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Reference

Fault codes (PCODEs) explained

A glossary of common diagnostic trouble codes that owners see around tuning, emissions and gearbox work. Each code is a starting point for diagnosis, not a verdict.

Boost

  • P0234Turbocharger overboost condition. Boost pressure went above the safe limit.
  • P0236Turbocharger boost sensor A range or performance. The boost sensor reading does not match the expected value.
  • P0237Turbocharger boost sensor A circuit low. The boost sensor signal is reading below its normal range, often a wiring or sensor fault.
  • P0299Turbocharger underboost condition. Boost pressure stayed below the target, so the ECU logs an underboost fault.
  • P2263Turbocharger boost system performance. Boost is not following the commanded target closely enough.

Airflow

  • P0101Mass air flow sensor range or performance. The airflow reading is outside the range the ECU expects for the operating point.

Fuel system

  • P0087Fuel rail or system pressure too low. Rail pressure fell below the value the ECU asked for.
  • P0088Fuel rail or system pressure too high. Rail pressure went above the commanded limit.

EGR

  • P0401Exhaust gas recirculation flow insufficient. The ECU sees less EGR flow than it expects.
  • P0402Exhaust gas recirculation flow excessive. The ECU sees more EGR flow than it expects.
  • P0403Exhaust gas recirculation control circuit. The EGR valve control circuit reports a fault.

DPF

  • P2002Diesel particulate filter efficiency below threshold (Bank 1). The ECU estimates the DPF is not trapping soot as well as expected.
  • P2452Diesel particulate filter pressure sensor circuit. The DPF differential pressure sensor reports a fault.
  • P2459Diesel particulate filter regeneration frequency. The DPF is regenerating more often than the ECU expects, which usually points to high soot loading.
  • P2463Diesel particulate filter soot accumulation. The ECU estimates a high soot load in the DPF.

SCR / AdBlue

  • P2201NOx sensor circuit (Bank 1). The NOx sensor signal is outside the expected range.
  • P20EESCR NOx catalyst efficiency below threshold (Bank 1). The AdBlue (SCR) catalyst is not reducing NOx as well as expected.

Catalyst

  • P0420Catalyst efficiency below threshold (Bank 1). The catalytic converter is not converting exhaust gases as well as the ECU expects.

Gearbox

  • P0700Transmission control system malfunction. The transmission control system has logged a fault. Read the stored codes in the TCU for detail.
  • P0715Input or turbine speed sensor circuit. The TCU does not see a valid speed signal from the input or turbine sensor.
  • P0730Incorrect gear ratio. The gear ratio measured from the speed sensors does not match the gear selected.
  • P0740Torque converter clutch circuit malfunction. The TCU reports a fault in the torque converter clutch circuit.

Checksum

  • P0601Internal control module memory checksum error. The ECU found a checksum error in its own calibration memory. This is common after a file is flashed without a valid checksum.
  • P0606Control module processor fault. The ECU reports an internal processor fault.

How to use this list

Read the codes stored in the ECU or TCU before you change anything, and record them so you can compare afterwards. A code describes what the ECU saw, so the cause can be a sensor, wiring, a leak, or the calibration itself.

See the guides for DPF off, EGR off and DTC off, or the FAQ.