THE CODE DIRECTORY

Diagnostic code references — Page 18.

Browse canonical reference entries. Each definition keeps its own source and vehicle-applicability limits.

Generic OBD-II

P040D

These are code references, not a list of confirmed faults on every vehicle. Read the source and applicability notes, then match the exact vehicle, year, engine and scan report.

The EGR temperature high-circuit result is a resistance-range finding

An erratic EGR temperature signal is assessed between consecutive samples

EGR temperature comparison after a long soak

Audi's secondary-air rationality monitor uses pressure deviation

Secondary-air flow compared with a pressure-based model

Secondary-air valve circuit feedback versus command

a small current threshold identifies an open secondary-air valve circuit

Audi's air-valve fault is evaluated as a ground-short voltage

AIR System Switching Valve B Circuit

valve B's open-circuit code needs electrical path identification

the CTUA valve-B fault points to a low electrical signal

Secondary-air pump control is an electrical test

AIR System Control B Circuit

EGR Temperature Sensor B Circuit

EGR temperature fails a post-soak response check

low-range EGR temperature feedback depends on sensor type

high-range EGR temperature circuit evidence

erratic EGR temperature changes between samples

AIR valve circuit low during an off command

Catalyst oxygen storage measured through a controlled mixture change

diesel oxidation-catalyst efficiency during regeneration

This catalyst efficiency check depends on an eligible regeneration event

Heated Catalyst Efficiency Below Threshold Bank 1

Heated Catalyst Temperature Below Threshold Bank 1

Catalyst Temperature Sensor Circuit Bank 1 Sensor 1

Catalyst Temperature Sensor Circuit Range/Performance Bank 1 Sensor 1

Catalyst Temperature Sensor Circuit Low Bank 1 Sensor 1

Catalyst Temperature Sensor Circuit High Bank 1 Sensor 1

Catalyst Heater Control Circuit/Open Bank 1

Catalyst Temperature Sensor Circuit Bank 1 Sensor 2

Catalyst Temperature Sensor Circuit Range/Performance Bank 1 Sensor 2

Catalyst Temperature Sensor Circuit Low Bank 1 Sensor 2

Catalyst Temperature Sensor Circuit High Bank 1 Sensor 2

high-pressure EGR remains open after drive removal

EGR stuck closed uses actuator-position evidence rather than flow alone

bank-2 catalyst oxygen-storage assessment

bank 2 warm-up catalyst efficiency and the firmware branch

main catalyst bank 2 efficiency needs exhaust and sensor evidence

Heated Catalyst Efficiency Below Threshold Bank 2

Heated Catalyst Temperature Below Threshold Bank 2

Catalyst Temperature Sensor Circuit Bank 2 Sensor 1

Catalyst Temperature Sensor Circuit Range/Performance Bank 2 Sensor 1

Catalyst Temperature Sensor Circuit Low Bank 2 Sensor 1

Catalyst Temperature Sensor Circuit High Bank 2 Sensor 1

Catalyst Heater Control Circuit/Open Bank 2

Catalyst Temperature Sensor Circuit Bank 2 Sensor 2

Catalyst Temperature Sensor Circuit Range/Performance Bank 2 Sensor 2

Catalyst Temperature Sensor Circuit Low Bank 2 Sensor 2

Catalyst Temperature Sensor Circuit High Bank 2 Sensor 2

Excess vacuum across the EVAP pump's reference orifice

Too little reference vacuum in an EVAP leak-detection check

Altima small-leak testing requires a working vent seal

when Titan purge flow fails the pressure-response check

A small EVAP leak judged against a measured reference opening

Purge-valve open-circuit logic in a row marked disabled

Titan purge-valve circuit open and the low-voltage branch

why Titan purge-solenoid high feedback needs circuit diagnosis

Vent restriction detected while purge operates normally

Titan vent-valve electrical fault versus a valve left open

Volkswagen EVAP vent circuit short classification

The EVAP vent open-load fault can arrive as a CAN status message

EGR Sensor C Circuit

EGR Sensor C Circuit Range/Performance

low-pressure EGR position signal below electrical range

upper-range low-pressure EGR SENT feedback

EGR Sensor C Circuit Intermittent/Erratic

AIR valve voltage high when commanded on

Maxima EVAP pressure sensing includes its hose connection

Fuel-tank pressure compared with the leak-pump pressure sensor

Fuel-tank pressure voltage below the electrical lower limit

Fuel-tank pressure voltage exceeds the permitted signal range

An apparent refueling event caused by an unstable pressure signal

Weak EVAP vacuum can mean a large leak or a purge blockage

A small EVAP leak can be detected after the engine is switched off

A refueling event changes the interpretation of an EVAP gross-leak result

Purge-solenoid control output low while the driver is off

Purge-solenoid high feedback during an active low-side command

low-pressure EGR command circuit open-load evidence

low-pressure EGR position tracking outside tolerance

low-pressure EGR short-to-ground indication during drive