P2646 Honda

P2646 Honda — Diagnose and fix

Shops are reporting a high incidence of P2646 Honda trouble codes on Honda CR-V, Honda Element and Accord models with the 2.4L engine. The P2646 Honda code definition is: P2646: VTEC Oil Pressure Switch Circuit Low Voltage. Honda has issued a service bulletin #13-021 to address the problem on the vehicles listed below. The vehicle may also have other trouble codes like:

P2646/P2651 (rocker arm oil pressure switch circuit low voltage).
P2647/P2652 (rocker arm oil pressure switch circuit high voltage).

Models affected by P2646 Honda and service bulletin #13-021

2003–12 Accord L4
2012–13 Civic ALL except Si and Hybrid ALL
2002–05 Civic Si
2002–09 CR-V
2011 CR-Z
2003–11 Element
2007–11 Fit

How to fix P2646 Honda trouble code

Based on the service bulletin, the rocker arm pressure switch can fail on an intermittent basis. Honda has issued an updated part; pressure switch 37250-PNE-G01 and O-ring 91319-PAA-A01.

How to test the VTEC oil switch

The VTEC oil switch is a normally closed style switch. The ECM provides a reference voltage to the switch on the blue/black wire and when the switch is closed, the reference voltage is grounded, so the ECM expects to see a large voltage drop. The ground wire is brown/yellow. Start your diagnosis by checking for good ground on the brown/yellow wire. Next, backprobe the connector to the oil switch and check for reference voltage on the blue/black wire with the engine running.

How the VTEC oil switch works

The VTEC system kicks in when RPMs reach the 2500-4000 range. Once RPMs reach that range, the ECM activates the VTEC solenoid which opens and allows oil pressure to reach the intake valve rocker arms. As the oil pressure rises, the oil pressure switch opens and stops the reference voltage from going to ground, so the ECM sees the full reference voltage instead of a large voltage drop.

What’s going on with the bad VTEC oil pressure switches

The defective switches are going into the open state at RPS 2500 and less when they should be in the closed position.

Other causes for  P2646 Honda

If you’ve replaced the VTEC oil pressure switch and still have code P2646 at RPMs in the 2500-400 range, you’ve got a low oil pressure issue, dirty oil, clogged VTEC screen or a problem with the VTEC assembly. In that case, make sure the oil filter is new to ensure the pressure isn’t restricted or in bypass mode.

©, 2020 Rick Muscoplat