2006 Dodge Dakota P0420 Issue: Cat Removal, Dummy O2 Sensors, and EVAP System Malfunction Diagnosis
I recently purchased a 2006 Dodge Dakota from an auction with 227k miles, originally suffering from two severely clogged catalytic converters—one completely blocked and one partially plugged. The truck exhibited poor performance: it lacked power, ran sluggishly, and would sputter under load. Initial diagnostic code was P0137 (Bank 1 Sensor 2), which pointed to a faulty oxygen sensor. To test drivability without emissions equipment, I hired a technician who removed both catalytic converters and installed straight pipes. He also replaced the downstream O2 sensors with dummy sensors to 'fool' the ECU and added new spark plugs as a precaution. The truck initially ran better but soon developed recurring issues: the check engine light returned after leaving the industrial park, power remained weak (only responding at 40 mph), and I noticed excessive fuel consumption—gas seemed to be being consumed rapidly with little engine response. After installing two new upstream O2 sensors, the P0420 code (Bank 1) appeared again. The ECU began stalling when the key was turned, though it would still crank and drive. I also started smelling gasoline when exiting the vehicle, along with noticeable sputtering and misfires. Three potential causes were identified: 1. Front and rear O2 sensors are reading identical values due to no catalytic converters between them—meaning dummy sensors aren’t fooling the ECU. 2. Spark plugs may be incorrect or faulty, causing misfires. 3. Both upstream O2 sensors are blackened and non-functional, leading to a rich fuel mixture. I believe the third option is most likely—the blackened sensors suggest they’re not working properly, which would explain both poor performance and the gasoline smell. I plan to replace both upstream O2 sensors tomorrow, though installation will be extremely difficult due to their location deep within the engine bay—far more challenging than on my G20 cargo van. Update: After installing new upstream sensors and clearing codes, I drove normally but noticed a return of issues by the next day. The P0230 code appeared along with a blinking 'EV' (Evaporative Emission) warning light. The EVAP system prevents fuel vapors from escaping into the atmosphere by storing them in a charcoal canister and purging them into the engine during combustion. A malfunction here could cause excessive vapor flow, leading to over-fueling or misfires. The P0230 code typically indicates issues with the fuel pump or pump module—yet I observe increased fuel consumption rather than reduced supply. This contradiction suggests either a faulty EVAP system or an incorrect high-flow fuel pump installed. I suspect that a leak in the EVAP system may be causing improper air density readings (affecting MAP and MAF sensors), leading to over-fueling, which explains both the gas smell and engine sputtering. This theory aligns with my observations of gasoline odor when exiting the vehicle. Next steps: I plan to conduct a full EVAP system inspection, including a smoke test, and verify fuel pump function. However, without proper diagnostic tools (like an OBD2 scanner that monitors live sensor data), it's difficult to confirm root causes. I’m considering taking the truck to a professional shop for further evaluation. I welcome any insights or experiences from others with similar issues on 2006 Dodge Dakotas or other vehicles with dual catalytic converters removed.
Before removing the catalytic converters, consider installing proper replacements. Always use OEM or Mopar-certified oxygen sensors—third-party sensors may not function correctly and can cause false readings or drivability issues. Use your OBD2 scanner to retrieve all stored codes and monitor real-time fuel trim values and upstream/downstream O2 sensor voltages for accurate diagnosis.