Computers/Controls - DTC P040 Diagnostics
DTC P0400 DiagnosisIn the event a pre-2000MY customer vehicle is presented with Diagnostic Trouble Code P0400, here are some helpful tips for proper diagnosis:
1) Does the car run rough and/or stall when opening the EGR valve manually?
Yes - OK. Go to next step.
No - Possible blockage or casting problem of the EGR port inside the intake manifold, may require replacement of the manifold.
2) Is there a good vacuum source coming through both the lines leading to the Back Pressure Transducer (BPT) from the throttle body?
Yes - OK. Go to next step.
No - Verify the vacuum source at the throttle body on both ports.
Yes - Clean or replace the metal lines and hoses connecting the throttle body & the BPT.
No - Clean both the throttle body vacuum ports or replace the assembly.
3) With the car running, remove the BPT and confirm a good exhaust pulse from the metal line leading to the underside of it.
Yes - OK. Go to next step.
No - Clean out the FGR pipes leading from the head to the intake and from the intake to the BPT then, re-check for the exhaust pulse.
4) With the BPT removed, check to see if liquid comes out of the hose connections on both the top and the boffom of it.
Yes - The diaphragm in the BPT has been perforated and needs to be replaced.
No - OK. Go to next step.
5) With the key on, engine of{ connect the green test mode connectors. Does the FGR solenoid cycle off and on?
No - Replace the solenoid (after confirming wiring to the solenoid is OK).
Yes - Shut the engine off and using a hand vacuum pump, confirm the solenoid holds vacuum when "off" and that vacuum is lost when the solenoid is cycled "on". Now would be a good time to vacuum-check the FGR valve diaphragm.
Yes - OK. Go to next step.
No - Replace FGR solenoid and/or FGR valve assembly.
6) With the green test mode connectors still connected, does the Pressure Sources Switching Solenoid cycle off and on?
No - Replace the solenoid (after confirming wiring to the solenoid is OK).
Yes - Confirm the valve switches from engine vacuum to atmosphere on the MAP sensor line when activated.
Yes - OK. Go to next step.
No - Replace the Pressure Sources Switching Solenoid.
7) Inspect the vacuum lines leading to the MAP sensor and the Pressure Sources Switching Solenoid for restriction or "pinching" from a cable tie or clamp.
Yes - Repair the restriction as necessary.
No - OK. Go to next step.
8) Inspect the Pressure Sources Switching Solenoid, MAP sensor, and all related hoses for any contamination from Charcoal Canister break down. Remove the component and tap the vacuum hose connection of the part on a white paper towel to see if any debris falls out that may have been preventing the valve from sealing properly when closed.
Yes - Clear memory and road test to see if code resets.
No - Continue inspection of other related components, lines, etc. for faults.