[Todd Jamison]

Greetings!

I have a 2011 Toyota Sienna 6 cylinder. For the past few weeks, I have had intermittent misfiring of the front bank of cylinders while getting up to speed on the highway.

Here are the symptoms:

When getting up to speed on the highway, going uphill, around 60-70 MPH and 3500 RPM (roughly), the flashing check engine light, ABS, and VSC lights come on and the van goes into limp mode. The vehicle also runs rough, as if it is going to stall. I am able to recreate the issue.
As soon as I restart the car, the check engine light goes off and the vehicle drives normally.

After running my OBD2 scanner, the following trouble codes were reported:

C1201: Engine Control System Malfunction
P0300: Random or Multiple Cylinder Misfire Detected
P0302: Cylinder 2 Misfire Detected
P0304: Cylinder 4 Misfire Detected
P0306: Cylinder 6 Misfire Detected

Steps I've taken so far:

The van has 110k miles on it, so I had our mechanic replace the coils, plugs, and intake plenum gaskets. All parts were OEM Denso. I hoped replacing the old plugs and coils would fix the issue but it did not...
To rule out electrical issues, I had the battery and alternator tested at two different auto stores. The scanner results indicated battery and alternator are good.

Before I take the car back in for more diagnosis, does anyone have suggestions as to what I might try next?

 

[vanlyfe2021]

I don't have experience with the 2gr fe misfiring myself, but this diagnostic flowchart might help.

Toyota Sienna Service Manual: Random / Multiple Cylinder Misfire Detected - Diagnostic trouble code chart - Sfi system - 2Gr-fe engine control system - Engine

www.tsienna.net

 

[greenskeeper]

I would inspect the wiring for that bank of cylinders, perhaps a varmit made a nest and chewed on some of the wiring?

 

[DC_Dave]

The really, really good news is that cylinders 2/4/6 are on the front. I think P0300/2/4/6 can be detected by your O2 sensor. The other good news is you needed plugs, and replacing the rear coils while you're back there isn't a bad idea if someone else is going to be spinning the wrenches. So really, you're only out the cost of the 3 front coils. Keep the old ones as spares, if you can.

You'll want to look at the front upstream O2 sensor based on these search results: P0300 P0302 P0304 P0306 o2 sensor site:toyotanation.com - Google Search

Good luck, and let us know what fixes it!