Allready retrived P0138 code, replaced with Toyota rear O2 sensor, day later CEL back on, flushed out badly gummed up EGR value and connection lines. Day later CEL comes back on. Exhaust sounds rough. Has only 52,000 km. Giving ~ 13 mpg = 18.1 liters per km Been to garage 3 times, diagnosing symtoms not cause ?