P28C1: Ion Sense Module Supply Voltage Low

What it means

Supply voltage on the ion sense module is below the expected window. Wiring shorted to ground, blown fuse, or upstream regulator fault.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

The ion-sense module's supply powers the circuit that biases the spark plug for ion-current measurement. This code sets when the engine control module detects that the module supply is below its expected range, such as from a blown fuse or a supply wire shorted to ground.

Common causes

Most likely first.

1. Module supply fuse blown
2. Supply wire shorted to ground

Symptoms

• Check-engine light
• Reduced power
• Hard starting or stalling

How to diagnose it

1. Review freeze-frame data and related supply codes to determine when the low-supply fault occurred and whether other circuits lost power at the same time.
2. Inspect the ion-sense module supply wiring and connector for damage, corrosion, a loose terminal, or contact with ground.
3. Check the relevant fuse and its supply path; if the fuse is blown, look for a short before fitting a replacement.
4. Measure voltage at the module supply under the conditions specified by the manufacturer and compare it with the manufacturer's specification.
5. If the supply remains low, test the disconnected circuit for a short to ground and check the upstream supply path using the wiring diagram.

Common fixes

• Repair a supply wire chafed or shorted to ground.
• Replace a blown ion-sense supply fuse after correcting the cause.
• Clean or repair corroded or loose module supply connector terminals.
• Repair a faulty upstream supply connection or regulator if testing identifies it.

Can I keep driving?

The engine may start poorly, stall, or lose power if the ion-sense module cannot operate. Avoid driving if stalling or reduced power makes the vehicle difficult to control, and have repeated fuse failures diagnosed before further use.