[DIAGNOSTIC] W205 C63S AMG -- LIN bus fault both aux water pumps not responding U02A9
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[DIAGNOSTIC] W205 C63S AMG -- LIN bus fault both aux water pumps not responding U02A9
Hi everyone,
I am currently diagnosing a W205 C63S AMG with a cooling system fault and I am convinced the root cause is a LIN bus issue. Looking for anyone who has dealt with something similar.
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**Fault codes:**
- U02A900 -- Communication with coolant pump 1 of the charge air cooler has a malfunction
- U121A87 -- LIN bus communication fault, missing message from fan
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**What happened:**
The car came in with the condenser fan not working. The car was previously working correctly with no prior intervention or recent repairs in that area. Fault developed on its own.
With Xentry I found that Pump 2 was not responding to actuator commands while Pump 1 was still operational with no fault logged. I confirmed 12V supply at the Pump 2 connector and replaced it. After replacement, Pump 2 still did not respond, and shortly after Pump 1 also dropped off communication with the same fault code.
Two pumps -- one original, one brand new -- both failing to communicate at the same time. This is not a component failure.
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**My theory:**
Both pumps share the same LIN bus segment. The U121A87 missing message suffix confirms the master is transmitting correctly but receiving no response from the slaves. There is likely a fault on the physical LIN line between the master and the pump junction -- either a short to ground, a damaged splice point, or a harness issue.
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**What I need:**
1. Wiring diagram for the low temperature coolant pump LIN circuit on the W205 C63S / M177 engine
2. Physical location of the LIN bus splice or junction point between the two pumps
3. Any known harness weak spots on this model in that circuit
4. Whether the condenser fan shares this same LIN segment or runs on a separate bus
5. Any similar cases and what the actual root cause turned out to be
Thanks in advance to anyone who can help.
I am currently diagnosing a W205 C63S AMG with a cooling system fault and I am convinced the root cause is a LIN bus issue. Looking for anyone who has dealt with something similar.
---
**Fault codes:**
- U02A900 -- Communication with coolant pump 1 of the charge air cooler has a malfunction
- U121A87 -- LIN bus communication fault, missing message from fan
---
**What happened:**
The car came in with the condenser fan not working. The car was previously working correctly with no prior intervention or recent repairs in that area. Fault developed on its own.
With Xentry I found that Pump 2 was not responding to actuator commands while Pump 1 was still operational with no fault logged. I confirmed 12V supply at the Pump 2 connector and replaced it. After replacement, Pump 2 still did not respond, and shortly after Pump 1 also dropped off communication with the same fault code.
Two pumps -- one original, one brand new -- both failing to communicate at the same time. This is not a component failure.
---
**My theory:**
Both pumps share the same LIN bus segment. The U121A87 missing message suffix confirms the master is transmitting correctly but receiving no response from the slaves. There is likely a fault on the physical LIN line between the master and the pump junction -- either a short to ground, a damaged splice point, or a harness issue.
---
**What I need:**
1. Wiring diagram for the low temperature coolant pump LIN circuit on the W205 C63S / M177 engine
2. Physical location of the LIN bus splice or junction point between the two pumps
3. Any known harness weak spots on this model in that circuit
4. Whether the condenser fan shares this same LIN segment or runs on a separate bus
5. Any similar cases and what the actual root cause turned out to be
Thanks in advance to anyone who can help.
Senior Member
Joined: Dec 2025
Posts: 422
Likes: 37
From: Keep it simple - keep it OEM
AMG C63s Coupe
Hello, can not help 1-5 - have no WIS ?!
Your Xentry does not have this ?
I was thinking its a complete System with all other MB programms.
My question is - have you replaced all of the two electric pumps ?
Last edited by C63s_Jack; Apr 29, 2026 at 01:35 AM.
