The ECM continuously monitors its internal processors (CPUs), heated oxygen sensor (HO2S) transistors. This self-check ensures that the ECM is functioning properly. This is diagnosed by internal "mirroring" of the main and sub CPUs to detect processor errors. If outputs from the processors deviate from the standards, the ECM will illuminate the MIL and set a DTC immediately.

DTC DETECTION CONDITION CHART

DTC No.	DTC Detection Condition	Trouble Area
P0606	ECM CPUs malfunction HO2S transistors malfunction	Exhaust gas leak HO2S ECM

HINT:

The MIL does not illuminate when a malfunction is detected in the CPU peripheral circuit of the ECM. It illuminate when a malfunction is detected in the ECM internal circuit: accelerator pedal position, brake switch, cruise control, heated oxygen sensor and throttle position.

NOTE: First check for an exhaust gas leak around the HO2S if P0606 is present. An exhaust gas leak generates noise in the HO2S output. The ECM may interpret this as an HO2S transistor malfunction and set P0606.

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.

• Brakes feel soft, pull hard to one side, or make grinding noises.

• The engine overheats, stalls repeatedly, or misfires under load.

• You are missing required tools, torque specs, or safe lifting equipment.

• You are not confident in the next step or safety outcome.

Safety Warning

DTC P0606: ECM / PCM Processor: Description

Use the Manual With the Right Hardware

When to See a Mechanic

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