Code
P1851
SEAT
P — Powertrain
Data bus powertrain: missing message from ABS control module
AI status
Completed
Completed
100%
Causes
- ABS control module not powered (fuse, relay, wiring, connector fault)
- Physical damage, corrosion or poor connection at ABS module connector
- Open/short/crossed wires on CAN high/low pair between ABS and bus
- CAN bus termination fault (missing or incorrect resistance)
- ABS module software fault or internal failure
- Module asleep or not woken by gateway/IGNITION/wake-up message
Symptoms
- MIL (check engine) or ABS/ESP warning lamp illuminated
- Loss of ABS, traction control or stability assist functions
- Cruise control may be disabled
- Possible drivability warnings if powertrain depends on ABS messages
- Intermittent or permanent communication errors reported by scanner
What to check
- Read all stored and pending DTCs and freeze frame data from all modules
- Confirm ABS module appears on the scanner/vehicle network (module list/message traffic)
- Check battery voltage and charging system; test with key on/engine off and during cranking
- Inspect fuses and relays related to ABS and vehicle CAN power circuits
- Visually inspect ABS connector and wiring for corrosion, bent pins or damage
- Measure CAN bus voltage levels and observe activity with a scope or CAN bus monitor
Signal parameters
- CAN high/low differential signaling (dominant and recessive states)
- Typical high-speed CAN nominal mid-line ~2.5 V per line, dominant pulls one line toward ~1.5 V and the other toward ~3.5 V (differential ~2 V)
- Expected bus activity when ignition on and when ABS sends wheel/vehicle speed messages
- Termination resistance ~60 ohms measured across CAN_H and CAN_L (two 120Ω terminators in parallel)
Diagnostic algorithm
- Capture and record all communication and ABS-related codes from all modules. Note whether code is permanent, stored or intermittent.
- Attempt to communicate with ABS module using a good OEM-level scan tool. Check if ABS module responds, provides live data or reports internal errors.
- Check vehicle battery voltage and charging system. Replace or charge battery if voltage is low; retest code.
- Inspect ABS module power and ground circuits: verify fuses, relays and pin voltages at ABS connector with ignition ON. Repair any missing supply or ground.
- Inspect ABS connector and harness for corrosion, damaged wires or pushed-out pins. Repair or replace damaged connector or wiring.
- With ignition ON, use oscilloscope or CAN bus tester to monitor CAN_H and CAN_L at ABS connector and at gateway/ECU. Look for correct differential signalling and message activity.
- Measure resistance between CAN_H and CAN_L with ignition OFF; expected ~60 ohms. If out of range, trace wiring for missing terminator or short.
- If a short to battery or ground is suspected, disconnect downstream modules one at a time (starting with ABS) to isolate the faulted segment and re-check bus behavior.
- If ABS module has no power or is bus-off and wiring/power verified good, try cycling power, clear codes and attempt a module wake-up; if it remains unresponsive, consider replacing or reprogramming ABS module.
- After repair, clear DTCs, perform relearns or coding if required, then retest by driving and verifying no recurring communications faults.
Likely causes
- Faulty or loose ABS module connector or power supply (fuse/relay)
- Broken/shorted CAN_H or CAN_L wiring at harness or pin
- ABS module has gone bus-off or failed to respond
- Missing termination resistor or incorrect bus resistance
Fault status
Status
Missing or lost message from ABS control module on the powertrain CAN/data bus — communication fault (intermittent or permanent).
Repair difficulty: Medium
Diagnostic time: 1.0 - 3.0 hours
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