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2007 Dodge Caravan 3.8L P0420 Code: Aftermarket Converter Causes Repeated DTC - Diagnosis & Fix

Model: 2007 Dodge Carvan Fault Code: P0420 Posted: 2012-04-18 15:50

I'm a fleet mechanic in Houston, TX, and I've been working on a 2007 Dodge Caravan 3.8L that's consistently throwing a P0420 diagnostic trouble code (DTC). The vehicle is used primarily for hospital site patrols, typically traveling at around 25 mph—only occasionally going faster when fueling or towing. The issue began when the MIL illuminated. After scanning with a basic OBD-II reader, I found a P0420 code, which I initially assumed was due to a failing catalytic converter. Given my experience with similar vehicles, I replaced the aftermarket Walker Brand converter (part #49948) that I purchased from Orilley’s. After installation, the vehicle appeared to run fine—until about a week later when the check engine light returned and the P0420 code reappeared. I suspected the catalytic converter might be defective or damaged, so I replaced it under warranty with a new one. However, just days after that, another call came in stating the MIL was on again. Upon inspection, I found no visible damage to the converter, no exhaust leaks, and both oxygen sensors were functioning properly. I then replaced both upstream and downstream O2 sensors with aftermarket Bosch units (cost-effective but not OEM), and also repaired a minor exhaust gasket leak. Despite these steps, the P0420 code persists. I’ve ruled out leaded fuel use, ECT sensor failure, and mechanical damage to the converter. I’m now seeking clarity on potential root causes of this persistent P0420 code in this specific 3.8L engine application—especially given that aftermarket converters may not be compatible with certain vehicle systems or may interfere with emissions monitoring. Any insights into possible underlying issues, such as fuel trim behavior, sensor calibration, or PCM adaptation would be greatly appreciated. Thanks for your help! - Justin

Related fault codes
P0420
Comments (9)
Anonymous 2012-04-19 07:01

Let's step back—what are the fuel trims at idle (2500 RPM) and wide-open throttle (WOT)? Are there any misfires detected on your scanner? I'd recommend removing the aftermarket Bosch O2 sensors and replacing them with OEM Dodge sensors. Also, ensure you're using the correct grade of Walker converter—some models are incompatible with certain engine configurations. Lastly, some PCM units require a specific procedure to recognize that a catalytic converter has been replaced; without this, the P0420 code may persist.

Anonymous 2012-04-19 08:38

Fuel trim is +4.7 at idle, 0.0 at 2500 RPM, and 0.0 WOT—no misfires or other codes detected on my scanner. I’ve kept the Bosch sensors due to budget constraints; my supervisor strictly controls spending and would not approve a full OEM sensor replacement. The converter I installed is confirmed correct by my parts supplier. Could you clarify how to properly 'tell' the PCM that a new converter has been installed? Is there a specific procedure or scan tool command?

Anonymous 2012-04-19 11:01

What scan tool are you using for diagnostics?

Anonymous 2012-04-19 11:14

I'm using an OTC 3111, a basic OBD-II scanner with generic data lists. I've been trying to upgrade to something like a Gynesis scanner that offers more detailed fuel and sensor readings—but my company doesn't allocate funds for such tools. They expect me to perform all diagnostics with minimal equipment.

Anonymous 2012-04-19 11:47

I spoke with another mechanic in our team—he also recommended switching out the Bosch sensors for OEM Dodge units. I’ve ordered both upstream and downstream O2 sensors and will update with results later today or tomorrow morning. Thanks for the suggestion!

Anonymous 2012-04-20 10:35

I've now swapped both O2 sensors with OEM Dodge parts. After a test drive, the P0420 code remains active—no improvement so far.

Anonymous 2012-04-20 13:27

Could you please share the part number of the catalytic converter you installed? This helps determine compatibility and potential issues with aftermarket units.

Anonymous 2012-04-20 13:56

The Walker converter is part #49948 from Orilley’s. Given that this issue persists, I'm now considering taking it to a Dodge dealership for professional diagnosis and testing.

Anonymous 2012-05-15 14:55

After further investigation, I discovered that the aftermarket catalytic converter was actually causing the P0420 code—likely due to improper emissions performance or failure to meet sensor feedback requirements. After installing an OEM Dodge converter and completing a test drive, the code has not returned. This confirms that aftermarket converters may not be suitable for this model year and engine configuration.