2002 Subaru Legacy

Tiny
MIKE OVERHOLT
  • MEMBER
  • 2002 SUBARU LEGACY
  • 4 CYL
  • AWD
  • AUTOMATIC
  • 91,000 MILES
I had a check engine light come on and had it checked. The code is P420 which they told me is a faulty catalytic converter. Will having this problem cause my car to overheat? Because this is a dealer only item I can't afford to spend 1300+ to get this fixed if it will mean more problems. Thank You Mike
Do you
have the same problem?
Yes
No
Tuesday, December 23rd, 2008 AT 8:41 AM

1 Reply

Tiny
ZACKMAN
  • MEMBER
P0420 OBD-II Trouble Code Technical Description

Catalyst System Efficiency Below Threshold (Bank 1)

What does that mean?

The catalytic converter has an oxygen sensor in front and behind it. When the vehicle is warm and running in closed loop mode, the upstream oxygen sensor waveform reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Typically the P0420 code triggers the Check Engine Light if the readings of the two sensors are similar. This is indicative of (among other things) a converter that is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.

Symptoms

You will likely not notice any drivability problems, although there may be symptoms.

Causes

A code P0420 may mean that one or more of the following has happened:

* Leaded fuel was used where unleaded was called for
* An oxygen sensor is not reading (functioning) properly
* The engine coolant temperature sensor is not working properly
* Damaged or leaking exhaust manifold / catalytic converter / exhaust pipe
* Retarded spark timing
* The oxygen sensors in front and behind the converter are reporting too similar of readings

Possible Solutions

Some suggested steps for troubleshooting a P0420 code include:

* Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
* Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
* Inspect the downstream heated oxygen sensor (HO2), replace if necessary
* Replace the catalytic converter

One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.

DTC P0420: CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD
1. Perform test procedure. If DTC P0420 is only DTC set, go to next step. If any other DTCs exist, diagnose and repair those DTCs. Do not diagnose DTC P0420. Clear memory. Validate repair by performing appropriate drive cycle.
2. Check for exhaust leaks or air suction caused by loose or missing nuts and bolts, or damage to
exhaust system. Check especially between cylinder head and front exhaust pipe, between front
exhaust pipe and front catalytic converter, and between front and rear catalytic converters. If no
damage is found, go to next step. If damage is found, repair or replace exhaust system. Clear
memory. Validate repair by performing appropriate drive cycle.
3. Separate rear catalytic converter from rear exhaust pipe. If there is no damage at rear face of rear
catalytic converter, go to next step. If there is damage at rear face of rear catalytic converter,
replace front and rear catalytic converters. Clear memory. Validate repair by performing appropriate drive cycle.
4. Remove front catalytic converter. If there is no damage to front or rear faces of front catalytic
converter, go to next step. If there is damage at front or rear faces of front catalytic converter,
replace front catalytic converter. Clear memory. Validate repair by performing appropriate drive cycle.
5. Check rear heated oxygen sensor shield wire. If shield wire is okay, go to next step. If shield wire
is not okay, repair as required. Clear memory. Validate repair by performing appropriate drive cycle.
6. Turn ignition off. Disconnect 30-pin ECM electrical harness connector B136. Using a DVOM,
measure resistance of Black/Red wires between 30-pin ECM electrical harness connector B136
terminals No. 21 and 22 and good engine ground point. If resistance is less than 5 ohms, contact
authorized Subaru dealership because probable cause is deterioration of multiple parts. If resistance is not less than 5 ohms, repair open circuit and check for poor terminal contact in all connectors between ECM and engine ground point. Engine ground point is located at center rear of engine. Clear memory. Validate repair by performing appropriate drive cycle.

***P0420 has nothing to with your vehicle overheat, at least not directly. I would check the engine coolant temp sensor first for your overheating problem. Faulty sensor won't turn on the cooling fans.
Was this
answer
helpful?
Yes
No
Wednesday, December 24th, 2008 AT 2:10 AM

Please login or register to post a reply.

Recommended Guides