Trouble code p0446 and reduced fuel efficiency gas problem in babies

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One of the more common engine trouble codes (aka OBD-II codes) is P0446, which indicates a problem with the EVAP system. This code recently came up, again, in our forums when user Branch1989 asked about poor fuel mileage on his Chevrolet Silverado.

We’ll start with P0446 because it’s the easiest to test/repair and the most likely culprit here. P0446 is a Evaporative Emission Control System Vent Control Circuit Malfunction. The EVAP system control the pressure in the system when gasoline in the closed-loop fuel system goes gaseous (evaporates). This is normal and it’s actually the "fumes" from gasoline that burn rather than the liquid, which is why gasoline in a puddle always burns at the top, rather than through.

When the vehicle is sitting, gasoline in the tank continuously evaporates into gaseous fumes. Until recently, these fumes were normally exhausted out of the car into the atmosphere, which is both inefficient and potentially dangerous. Now, they are captured and held through the EVAP system, which then feeds them into the fuel injection when the vehicle is started and running. P0446 specifically points to the vent control circuit, which controls the circuit ground which closes the vent. When the key is off, the ground is automatically made and the vent closes, allowing the system to pressurize. When the key is switched on, the vent opens and allows vapors to enter the fuel stream.

When P0446 is set, it usually means there is a leak in the system somewhere. Most likely it’s a loose or mis-aligned filler cap. Often, just taking the filler cap off then putting it back on will fix the problem. Sometimes, the cap may need to be replaced with a new one (always buy OEM specification). Clear the codes after each try (remove/replace) and drive the vehicle for a few miles, then park and turn off the engine to see if they come back. If not, you’ve fixed the issue.

Other issues could be a broken or faulty vent valve, a short in the wiring, a blockage in the valve, or a bad PCM. Try replacing the vent valve first, as it is cheapest and easiest, and check the circuit while removing the old one (and before replacing it with a new one, so you can return it if the wiring is at fault). P0430

This is likely related to the EVAP problem above, but could mean something else and may be more likely to cause more noticeable short-term fuel efficiency losses. P0430 is a Catalyst System Efficiency Below Threshold (Bank 2) code. It indicates that the oxygen sensor below the catalytic converter is reading high. This indicates that either the O2 sensor is bad or the catalytic converter is not working properly.

The first step is to test the O2 sensors – there will be one on each exhaust manifold and one below the catalytic converter. Many auto repair stores can test these for you as can any competent mechanic. Test the circuits running to and from the sensors as well, to see if there is resistance or fault. Replacing defective O2 sensors will likely remedy your problem.

If not, the problem will be a bad catalytic converter, which you are not likely to be legally allowed to replace on your own unless you are certified to do so (check local and state laws). Be sure to check your warranty as well as many vehicle manufacturers offer longer warranties on emissions parts (esp. in California-spec vehicles) and your issue may be covered. P0116

P0116 is one of those codes that usually throws along with several others, as it did here. This is a Engine Coolant Temperature Circuit Range/Performance Problem. The ECT sensor measures the temperature of the "water" (engine coolant) and is usually located on the engine block where it dips into the coolant passage. It’s usually a simple two-wire sensor with one being power supply and the other being output to the PCM (computer). The outgoing wire will change voltage based on the temperature of the coolant, thus giving a reading of current ECT.

In our current case, this may be what is causing the largely noticeable mileage loss in Branch1989’s truck. A bad sensor here will result in a lot of obvious problems, some of which include poor fuel economy, possible misfires, idle loss, and tailpipe smoke.

Several causes may be to blame besides the sensor. Usually, it’s a broken or stuck thermostat, which is allowing free-flow of the coolant through the engine no matter the temperature. This is easily fixed by replacing the thermostat and should be the first thing checked after a circuit test.

Test the circuit to and from the ECT sensor for grounding issues or breaks. If all is OK, remove the thermostat and test it or replace it with a new one. Clear the codes and try again. If this doesn’t fix it, re-test the thermostat (even if new) to be sure it’s not stuck. If that doesn’t fix it, remove the ECT sensor and replace (some shops can test these, but they are often manufacturer-specific).