R34 25GT Engine Light 21

[CF.Panda]

Hello, first of, I’ve looked at many threads on this code but none have helped me, if you know one that will help me feel free to post a link.

 

Now to my problem. Recently I was driving and noticed the check engine light came on. I went home and did the test to see what code it gave. I got the 21 code but everything seems fine with the car, no misfire, idle is good, acceleration is the same as when I bought it. I mean no signs of obvious failure. If no misfire is happening I can rule out the coil packs right? Which is about 98% of posts saying it is. If anyone has had this happen and found the issues let me know please. No other lights are on.

 

The car WAS due for an oil change. I since then changed it so idk if that triggers code 21. Either way it hasn’t turned off.

[GTSBoy]

Have you done 10x restarts since it came on?

[joshuaho96]

4 hours ago, CF.Panda said:

Hello, first of, I’ve looked at many threads on this code but none have helped me, if you know one that will help me feel free to post a link.

 

Now to my problem. Recently I was driving and noticed the check engine light came on. I went home and did the test to see what code it gave. I got the 21 code but everything seems fine with the car, no misfire, idle is good, acceleration is the same as when I bought it. I mean no signs of obvious failure. If no misfire is happening I can rule out the coil packs right? Which is about 98% of posts saying it is. If anyone has had this happen and found the issues let me know please. No other lights are on.

 

The car WAS due for an oil change. I since then changed it so idk if that triggers code 21. Either way it hasn’t turned off.

Oil change does not trigger code 21. Code 21 is for coilpacks primary side connection. You can try to clear the code with a battery disconnect, hold down the brake pedal to drain capacitors through the brake lights with the ignition on for 10-15 seconds before you reconnect the battery. I have seen R35 coil conversion permanently cause this code with no ill effects so it might be the resistance it wants to see isn't quite right on one or more coilpacks. Could be inside the ECU, could be the harness, could be a coil. You can test it all if you want or just ignore until the car actually starts misfiring.

[CF.Panda]

21 hours ago, GTSBoy said:

Have you done 10x restarts since it came on?

Yes I have. Probably on my 15x restart.

[CF.Panda]

17 hours ago, joshuaho96 said:

Oil change does not trigger code 21. Code 21 is for coilpacks primary side connection. You can try to clear the code with a battery disconnect, hold down the brake pedal to drain capacitors through the brake lights with the ignition on for 10-15 seconds before you reconnect the battery. I have seen R35 coil conversion permanently cause this code with no ill effects so it might be the resistance it wants to see isn't quite right on one or more coilpacks. Could be inside the ECU, could be the harness, could be a coil. You can test it all if you want or just ignore until the car actually starts misfiring.

So 10/10 times that code is coil packs? I mean if that's the case ill drop the $400USD or so to replace them. I didn't know these cars threw codes before the issue became an issue.

[GTSBoy]

5 hours ago, CF.Panda said:

Yes I have. Probably on my 15x restart.

If it (the code) wasn't still current, it should have gone away by itself by now.

5 hours ago, CF.Panda said:

So 10/10 times that code is coil packs?

No, nothing it 10/10. But it is quite likely. Everything else to do with the ignition could still be responsible (which is wires and connectors). The car is an old piece of shit now, so all the wires and connectors are also old pieces of shit.

[joshuaho96]

6 hours ago, CF.Panda said:

So 10/10 times that code is coil packs? I mean if that's the case ill drop the $400USD or so to replace them. I didn't know these cars threw codes before the issue became an issue.

It's not that straightforward. For example I have had this code because my ignitor/power transistor wires were unplugged. It knows something is wrong but has no idea what it means. If you want to actually diagnose this I recommend following the service manual flowchart for code 21. Ohm out the harness, coilpacks, inspect the ECU, etc. And again, it's entirely possible for there to be no problem at all but if you're running a different coilpack it'll trigger the code because it doesn't like what it sees.

[CF.Panda]

21 hours ago, joshuaho96 said:

It's not that straightforward. For example I have had this code because my ignitor/power transistor wires were unplugged. It knows something is wrong but has no idea what it means. If you want to actually diagnose this I recommend following the service manual flowchart for code 21. Ohm out the harness, coilpacks, inspect the ECU, etc. And again, it's entirely possible for there to be no problem at all but if you're running a different coilpack it'll trigger the code because it doesn't like what it sees.

shot myself in the leg and now last night I have shaking when I idle lol

[CF.Panda]

23 hours ago, GTSBoy said:

If it (the code) wasn't still current, it should have gone away by itself by now.

No, nothing it 10/10. But it is quite likely. Everything else to do with the ignition could still be responsible (which is wires and connectors). The car is an old piece of shit now, so all the wires and connectors are also old pieces of shit.

Just yesterday the car started to shake a bit when idle. So tomorrow ima test the some wires and packs with the multimeter and see what it is. Hopefully is a pack because thats the easiest thing to buy same day here where I live in the USA.

[MBS206]

1 hour ago, CF.Panda said:

Just yesterday the car started to shake a bit when idle. So tomorrow ima test the some wires and packs with the multimeter and see what it is. Hopefully is a pack because thats the easiest thing to buy same day here where I live in the USA.

Does it also misfire equally when revving?

 

Josh is very correct in what you should do. The coilpack harness wiring loom itself is a known problem due to its age and the number of heat cycles it has gone through.

Throwing parts at a vehicle to diagnose the issue isn't a smart or good way to do it. Secondly, you may have a bad coil pack, you pop replacements in, they fix that issue, but messing with the harness breaks it, so the issue persists. So now you think "well it wasn't the coil packs" and have to continue chasing your tail, potentially swapping back in your shit coil packs and returning the good ones (yes, I've seen people do this because 'it wasn't the problem' and they want to save money).

And suddenly, you've got two issues with the same symptoms...

 

Diagnose, don't use the spare parts shotgun.

[CF.Panda]

20 hours ago, MBS206 said:

Does it also misfire equally when revving?

 

Josh is very correct in what you should do. The coilpack harness wiring loom itself is a known problem due to its age and the number of heat cycles it has gone through.

Throwing parts at a vehicle to diagnose the issue isn't a smart or good way to do it. Secondly, you may have a bad coil pack, you pop replacements in, they fix that issue, but messing with the harness breaks it, so the issue persists. So now you think "well it wasn't the coil packs" and have to continue chasing your tail, potentially swapping back in your shit coil packs and returning the good ones (yes, I've seen people do this because 'it wasn't the problem' and they want to save money).

And suddenly, you've got two issues with the same symptoms...

 

Diagnose, don't use the spare parts shotgun.

Only at idle. Isn’t a problem when rev it seems.

[CF.Panda]

20 hours ago, MBS206 said:

Does it also misfire equally when revving?

 

Josh is very correct in what you should do. The coilpack harness wiring loom itself is a known problem due to its age and the number of heat cycles it has gone through.

Throwing parts at a vehicle to diagnose the issue isn't a smart or good way to do it. Secondly, you may have a bad coil pack, you pop replacements in, they fix that issue, but messing with the harness breaks it, so the issue persists. So now you think "well it wasn't the coil packs" and have to continue chasing your tail, potentially swapping back in your shit coil packs and returning the good ones (yes, I've seen people do this because 'it wasn't the problem' and they want to save money).

And suddenly, you've got two issues with the same symptoms...

 

Diagnose, don't use the spare parts shotgun.

Yea that’s why I said ima test them with multimeter and see the reads.

[CF.Panda]

On 23/08/2025 at 9:31 PM, MBS206 said:

Does it also misfire equally when revving?

 

Josh is very correct in what you should do. The coilpack harness wiring loom itself is a known problem due to its age and the number of heat cycles it has gone through.

Throwing parts at a vehicle to diagnose the issue isn't a smart or good way to do it. Secondly, you may have a bad coil pack, you pop replacements in, they fix that issue, but messing with the harness breaks it, so the issue persists. So now you think "well it wasn't the coil packs" and have to continue chasing your tail, potentially swapping back in your shit coil packs and returning the good ones (yes, I've seen people do this because 'it wasn't the problem' and they want to save money).

And suddenly, you've got two issues with the same symptoms...

 

Diagnose, don't use the spare parts shotgun.

Used 60K in Ohms and got these numbers:

1.45 Ω

1.38 Ω

1.47 Ω

1.43 Ω

1.56 Ω

1.45 Ω

 

I haven't been able to find what the correct range is though so they are within spec /:

[CF.Panda]

3 minutes ago, CF.Panda said:

Used 60K in Ohms and got these numbers:

1.45 Ω

1.38 Ω

1.47 Ω

1.43 Ω

1.56 Ω

1.45 Ω

 

I haven't been able to find what the correct range is though so they are within spec /:

On the 600k Ohms:

1.4 Ω

1.4 Ω

1.4 Ω

1.5 Ω

1.5 Ω

1.5 Ω

[MBS206]

29 minutes ago, CF.Panda said:

Used 60K in Ohms and got these numbers:

1.45 Ω

1.38 Ω

1.47 Ω

1.43 Ω

1.56 Ω

1.45 Ω

 

I haven't been able to find what the correct range is though so they are within spec /:

You want the service manual that Josh references so you know what to measure, and how it needs to be connected doing the measurement, AND the expected value.

[GTSBoy]

And, when measuring and getting <2 ohms, perhaps chose the next larger range than that, rather than something with a k in it!

[CF.Panda]

18 minutes ago, MBS206 said:

You want the service manual that Josh references so you know what to measure, and how it needs to be connected doing the measurement, AND the expected value.

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.

[CF.Panda]

9 minutes ago, GTSBoy said:

And, when measuring and getting <2 ohms, perhaps chose the next larger range than that, rather than something with a k in it!

I tried the next one up from 600k ohms which is 60M on my multimeter and all readings came out as 0.00 ohms so right now the most accurate is 600k ohms with only a 0.1 ohms difference between coils. Three coils read 1.4 ohms and the other three read 1.5 ohms.

[GTSBoy]

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