 is infinite or very high resistance, -3, +2 should do the same. (-1, +3), (+3, -2), (+1, -2), and (-1, +2) should all be somewhere between 0 ohms and high resistance. If you trace the path from the coil pin all the way back to the ECU in the diagram the resistance you measure at the coil vs connected to the harness should be pretty much the same. 

CF.Panda Apprentice

CF.Panda

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1 hour ago, GTSBoy said:

Could be correct. Meter might be that far out. Compare against a known 5 ohm 1% resistor.

I bought 6 new spark plugs to compare readings. They will be here tomorrow and ill see if meter is far off if it all. I know one is at least reading 0 so its best replace all anyway if one is bad, from experience. I still have to test the harness just to be sure its not a double issue and I waste my time haha-the old plugs are iridium I always thought those lasted more than 50,000 km.

MBS206 Grand Master

MBS206

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1 hour ago, CF.Panda said:

I bought 6 new spark plugs to compare readings. They will be here tomorrow and ill see if meter is far off if it all. I know one is at least reading 0 so its best replace all anyway if one is bad, from experience. I still have to test the harness just to be sure its not a double issue and I waste my time haha-the old plugs are iridium I always thought those lasted more than 50,000 km.

Ha, I stopped running Iridiums and fancy spark plugs in RBs as they last just as long as coppers, and coppers are way way cheaper.

RBs foul the plugs well before "they expire the plug"

CF.Panda Apprentice

CF.Panda

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22 hours ago, MBS206 said:

Ha, I stopped running Iridiums and fancy spark plugs in RBs as they last just as long as coppers, and coppers are way way cheaper.

RBs foul the plugs well before "they expire the plug"

Yea got me coppers. Bad news is I just put them in and the car still has the light and shakes. I also noticed I hear like rattling coming from the exhaust piping right under me. Does 21 also have a possibility of being O2 sensors?

 

So, coil packs are good, the harness is good, the MAF is working, and obviously the spark plugs just replaced and working.

MBS206 Grand Master

MBS206

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34 minutes ago, CF.Panda said:

Yea got me coppers. Bad news is I just put them in and the car still has the light and shakes. I also noticed I hear like rattling coming from the exhaust piping right under me. Does 21 also have a possibility of being O2 sensors?

 

So, coil packs are good, the harness is good, the MAF is working, and obviously the spark plugs just replaced and working.

21 is explicitly the coils detected as the issue.

Is it guaranteed the actual issue? No. Just the ECU thinks it is.

When you say rattling, describe it better, get an audio clip as best you can if possible.

CF.Panda Apprentice

CF.Panda

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8 hours ago, MBS206 said:

21 is explicitly the coils detected as the issue.

Is it guaranteed the actual issue? No. Just the ECU thinks it is.

When you say rattling, describe it better, get an audio clip as best you can if possible.

I wasn’t able to replicate the sound anymore. It was a one time sound so maybe it was just coincidence but it was like small pebbles being dropped in a coke can.

 

should I try cleaning the MAF? It could be it’s functioning but dirty? I don’t know.

joshuaho96 Veteran

joshuaho96

Posted
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On 26/08/2025 at 11:18 PM, CF.Panda said:

Yea got me coppers. Bad news is I just put them in and the car still has the light and shakes. I also noticed I hear like rattling coming from the exhaust piping right under me. Does 21 also have a possibility of being O2 sensors?

 

So, coil packs are good, the harness is good, the MAF is working, and obviously the spark plugs just replaced and working.

No, code 21 is very straightforward. It can only be the things described in that diagnostic flow. In fact it has no way of knowing that the spark plug resistance is out of spec.

CF.Panda Apprentice

CF.Panda

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2 hours ago, joshuaho96 said:

No, code 21 is very straightforward. It can only be the things described in that diagnostic flow. In fact it has no way of knowing that the spark plug resistance is out of spec.

By popular demand.. it was a coil. Got my hands on 1 new OEM coil, replaced with the one that made the less noise difference when I unplugged it while the car was running and started the car up. No stutter and the engine light was gone. I guess I’ll buy the other 5 they have lol

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