Reset the codes and now PO421 has come back. What is causing this?

RACHELLE

MEMBER
2002 MITSUBISHI LANCER
146,500 MILES

We took the car in for emissions and the cat and sensors were bad. We replaced them with Exact OEM direct fit parts and did two recalls on the car( PCM reflash and fan controller) and replaced the spark-plugs, PCV valve, pulley belts, valve cover gasket, air filter, oil-change, and fuel-injection flush service and a fuel injection hose we found was broken. I replaced the front and rear O2 sensors and the catalytic converter. We reset the codes, and drove the car approximately 81 miles and the service engine light came on again. Took it to autozone and came out with the PO421 code again. They said we either didnt get the converter/rear sensor on tight enough or its an exhaust leak. The service engine light came on aprx. 10 min after going through the car wash. If the converter/sensor wasnt tight enough is that going to cause the problem? Could water get into the converter? Or is it a exhaustleak somewhere? What do you recommend now? Also when I turned into autozone, the car stalled, but started right back up. Thanks

1 Reply

KHLOW2008

MECHANIC
41,814 POSTS

DTC P0421 : WARM UP CATALYST EFFICIENCY BELOW THRESHOLD

Description

The signal from the rear heated oxygen sensor differs from the front heated oxygen sensor. That is because the catalytic converter purifies the exhaust gas. When the catalytic convener has deteriorated, the signal from the front heated oxygen sensor becomes similar to the rear heated oxygen sensor. The ECM (M/T) or PCM (A/T) compares the output of the front and rear heated oxygen sensor signals.

DTC Set Conditions

Check conditions are when engine speed is less than 3000 RPM, volume airflow sensor output frequency is 63-169 Hz, intake air temperature is more than 14°F (-10°C) barometric pressure is more than 11 psi (76 kPa), the throttle valve is open, vehicle is under the closed loop air/fuel ratio control and vehicle speed is 0.93 MPH (1.5 km/h) or more. Monitoring time is 10 seconds by 7 times, short-term fuel trim is more than -25 percent and less than 25 percent, and the cumulative volume airflow sensor output frequency is more than 4000 Hz. Judgment criteria is when the heated oxygen
sensor (rear) signal frequency plus heated oxygen sensor (front) signal frequency equals 0.8 or more.

Troubleshooting Hints

The most likely causes for this DTC to be set are:

Catalytic converter deteriorated.

Heated oxygen sensor failed.

ECM (M/T) or PCM (A/T) failed.

Diagnostic Procedure
CAUTION:
To prevent damage to scan tool, always turn ignition switch to LOCK position before connecting or disconnecting scan tool.

1. Check for exhaust leaks. Repair as necessary. After repairs, go to step 7. If no leaks are found, go to next step.

2. Connect scan tool to data link connector. Turn ignition on. Set scan tool to check data list item 59: HEATED OXYGEN SENSOR (REAR). Start and operate engine at idle. Warm engine to normal operating temperature. Observe scan tool. When engine is revved up, the output voltage should repeat zero volt and 0.6-1.0 volt alternately. Turn ignition switch to LOCK position. If sensor is operating properly, go to next step. If sensor is not operating properly, refer to DTC P0137: HEATED OXYGEN SENSOR CIRCUIT LOW VOLTAGE (SENSOR 2), DTC P0138: HEATED OXYGEN SENSOR CIRCUIT HIGH
VOLTAGE (SENSOR 2) and DTC P0139: HEATED OXYGEN SENSOR CIRCUIT SLOW RESPONSE (SENSOR 2).

3. Start and operate engine at idle. Set scan tool to check data list item 11: HEATED OXYGEN SENSOR (FRONT). Observe scan tool. When engine is revved, the output voltage should be 0.6-1.0 volt. Turn ignition switch to LOCK position. If sensor is operating properly, go to next step. If sensor is not operating properly, refer to
DTC P0132: HEATED OXYGEN SENSOR CIRCUIT HIGH VOLTAGE (SENSOR 1),
DTC P0133: HEATED OXYGEN SENSOR CIRCUIT SLOW RESPONSE (SENSOR 1) and DTC P0134: HEATED OXYGEN SENSOR CIRCUIT NO ACTIVITY DETECTED(SENSOR 1).

4. Start and operate engine at 2000 RPM. Observe scan tool. Output voltage should be 0.0-0.4 and 0.6-1.0 volt and alternating 15 times or more within 10 seconds. Turn ignition switch to LOCK position. If sensor is operating properly, go to next step. If sensor is not operating properly, replace heated oxygen sensor (front). After replacing sensor, go to step 7.

5. Test the OBD-II drive cycle. Carry out test drive with the drive cycle pattern. See CATALYTIC CONVERTER MONITOR and OTHER MONITOR under VERIFYING REPAIRS. Check for DTCs. If DTC P0421 is output, replace catalytic converter. After replacing catalytic converter, go to step 7. If DTC P0421 is not output, inspection is complete.

6. Test the OBD-II drive cycle. Carry out test drive with the drive cycle pattern. See CATALYTIC CONVERTER MONITOR and OTHER MONITOR under VERIFYING REPAIRS. Check for DTCs. If DTC P0421 is output, replace ECM or PCM. After replacing ECM or PCM, go to next step. If DTC P0421 is not output, inspection is complete.

7. Test the OBD-II drive cycle. Carry out a test drive with the drive cycle pattern. See OTHER MONITOR under VERIFYING REPAIRS. Check for DTCs. If DTC P0421 is output, repeat diagnosis. If DTC
P0421 is not output, inspection is complete.