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Hi. İ own a Alfa 147 1.9 JTD 16v. The problem that İ have been getting the code PO237. This problem appears when İ drive my car more than 160kmh. After 160kmh the car goes on limp mode and the check engine light comes on. When İ stop the car and wait for like 10 mins the car resets itself and it's not on limp mode but check engine light will remain and İ have to clear the codes out. İ have changed the map sensor and İ have cleaned the turbo. What can be the cause of this problem ?
 

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P0237 is Boost pressure circuit A low signal.

Your boost pressure sensor is returning a lower value than expected.
Check all vacuum hoses from vacuum pump to turbo actuator.

Check boost/induction pressure hoses and intercooler for leaks. Liberally dousing it with a soapy solution and then running engine with short, sharp full throttle stabs to build up induction boost would allow any leaks to form bubbles which would be visible.

If all that checks out, check air filter.
Check turbo actuator rod moves all the way.
Diagnostically check maximum MAF reading on a maximum acceleration run. From memory it should be 550-600kg/hr.

If all that checks out ok, clean EGR valve to ensure it closes fully and is not loosing boost pressure.
Finally, if that is ok, get a pressure gauge to manually test boost pressure and compare with that given by ECU via boost pressure sensor. If there is a discrepancy, check out boost pressure sensor.

If none of the above sorts it, suspect worn turbo. Check for turbine shaft movement and look for casing scoring.
 

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Before replacing turbo, it may be wise to ensure exhaust is not restricted as this would slow turbine speed due to pressure build up. You may need to disconnect exhaust to see if that makes a difference.
 
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