Vehicle: Ford Focus Lacks Consistent Turbo Boost

Mileage: 109,984 Miles

Problem: This little hot rod Focus was dropped off at the shop because the malfunction indicator light (MIL) was on and it had some turbo issues.

Case Details: While driving and shifting from first gear to second gear, excess turbo pressure would blow off once then the turbo boost would stop.

The technician pulled a couple of diagnostic trouble codes (DTCs) with his scan tool: P0034 - Turbocharger/Supercharger Bypass Valve A Control Circuit Low. This DTC sets when the PCM detects a short to ground in the TCBY circuit.

P0035 - Turbocharger/Supercharger Bypass Valve A Control Circuit High. This DTC sets when the PCM detects an open circuit or high voltage in the TCBY circuit.

Based only on the two DTCs and observing no wiring issues, the technician replaced the turbo boost valve solenoid and cleared the DTCs. During the test drive, the vehicle would build boost on the first three shifts, then the MIL came on and the same two DTCs returned. The technician noticed that sometimes the DTCs would not clear on the first try; other times they would. It was time to call an ALLDATA Tech-Assist consultant.

The consultant suggested checking for B+ on the grey wire, pin2 of the bypass valve with the key on, with the engine off to confirm it had battery voltage. He noted that the violet/green wire was the ground control, though it should only have ground when the bypass valve is commanded on.

Also, with the battery disconnected, the consultant suggested checking for continuity between pins 1 and 2 of the bypass valve connector. There should be no continuity, an infinite reading.

Confirmed Repair: While testing, the technician found no battery voltage on the grey wire to the turbo charger bypass valve. He ran a new wire from the PCM to the turbo charger bypass connector. On the test drive, the turbo consistently boosted as designed. Interesting fix!