U0901: Invalid Data from Ion Sense Module

What it means

A receiving module detected implausible data from the ion sense module (used for misfire/knock detection via ionisation current at the spark plug). Per-cylinder ion current values fail plausibility checks and are rejected.

At a glance

• Usually lights the warning light of the system that lost communication
• Affects emissions; the car can fail an emissions test

How it works

The ion sense module evaluates electrical current at the spark plugs to help detect combustion problems such as misfire or knock. U0901 sets when another module receives per-cylinder ion-current data that fails its plausibility checks, so those readings are rejected and combustion monitoring may be less reliable.

Common causes

Most likely first.

1. Module internal fault or sensor input drift
2. Software version mismatch after replacement
3. Gateway module routing CAN messages incorrectly

Symptoms

• Multiple warning lights may illuminate
• Affected modules may go into limp mode or shut down
• Specific functions inoperative depending on module
• Diagnostic tools may show communication faults

How to diagnose it

1. Scan all modules for ion sense, misfire, knock, and communication codes, and review freeze-frame data for the conditions when the invalid data was recorded.
2. Check scan-tool data for per-cylinder ion-current values and note whether one cylinder or all reported values are implausible or missing.
3. Inspect the ion sense module connectors, grounds, and wiring, including any wiring disturbed during recent ignition or module work.
4. Check for a software or configuration mismatch if the ion sense module or a related module was recently replaced, using the manufacturer's service information.
5. Test the module's inputs, power, ground, and network messages according to the manufacturer's procedure before deciding the ion sense module or gateway is faulty.

Common fixes

• Reconnect or repair damaged ion sense module wiring and ground connections.
• Correct module software or configuration incompatibility after replacement using the manufacturer's approved procedure.
• Repair a gateway or network wiring fault if it is routing the ion sense messages incorrectly.
• Replace the ion sense module if its inputs, power, ground, and network are correct but it continues to send implausible per-cylinder data.

Can I keep driving?

If the engine runs smoothly and no warning indicates active misfire, drive gently to arrange diagnosis; avoid heavy acceleration because combustion monitoring may not reliably identify knock or misfire. Stop driving if the engine shakes, loses power, or the check-engine light flashes.