Exhaust, turbo and emissions fault codes

Draft guidance: awaiting Claude spot-check, qualified-mechanic review and named-editor approval. Do not use a fault code alone to replace parts.

Catalytic converter, EGR and turbo codes. They can mean an MOT emissions fail, and a turbo fault can put the car into limp mode. Most start with a cheap check before anything expensive.

• P0234: Turbocharger or supercharger overboost condition

  The engine control unit has measured, or believes it has measured, more boost pressure than it commanded.

• P0299: Turbocharger or supercharger underboost condition

  The engine control unit has detected less boost pressure than it requested under the relevant operating conditions.

• P0400: Exhaust gas recirculation flow malfunction

  The engine computer has detected that exhaust gas recirculation (EGR) flow is not behaving as expected.

• P0401: Exhaust gas recirculation flow insufficient detected

  The control module commanded EGR operation but measured or inferred less flow than expected.

• P0402: Exhaust gas recirculation flow excessive detected

  The engine computer has measured or inferred more EGR flow than it commanded.

• P0420: Catalyst system efficiency below threshold, Bank 1

  The control module believes the Bank 1 catalytic converter is storing and treating oxygen less effectively than its calibrated threshold.

• P0430: Catalyst system efficiency below threshold, Bank 2

  The control module believes the catalytic converter on Bank 2 is working below its calibrated efficiency threshold.