Technical Information K20 Swap Wiring

For those with short reading spans

  • You must use a main relay.
  • Do not wire the main relay (E7) or fuel pump (E1) to 12V. This will damage the ECU. Both E1 and E7 are outputs which switch on relays by grounding the ECU output and do not like being forced to eat 12V.
  • Do not supply the O2 sensor heater relay with unswitched 12V. It must be a switched power source. Otherwise the relay will latch on and never switch off, so your battery will go flat quickly and the O2 sensor will burn out.
  • Do not supply the main relay with switched power. It must be unswitched power otherwise the ECU will not get backup power via the main relay coil.
  • Make sure that the intake manifold ground is good.
  • Do not rely on modified or adapted harnesses from any source - most do not work - check the wiring yourself.
  • Switch off OBDII and the multiplexer functions.


This is a reference for wiring a K20A into a race car. Incorrect wiring is cause of many problems with engine swaps and you must follow the wiring diagram below. In general you need to K20A engine wiring harness. This harness plugs straight into the ECU A and B connectors, leaving only the ECU E connector and the engine harness 20 pin connector for you to wire in.

Wiring diagram

This is a valid, tested wiring diagram from our race car. It runs perfectly without any codes (with OBDII disabled). Closed loop works correctly.

This diagram is the intellectual property of Hondata and may not be reproduced elsewhere.

Transmission Issues

Any K-series engine can be used with K-Pro in an engine swap.  But the transmission will need to be from an 02-04.  Using an 05-06 transmission will mean that the speedometer will not work, and VTEC will not engage. 

You must use the 02-04 engine harness and oxygen sensor. 


  • Important: You must use a main relay. The ECU will not work correctly otherwise as it requires constant power via the coil side of the main relay. There is no way around this, you must wire in a main relay. This cannot be stressed enough.
  • Do not wire power to the E1 'Fuel Pump Relay' output from the ECU. This will smoke the ECU up quickly.
  • You don't need to touch the engine harness. Plug it into the engine, and the A and B connectors into the ECU.
  • Make sure you feed power from the main relay to all the points in the 20 pin connector - there are three, pins 9,14 and 20.
  • The ground from the engine wiring harness to the intake manifold is critical. Without a good ground the ECU will do all sorts of weird things. It is better to put the ground on the stud from the cylinder head than the intake manifold.
  • The brake switch input is not critical. It just idles the engine up slightly at idle.
  • The O2 sensor and O2 sensor heater relay is optional for a race car. 
  • You must use a relay if you want to use the o2 sensor. The ECU looks at the voltage on the relay via pin 15 so there is no way around this.
  • E9 draws little current and doesn't need a thick wire.
  • Take the time to wire up the MIL, oil pressure and charge light. Remember on a K if your charge light comes on, that means you have no cooling as well.