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You need a different mechanic. Engine computers do not "reboot". The codes don't tell you what to change. They tell you about a situation and you have to diagnose what caused it. It appears they just don't know what they are doing.
Here's a TSB on that code.
Bulletin No: 01-007/02
Last Issued: 2/25/2002
Subject:MIL ILLUMINATES WITH DTC P0402 STORED IN MEMORY
BULLETIN NOTE This bulletin supersedes 01-012/01 issued 6/28/2001. The SRT (Suggested Repair Times) have been revised.
APPLICABLE MODEL(S)/VINsAll 1999-2001 Protege (1.6L)All 1999-2000 Miata
The Malfunction Indicator Lamp (MIL) may illuminate with Diagnostic Trouble Code (DTC) P0402 stored in memory. This may be caused by carbon deposits accumulating at the Exhaust Gas Recirculation (EGR) exhaust outlet port, inside the intake manifold. This may cause pressure to increase inside the exhaust gas flow circuit, resulting in stored DTC P0402.
Customers having this concern should have their vehicle repaired using the following procedure.
Verify customer concern.
With compressed air, blow loosened carbon through EGR inlet port.
Reassemble components in reverse order.
Note: This information applies to verified customer complaints on vehicles covered under normal warranty. Refer to the SRT microfiche for warranty term information.