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Doug C.
Doug C., ASE Certified Technician
Category: Mitsubishi
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Mitsubishi Lancer Im getting this code (P0421) after changing

Customer Question

Im getting this code (P0421) after changing my o2 sensor. When i clear it, it just comes back on! I need to get this thing inspected. What do I do?
Submitted: 3 years ago.
Category: Mitsubishi
Expert:  Jerry replied 3 years ago.

Hello and welcome to Just answer.com, Super mechanic here. Please DON’T hit accept until you are completely satisfied with the answer. About your vehicle.

That could be a number of things. There was a recall on some of them though. You may want to call the dealer and see if it is covered.

Recall - O2 Sensor Harness Corrosion

No: EMR-03-001

DATE: July, 2003

MODEL:
2003 Lancer Evolution SUBJECT:
OXYGEN SENSOR HARNESS CORROSION - EMISSIONS RECALL


PURPOSE

Mitsubishi Motors North America, Inc. (MMNA) is recalling certain 2003 Lancer Evolution vehicles for possible electrical corrosion in the rear oxygen sensor harness. This condition could cause the sensor to malfunction, set a diagnostic trouble code and illuminate the MIL. The Environmental Protection Agency (EPA) and the California Air Resources Board (CARB) have been notified about this recall.

A sample copy of the customer notification letter is shown. (Customers in California may receive a slightly modified version.)

BACKGROUND INFORMATION

 

 

Symptom/Condition

Water may enter the floorboard from body seam seal openings at the side sills and/or bulkhead. This water may cause electrical corrosion to occur in the heated rear oxygen sensor harness.

 

 

Remedy

Inspect for water leakage at the side sills and bulkhead, and apply sealant if needed. Also inspect the heated rear oxygen sensor connector for signs of electrical corrosion. If necessary, install a subharness and replace the sensor.

 

AFFECTED VEHICLES
2003 Lancer Evolution vehicles produced between 8/20/02 - 4/11/03

P0421

DTC P0421: Warm Up Catalyst Efficiency Below Threshold

TECHNICAL DESCRIPTION

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

DTC SET CONDITIONS

Check Conditions

Judgment Criteria

  • The heated oxygen sensor (rear) signal frequency divided by heated oxygen sensor (front) signal frequency = 0.75 or 0.8 or more.

TROUBLESHOOTING HINTS (The most likely causes for this code to be set are:)

DIAGNOSIS

Required Special Tool:

  • MB991502: Scan Tool (MUT-II)

STEP 1. Check for exhaust leaks.

Q: Are there any abnormalities?

YES: Go to Step 2.

NO: Repair it. Then go to Step 7.

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STEP 2. Using scan tool MB991502, check data list item 59: Heated Oxygen Sensor (rear).

CAUTION: To prevent damage to scan tool MB991502, always turn the
ignition switch to the "LOCK" (OFF) position before connecting or disconnecting scan tool MB991502.

  1. Connect scan tool MB991502 to the data link connector.
  2. Start the engine and run at idle.
  3. Set scan tool MB991502 to the data reading mode for item 59, Heated Oxygen Sensor (rear).
    • Warm up the engine. When the engine is revved, the output voltage should repeat 0 volt and 0.6 to 1.0 volt alternately.
  1. Turn the ignition switch to the "LOCK" (OFF) position.

Q: Is the sensor operating properly?

YES: Go to Step 3.

NO: Refer to, DTC P0136 - Heated Oxygen Sensor Circuit (sensor 2), DTC P0137- Heated Oxygen Sensor Circuit Low Voltage (sensor 2), DTC P0138 - Heated Oxygen Sensor Circuit High Voltage (sensor 2), DTC P0139- Heated Oxygen Sensor Circuit Slow Response (sensor 2).

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STEP 3. Using scan tool MB991502, check data list item 11: Heated Oxygen Sensor (front).

  1. Start the engine and run at idle.
  2. Set scan tool MB991502 to the data reading mode for item 11, Heated Oxygen Sensor (front).
    • Warm up the engine. When the engine is revved, the output voltage should measure 0.6 to 1.0 volt .
  1. Turn the ignition switch to the "LOCK" (OFF) position.

Q: Is the sensor operating properly?

YES: Go to Step 4.

NO: Refer to, DTC P0130 - Heated Oxygen Sensor Circuit (sensor 1), DTC P0131 - Heated Oxygen Sensor Low Voltage (sensor 1), DTC P0132 - Heated Oxygen Sensor Circuit High Voltage (sensor 1), DTC P0133 - Heated Oxygen Sensor Circuit Slow Response (sensor 1), DTC P0134 - Heated Oxygen Sensor Circuit No Activity Detected (sensor 1).

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STEP 4. Using scan tool MB991502, check data list item 11: Heated Oxygen Sensor (front).

  1. Start the engine and run at idle.
  2. Set scan tool MB991502 to the data reading mode for item 11, Heated Oxygen Sensor (front).
  3. Keep the engine speed at 2,000 r/min .
    • 0 - 0.4 and 0.6 - 1.0 volt should alternate 15 times or more within 10 seconds .
  1. Turn the ignition switch to the "LOCK" (OFF) position.

Q: Is the sensor operating properly?

YES: Go to Step 5.

NO: Replace the heated oxygen sensor (front). Then go to Step 7.

STEP 5. Perform the OBD-II drive cycle.

  1. Carry out a test drive with the drive cycle pattern. Refer to Trouble Code Diagnosis- OBD-II Drive Cycle -Procedure 3 - Catalytic Converter Monitor.
  2. Check the diagnostic trouble code (DTC).

Q: Is DTC P0421 set?

YES: Replace the catalytic converter. Then go to Step 6.

NO: The procedure is complete.

STEP 6. Perform the OBD-II drive cycle.

  1. Carry out a test drive with the drive cycle pattern. Refer to Trouble Code Diagnosis- OBD-II Drive Cycle -Procedure 3 - Catalytic Converter Monitor and Procedure 6 - Other Monitor.
  2. Check the diagnostic trouble code (DTC).

Q: Is DTC P0421 set?

YES: Replace the ECM or PCM. Then go to Step 7.

NO: The procedure is complete.

STEP 7. Perform the OBD-II drive cycle.

  1. Carry out a test drive with the drive cycle pattern. Refer to Trouble Code Diagnosis- OBD-II Drive Cycle - Procedure 3 - Catalytic Converter Monitor.
  2. Check the diagnostic trouble code (DTC).
Q: Is DTC P0421 set?

YES: Repeat the troubleshooting.

NO: The procedure is complete.

Customer: replied 3 years ago.
why does the code say pending?
Expert:  Jerry replied 3 years ago.
You may have to do a drive cycle on it one or two times to get it to go through the test so it can pass.


Readiness Test Status

READINESS TEST STATUS

PURPOSE
The Readiness function also referred as I/M Readiness or I/M Flags indicate if a full diagnostic check has been "Completed" (is "Ready") for each non-continuous monitor. Enhanced I/M State Emission Programs will use the Readiness status (Codes) to see if the vehicle is ready for OBD-II testing. "Incomplete" (Not Ready) codes will be one of the triggers for I/M failure.

OVERVIEW
The engine control module (ECM) or power-train control module (PCM) monitors the following main diagnosis items and records whether the evaluation was completed or is incomplete. The Readiness codes were established for the I/M programs, thereby confirming that the vehicle was not tampered with by erasing the diagnostic trouble code(s) (DTC's) before I/M testing. The Readiness and DTC codes can be reset by disconnecting the battery or by erasing the codes with a scan tool. For this reason all Readiness codes must read "Complete" before I/M testing.

When the monitors run and complete, the MUT-II will record the Readiness Code as "Complete" (General Scan Tools record as "Ready"). If the monitor did not run completely, the system then reads as "Incomplete" (General Scan Tools record as "Not Ready"). When the vehicle is operating normally and the OBD-II Drive Cycle is carried out, Readiness Code will set as "Complete" on the first drive cycle. If during the first drive cycle a fault is detected then, a second drive is required before the Readiness Code will "Complete." If the fault is still there, then a DTC will set.

After the Readiness is "Complete," the technician is assured that any DTC's associated with that monitor will be displayed if the system has a problem. That is why some State's I/M programs require the Readiness Code as "Complete" before they check for DTC's.

NOTE: After a repair is mode fora DTC the technician should drive the OBD-II drive cycle checking that the MUT-II records all Readiness as "Complete".


OBD-II Drive Cycle Pattern Lists
The contents of this article reflect the changes called out by TSB 04-13-12

CAUTION:
Two technicians should always be in the vehicle when carrying out a test.

NOTE:
Check that the diagnosis trouble code (DTC) is not output before driving the OBD-II drive cycle. Erase the DTC if it has been output.



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Customer: replied 3 years ago.
The first code that I got was P0135 (o2 sensor up-stream) so I changed it. Took it for a long ride and now the engine light goes back on and im reading a P0421 (the second sensor is reading back catalyst. Seeing the fact that i never had that P0421 with the orig. bad sensor installed and im reading that code after i install a new sensor. Do you think the new sensor is bad? I reinstalled the old sensor and I only read p0135 .
Expert:  Jerry replied 3 years ago.

I never had that type of problem with one, but wait, there may be someone who has. Let me open it back up. Thank you for your patience, no need to reply as that will lock the question back to me, someone should be along shortly, I'm sure.

Expert:  Doug C. replied 3 years ago.
Hi,

You need to exchange the new sensor you received for a different one, or if it is aftermarket replace it with a genuine Mitsubishi sensor.

The initial P0135 is just a simple blown out heater circuit in the sensor; very common problem and you did right by replacing the top sensor. The problem here is in the results you are getting from the new sensor skewing the catalyst monitor, throwing the P0421 catalyst efficiency code.

To understand what is going on it helps to know how the catalyst efficiency is monitored and determined good or bad.

While the engine is running, the front oxygen sensor is constantly fluctuating between roughly 0-200mV and 600-800mV about twice per second. This is indicating the oxygen content of your exhaust as it is adjusted on th e fly to maintain a stoich fuel mixture. The secondary usage of this reading however is a baseline reading for the oxygen content before the catalyst.
After the exhaust runs through the manifold and pre-cats, it is read again by the secondary oxygen sensor. Under a normal idling condition, this sensor would read about 600-800mV at all times idling, no fluctuation. This would be considered a functioning catalyst. When the two sensors are compared to each other, it is not only looking for that target voltage on the second sensor, but also a ratio comparison to the front sensor. If the comparison of the two sensors is within 80% of one another for more than 12 seconds, it sets the P0421 pending, and the second time it occurs it will set a hard fault.
Under normal circumstances, this would indicate a failing converter, and you would see mimicked behavior from both sensors, showing the 200-800 fluctuation on both, signaling a bad cat.
Because your situation goes away with the front sensor being changed, we know this is not the case here, and what the system is seeing is not the target range of the rear sensor being out, but rather the ratio being too similar. What is likely here is that your new sensor is stuck at high voltage, showing 600-800mV at all times, and then the catalyst which is working normally is showing a correct 600-800mV on the rear sensor but the ratio is too close and triggering the P0421 pending/hard.

There is no other reasonable explanation for this. The new sensor is either defective (stuck), of low quality (aftermarket sensors are notorious for behavior like this on Mitsubishis), or a cut and splice sensor that is wired incorrectly.

Put a new front sensor in place of the malfunctioning one and you should see both your P0421 go away as well as the P0135 heater fault.

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