U1794

SCP (J1850) Lack of Acknowledgment for Mirrors

Network / Communication Network/Communication SCP Bus Acknowledgment Failure 🟡 Moderate — Fix within a week ⚠️ Drive with Care
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What This Actually Means

In plain language — no jargon

Your vehicle's communication network isn't getting a confirmation signal from the mirror control module, like sending a text message that never gets a 'read receipt.' The SCP bus expects acknowledgment within a specific time window, and when it doesn't arrive, this fault is triggered.

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Symptoms You May Notice

3 known symptoms for this code
Power mirror controls unresponsive or intermittent
Mirror adjustment features not working on one or both sides
Dashboard warning light or communication fault indicator illuminated
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How Your ECU Detects This

Technical sensor logic and voltage thresholds

The body control module (BCM) transmits mirror adjustment commands over the J1850 SCP (Standard Corporate Protocol) bus and monitors for acknowledgment responses from the mirror control module within 50-100ms. If no ACK signal is received after multiple retries, the ECU logs a lack-of-acknowledgment fault.

Voltage & Parameter Thresholds

ParameterNormal RangeFault Condition
SCP Bus ACK Response Time 50-100ms after command transmission No acknowledgment received after timeout threshold
Bus Communication Attempts Successful ACK on first or second attempt Failure after 3+ consecutive retries
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Diagnostic & DIY Fix Guide

Check these in order — from cheapest to most complex
1
Battery terminal connections
Clean and tighten both positive and negative battery terminals to ensure solid ground and power distribution to all modules.
2
SCP bus wiring harness
Inspect the J1850 bus wiring between BCM and mirror modules for corrosion, loose connectors, or damage and reseat connectors firmly.
3
Mirror control module
If wiring and connections are intact, the mirror control module may need replacement as it may not be responding to bus commands.
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When to See a Professional Mechanic

Not all fault codes are safe to DIY

Code U1794 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.

Safety note: OBD-II codes identify the system or circuit where a fault was detected — they do not always identify the exact failed component. A professional mechanic using live sensor data will diagnose the root cause more accurately than replacing parts based on the code alone.
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How to Clear Code U1794

What happens after you fix the fault

Once the fault is repaired, U1794 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.

The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.

✅ Safe to Clear When
  • Fault has been diagnosed and repaired
  • You want to confirm the repair worked
  • Code appeared after a sensor was cleaned
⚠️ Do Not Clear When
  • Preparing for an emissions/PUC test
  • Root cause is still undiagnosed
  • Check engine light is flashing
Emissions test note: Clearing codes resets OBD readiness monitors. Most vehicles need 50–100 km of mixed driving before monitors complete. Do not clear codes immediately before an emissions or PUC inspection.