Aston Martin diagnostic trouble code
P0420: Catalyst System Efï¬ciency Below Threshold (Bank 1)
Use this reference to understand the recorded condition, confirm the available V8 Vantage or DB9 manual coverage, and follow the documented diagnostic checks before replacing parts.
- Code family
- Powertrain
- Vehicle system
- Emissions and EVAP
- Manual coverage
- V8 Vantage, DB9
- Warning-lamp status
- V8 Vantage: MIL; DB9: MIL
What Aston Martin code P0420 means
Catalyst System Efï¬ciency Below Threshold (Bank 1)
DB9 preliminary diagnostic manual · section 2-86
Manufacturer fault-analysis procedure
Manual fault definition
P0420 - Indicates that cylinder 1-3 catalyst system
efficiency is below the acceptable threshold.
efficiency is below the acceptable threshold.
Cautions and notes
CAUTION: This code may be generated in the
Primary or Secondary Powertrain Control Module
(PCM). Check that you are working on the correct
half of the engine.
NOTE: Fuel pressure outside specification can cause
incorrect fuel air mixture leading to higher than normal
catalyst temperatures.
Run the engine at idle. Record the fuel pressure.
Increase engine speed to 2500 rpm and maintain
for one minute. Record the fuel pressure.
The nominal fuel pressure specification is 40 psi
above manifold pressure.
If fuel pressure is in specification, go on to step 7.
If fuel pressure is out of specification, service as
necessary. Reset the PCM and run a KOER test to
ensure that the problem is resolved.
NOTE: The vacuum gauge reading may be normal when
the engine is first started and idled. However, excessive
restriction in the exhaust system will cause the vacuum
to diminish even when the engine is idling.
Note the initial gauge reading and any change in
the reading during the initial period at idle speed.
Steadily increase engine speed to 2000 rpm, then
allow it to slowly decrease back to idle speed.
The manifold vacuum must rise to at least 16
inches Hg with the engine at 2000 rpm.
If a vacuum of less than 16 inches Hg at 2000
rpm is observed, remove the exhaust system and
check for restrictions or blockage.
If the manifold vacuum met the basic specification, the P0420/P0430 codes are caused by poor
catalyst performance, change the catalytic covertors and run the drive cycle to ensure that the
problem is resolved.
Primary or Secondary Powertrain Control Module
(PCM). Check that you are working on the correct
half of the engine.
NOTE: Fuel pressure outside specification can cause
incorrect fuel air mixture leading to higher than normal
catalyst temperatures.
Run the engine at idle. Record the fuel pressure.
Increase engine speed to 2500 rpm and maintain
for one minute. Record the fuel pressure.
The nominal fuel pressure specification is 40 psi
above manifold pressure.
If fuel pressure is in specification, go on to step 7.
If fuel pressure is out of specification, service as
necessary. Reset the PCM and run a KOER test to
ensure that the problem is resolved.
NOTE: The vacuum gauge reading may be normal when
the engine is first started and idled. However, excessive
restriction in the exhaust system will cause the vacuum
to diminish even when the engine is idling.
Note the initial gauge reading and any change in
the reading during the initial period at idle speed.
Steadily increase engine speed to 2000 rpm, then
allow it to slowly decrease back to idle speed.
The manifold vacuum must rise to at least 16
inches Hg with the engine at 2000 rpm.
If a vacuum of less than 16 inches Hg at 2000
rpm is observed, remove the exhaust system and
check for restrictions or blockage.
If the manifold vacuum met the basic specification, the P0420/P0430 codes are caused by poor
catalyst performance, change the catalytic covertors and run the drive cycle to ensure that the
problem is resolved.
Test sequence and decision path
1. Connect the WDS and note all logged DTCs. If
any HO2S DTCs are recorded, analyse these
problems, then return to this procedure.
2. If any ECT sensor DTCs are recorded P0116,
P0117, P0118, P0119, or P1116, analyse these
problems, then return to this procedure.
3. If any other DTCs are recorded (not including the
initial P0420) analyse these codes, then return to
this procedure.
4. Check the wiring of each catalyst monitor oxygen
sensor. If any wiring error is found, correct it, then
reset the PCM memory and rerun the KOER Test
to check that the problem is resolved. If no wiring
errors are found, go to step 6.
5. Whilst running the engine, use datalogger to
compare the upstream and downstream oxygen
sensor signals for each engine bank.
If the signals are significantly different, indicating
a correctly working catalyst, go to step 6.
If the upstream and downstream signals on either
bank are identical, the catalyst on that bank is
defective and must be replaced. Investigate for
possible causes before running the engine on the
new catalyst.
6. Using datalogger, monitor the fuel pressure in
both fuel rails.
7. Check for leaks/damage in the exhaust system.
Examine all flanges for security and leaks. Examine all components for splits or corrosion. Examine supports for security to avoid any flexing
under road shock. Check the complete system for
damage which could cause restrictions.
If any defects are identified, repair or replace
components as necessary and retest the vehicle to
check for complete resolution of the problem. If
no defects are found, go to step 8.
8. Check the inlet manifold vacuum:
Attach a vacuum gauge to the inlet manifold.
Start the engine and allow it to run at idle speed
whilst observing the vacuum gauge.
Open the corresponding manual page for wiring diagrams and figures
any HO2S DTCs are recorded, analyse these
problems, then return to this procedure.
2. If any ECT sensor DTCs are recorded P0116,
P0117, P0118, P0119, or P1116, analyse these
problems, then return to this procedure.
3. If any other DTCs are recorded (not including the
initial P0420) analyse these codes, then return to
this procedure.
4. Check the wiring of each catalyst monitor oxygen
sensor. If any wiring error is found, correct it, then
reset the PCM memory and rerun the KOER Test
to check that the problem is resolved. If no wiring
errors are found, go to step 6.
5. Whilst running the engine, use datalogger to
compare the upstream and downstream oxygen
sensor signals for each engine bank.
If the signals are significantly different, indicating
a correctly working catalyst, go to step 6.
If the upstream and downstream signals on either
bank are identical, the catalyst on that bank is
defective and must be replaced. Investigate for
possible causes before running the engine on the
new catalyst.
6. Using datalogger, monitor the fuel pressure in
both fuel rails.
7. Check for leaks/damage in the exhaust system.
Examine all flanges for security and leaks. Examine all components for splits or corrosion. Examine supports for security to avoid any flexing
under road shock. Check the complete system for
damage which could cause restrictions.
If any defects are identified, repair or replace
components as necessary and retest the vehicle to
check for complete resolution of the problem. If
no defects are found, go to step 8.
8. Check the inlet manifold vacuum:
Attach a vacuum gauge to the inlet manifold.
Start the engine and allow it to run at idle speed
whilst observing the vacuum gauge.
Supporting owner context
What owners have reported
Owner discussions include catalyst-efficiency codes after aftermarket catalyst changes. Check the exhaust, sensors and calibration context; a forum workaround should not be treated as a repair.
This is supporting owner-reported context, not a replacement for the manufacturer procedure or a diagnosis of every vehicle.
Read the related owner discussion