diff --git a/src/prebid.js b/src/prebid.js
index 94391343cd3..0fff218a32f 100644
--- a/src/prebid.js
+++ b/src/prebid.js
@@ -622,7 +622,7 @@ $$PREBID_GLOBAL$$.removeAdUnit = function (adUnitCode) {
  * @alias module:pbjs.requestBids
  */
 $$PREBID_GLOBAL$$.requestBids = (function() {
-  const delegate = hook('sync', function ({ bidsBackHandler, timeout, adUnits, adUnitCodes, labels, auctionId, ttlBuffer, ortb2, metrics } = {}) {
+  const delegate = hook('async', function ({ bidsBackHandler, timeout, adUnits, adUnitCodes, labels, auctionId, ttlBuffer, ortb2, metrics } = {}) {
     events.emit(REQUEST_BIDS);
     const cbTimeout = timeout || config.getConfig('bidderTimeout');
     logInfo('Invoking $$PREBID_GLOBAL$$.requestBids', arguments);
@@ -646,7 +646,7 @@ $$PREBID_GLOBAL$$.requestBids = (function() {
   });
 })();
 
-export const startAuction = hook('sync', function ({ bidsBackHandler, timeout: cbTimeout, adUnits, ttlBuffer, adUnitCodes, labels, auctionId, ortb2Fragments, metrics } = {}) {
+export const startAuction = hook('async', function ({ bidsBackHandler, timeout: cbTimeout, adUnits, ttlBuffer, adUnitCodes, labels, auctionId, ortb2Fragments, metrics } = {}) {
   const s2sBidders = getS2SBidderSet(config.getConfig('s2sConfig') || []);
   adUnits = useMetrics(metrics).measureTime('requestBids.validate', () => checkAdUnitSetup(adUnits));
 
@@ -658,85 +658,80 @@ export const startAuction = hook('sync', function ({ bidsBackHandler, timeout: c
     adUnitCodes = adUnits && adUnits.map(unit => unit.code);
   }
 
-  return new Promise((resolve) => {
-    function auctionDone(bids, timedOut, auctionId) {
-      if (typeof bidsBackHandler === 'function') {
-        try {
-          bidsBackHandler(bids, timedOut, auctionId);
-        } catch (e) {
-          logError('Error executing bidsBackHandler', null, e);
-        }
-      }
-      resolve({bids, timedOut, auctionId});
-    }
+  /*
+   * for a given adunit which supports a set of mediaTypes
+   * and a given bidder which supports a set of mediaTypes
+   * a bidder is eligible to participate on the adunit
+   * if it supports at least one of the mediaTypes on the adunit
+   */
+  adUnits.forEach(adUnit => {
+    // get the adunit's mediaTypes, defaulting to banner if mediaTypes isn't present
+    const adUnitMediaTypes = Object.keys(adUnit.mediaTypes || { 'banner': 'banner' });
+
+    // get the bidder's mediaTypes
+    const allBidders = adUnit.bids.map(bid => bid.bidder);
+    const bidderRegistry = adapterManager.bidderRegistry;
+
+    const bidders = allBidders.filter(bidder => !s2sBidders.has(bidder));
 
-    /*
-     * for a given adunit which supports a set of mediaTypes
-     * and a given bidder which supports a set of mediaTypes
-     * a bidder is eligible to participate on the adunit
-     * if it supports at least one of the mediaTypes on the adunit
-     */
-    adUnits.forEach(adUnit => {
-      // get the adunit's mediaTypes, defaulting to banner if mediaTypes isn't present
-      const adUnitMediaTypes = Object.keys(adUnit.mediaTypes || { 'banner': 'banner' });
-
-      // get the bidder's mediaTypes
-      const allBidders = adUnit.bids.map(bid => bid.bidder);
-      const bidderRegistry = adapterManager.bidderRegistry;
-
-      const bidders = allBidders.filter(bidder => !s2sBidders.has(bidder));
-
-      const tid = adUnit.ortb2Imp?.ext?.tid || generateUUID();
-      adUnit.transactionId = tid;
-      if (ttlBuffer != null && !adUnit.hasOwnProperty('ttlBuffer')) {
-        adUnit.ttlBuffer = ttlBuffer;
+    const tid = adUnit.ortb2Imp?.ext?.tid || generateUUID();
+    adUnit.transactionId = tid;
+    if (ttlBuffer != null && !adUnit.hasOwnProperty('ttlBuffer')) {
+      adUnit.ttlBuffer = ttlBuffer;
+    }
+    // Populate ortb2Imp.ext.tid with transactionId. Specifying a transaction ID per item in the ortb impression array, lets multiple transaction IDs be transmitted in a single bid request.
+    deepSetValue(adUnit, 'ortb2Imp.ext.tid', tid);
+
+    bidders.forEach(bidder => {
+      const adapter = bidderRegistry[bidder];
+      const spec = adapter && adapter.getSpec && adapter.getSpec();
+      // banner is default if not specified in spec
+      const bidderMediaTypes = (spec && spec.supportedMediaTypes) || ['banner'];
+
+      // check if the bidder's mediaTypes are not in the adUnit's mediaTypes
+      const bidderEligible = adUnitMediaTypes.some(type => includes(bidderMediaTypes, type));
+      if (!bidderEligible) {
+        // drop the bidder from the ad unit if it's not compatible
+        logWarn(unsupportedBidderMessage(adUnit, bidder));
+        adUnit.bids = adUnit.bids.filter(bid => bid.bidder !== bidder);
+      } else {
+        adunitCounter.incrementBidderRequestsCounter(adUnit.code, bidder);
       }
-      // Populate ortb2Imp.ext.tid with transactionId. Specifying a transaction ID per item in the ortb impression array, lets multiple transaction IDs be transmitted in a single bid request.
-      deepSetValue(adUnit, 'ortb2Imp.ext.tid', tid);
-
-      bidders.forEach(bidder => {
-        const adapter = bidderRegistry[bidder];
-        const spec = adapter && adapter.getSpec && adapter.getSpec();
-        // banner is default if not specified in spec
-        const bidderMediaTypes = (spec && spec.supportedMediaTypes) || ['banner'];
-
-        // check if the bidder's mediaTypes are not in the adUnit's mediaTypes
-        const bidderEligible = adUnitMediaTypes.some(type => includes(bidderMediaTypes, type));
-        if (!bidderEligible) {
-          // drop the bidder from the ad unit if it's not compatible
-          logWarn(unsupportedBidderMessage(adUnit, bidder));
-          adUnit.bids = adUnit.bids.filter(bid => bid.bidder !== bidder);
-        } else {
-          adunitCounter.incrementBidderRequestsCounter(adUnit.code, bidder);
-        }
-      });
-      adunitCounter.incrementRequestsCounter(adUnit.code);
     });
+    adunitCounter.incrementRequestsCounter(adUnit.code);
+  });
 
-    if (!adUnits || adUnits.length === 0) {
-      logMessage('No adUnits configured. No bids requested.');
-      auctionDone();
-    } else {
-      const auction = auctionManager.createAuction({
-        adUnits,
-        adUnitCodes,
-        callback: auctionDone,
-        cbTimeout,
-        labels,
-        auctionId,
-        ortb2Fragments,
-        metrics,
-      });
-
-      let adUnitsLen = adUnits.length;
-      if (adUnitsLen > 15) {
-        logInfo(`Current auction ${auction.getAuctionId()} contains ${adUnitsLen} adUnits.`, adUnits);
+  if (!adUnits || adUnits.length === 0) {
+    logMessage('No adUnits configured. No bids requested.');
+    if (typeof bidsBackHandler === 'function') {
+      // executeCallback, this will only be called in case of first request
+      try {
+        bidsBackHandler();
+      } catch (e) {
+        logError('Error executing bidsBackHandler', null, e);
       }
-
-      adUnitCodes.forEach(code => targeting.setLatestAuctionForAdUnit(code, auction.getAuctionId()));
-      auction.callBids();
     }
+    return;
+  }
+
+  const auction = auctionManager.createAuction({
+    adUnits,
+    adUnitCodes,
+    callback: bidsBackHandler,
+    cbTimeout,
+    labels,
+    auctionId,
+    ortb2Fragments,
+    metrics,
   });
+
+  let adUnitsLen = adUnits.length;
+  if (adUnitsLen > 15) {
+    logInfo(`Current auction ${auction.getAuctionId()} contains ${adUnitsLen} adUnits.`, adUnits);
+  }
+
+  adUnitCodes.forEach(code => targeting.setLatestAuctionForAdUnit(code, auction.getAuctionId()));
+  auction.callBids();
 }, 'startAuction');
 
 export function executeCallbacks(fn, reqBidsConfigObj) {
diff --git a/test/spec/unit/pbjs_api_spec.js b/test/spec/unit/pbjs_api_spec.js
index b8b82f7ca96..68fba532252 100644
--- a/test/spec/unit/pbjs_api_spec.js
+++ b/test/spec/unit/pbjs_api_spec.js
@@ -21,8 +21,7 @@ import {hook} from '../../../src/hook.js';
 import {reset as resetDebugging} from '../../../src/debugging.js';
 import $$PREBID_GLOBAL$$ from 'src/prebid.js';
 import {resetAuctionState} from 'src/auction.js';
-import {stubAuctionIndex} from '../../helpers/indexStub.js';
-import {createBid} from '../../../src/bidfactory.js';
+
 var assert = require('chai').assert;
 var expect = require('chai').expect;
 
@@ -1468,6 +1467,7 @@ describe('Unit: Prebid Module', function () {
     after(function () {
       clock.restore();
     });
+    let bidsBackHandlerStub = sinon.stub();
 
     const BIDDER_CODE = 'sampleBidder';
     let bids = [{
@@ -1504,12 +1504,11 @@ describe('Unit: Prebid Module', function () {
       'start': 1000
     }];
 
-    let spec, indexStub, auction, completeAuction;
-
     beforeEach(function () {
       logMessageSpy = sinon.spy(utils, 'logMessage');
       makeRequestsStub = sinon.stub(adapterManager, 'makeBidRequests');
       makeRequestsStub.returns(bidRequests);
+
       adUnits = [{
         code: 'adUnit-code',
         mediaTypes: {
@@ -1517,53 +1516,45 @@ describe('Unit: Prebid Module', function () {
             sizes: [[300, 250]]
           }
         },
-        transactionId: 'mock-tid',
         bids: [
           {bidder: BIDDER_CODE, params: {placementId: 'id'}},
         ]
       }];
-      indexStub = sinon.stub(auctionManager, 'index');
-      indexStub.get(() => stubAuctionIndex({adUnits, bidRequests}))
-      sinon.stub(adapterManager, 'callBids').callsFake((_, bidrequests, addBidResponse, adapterDone) => {
-        completeAuction = (bidsReceived) => {
-          bidsReceived.forEach((bid) => addBidResponse(bid.adUnitCode, Object.assign(createBid(), bid)));
-          bidRequests.forEach((req) => adapterDone.call(req));
-        }
-      })
-      const origNewAuction = auctionModule.newAuction;
-      sinon.stub(auctionModule, 'newAuction').callsFake(function (opts) {
-        auction = origNewAuction(opts);
-        return auction;
-      })
-      spec = {
+      let adUnitCodes = ['adUnit-code'];
+      let auction = auctionModule.newAuction({
+        adUnits,
+        adUnitCodes,
+        callback: bidsBackHandlerStub,
+        cbTimeout: 2000
+      });
+      let createAuctionStub = sinon.stub(auctionModule, 'newAuction');
+      createAuctionStub.returns(auction);
+    });
+
+    afterEach(function () {
+      clock.restore();
+      adapterManager.makeBidRequests.restore();
+      auctionModule.newAuction.restore();
+      utils.logMessage.restore();
+    });
+
+    it('should execute callback after timeout', function () {
+      let spec = {
         code: BIDDER_CODE,
         isBidRequestValid: sinon.stub(),
         buildRequests: sinon.stub(),
         interpretResponse: sinon.stub(),
         getUserSyncs: sinon.stub(),
-        onTimeout: sinon.stub(),
-        onSetTargeting: sinon.stub(),
+        onTimeout: sinon.stub()
       };
 
       registerBidder(spec);
       spec.buildRequests.returns([{'id': 123, 'method': 'POST'}]);
       spec.isBidRequestValid.returns(true);
       spec.interpretResponse.returns(bids);
-    });
-
-    afterEach(function () {
-      clock.restore();
-      adapterManager.makeBidRequests.restore();
-      adapterManager.callBids.restore();
-      indexStub.restore();
-      auction.getBidsReceived = () => [];
-      auctionModule.newAuction.restore();
-      utils.logMessage.restore();
-    });
 
-    it('should execute callback after timeout', function () {
       let requestObj = {
-        bidsBackHandler: sinon.stub(),
+        bidsBackHandler: null, // does not need to be defined because of newAuction mock in beforeEach
         timeout: 2000,
         adUnits: adUnits
       };
@@ -1576,13 +1567,26 @@ describe('Unit: Prebid Module', function () {
       clock.tick(1);
       assert.ok(logMessageSpy.calledWith(sinon.match(re)), 'executeCallback called');
 
-      expect(requestObj.bidsBackHandler.getCall(0).args[1]).to.equal(true,
+      expect(bidsBackHandlerStub.getCall(0).args[1]).to.equal(true,
         'bidsBackHandler should be called with timedOut=true');
 
       sinon.assert.called(spec.onTimeout);
     });
 
-    it('should execute `onSetTargeting` after setTargetingForGPTAsync', function () {
+    it('should execute callback after setTargeting', function () {
+      let spec = {
+        code: BIDDER_CODE,
+        isBidRequestValid: sinon.stub(),
+        buildRequests: sinon.stub(),
+        interpretResponse: sinon.stub(),
+        onSetTargeting: sinon.stub()
+      };
+
+      registerBidder(spec);
+      spec.buildRequests.returns([{'id': 123, 'method': 'POST'}]);
+      spec.isBidRequestValid.returns(true);
+      spec.interpretResponse.returns(bids);
+
       const bidId = 1;
       const auctionId = 1;
       let adResponse = Object.assign({
@@ -1591,7 +1595,6 @@ describe('Unit: Prebid Module', function () {
         width: 300,
         height: 250,
         adUnitCode: bidRequests[0].bids[0].adUnitCode,
-        transactionId: 'mock-tid',
         adserverTargeting: {
           'hb_bidder': BIDDER_CODE,
           'hb_adid': bidId,
@@ -1600,77 +1603,20 @@ describe('Unit: Prebid Module', function () {
         },
         bidder: bids[0].bidderCode,
       }, bids[0]);
+      auction.getBidsReceived = function() { return [adResponse]; }
+      auction.getAuctionId = () => auctionId;
 
       let requestObj = {
-        bidsBackHandler: null,
+        bidsBackHandler: null, // does not need to be defined because of newAuction mock in beforeEach
         timeout: 2000,
         adUnits: adUnits
       };
 
       $$PREBID_GLOBAL$$.requestBids(requestObj);
-      completeAuction([adResponse]);
       $$PREBID_GLOBAL$$.setTargetingForGPTAsync();
 
       sinon.assert.called(spec.onSetTargeting);
     });
-
-    describe('returns a promise that resolves', () => {
-      Object.entries({
-        'immediately, without bidsBackHandler': (req) => $$PREBID_GLOBAL$$.requestBids(req),
-        'after bidsBackHandler': (() => {
-          const bidsBackHandler = sinon.stub();
-          return function (req) {
-            return $$PREBID_GLOBAL$$.requestBids({...req, bidsBackHandler}).then(({bids, timedOut, auctionId}) => {
-              sinon.assert.calledWith(bidsBackHandler, bids, timedOut, auctionId);
-              return {bids, timedOut, auctionId};
-            })
-          }
-        })(),
-        'after a bidsBackHandler that throws': (req) => $$PREBID_GLOBAL$$.requestBids({...req, bidsBackHandler: () => { throw new Error() }})
-      }).forEach(([t, requestBids]) => {
-        describe(t, () => {
-          it('with no args, when no adUnits are defined', () => {
-            return requestBids({}).then((res) => {
-              expect(res).to.eql({
-                bids: undefined,
-                timedOut: undefined,
-                auctionId: undefined
-              });
-            });
-          });
-
-          it('on timeout', (done) => {
-            requestBids({
-              auctionId: 'mock-auctionId',
-              adUnits,
-              timeout: 10
-            }).then(({timedOut, bids, auctionId}) => {
-              expect(timedOut).to.be.true;
-              expect(bids).to.eql({});
-              expect(auctionId).to.eql('mock-auctionId');
-              done();
-            });
-            clock.tick(12);
-          });
-
-          it('with auction result', (done) => {
-            const bid = {
-              bidder: 'mock-bidder',
-              adUnitCode: adUnits[0].code,
-              transactionId: adUnits[0].transactionId
-            }
-            requestBids({
-              adUnits,
-            }).then(({bids}) => {
-              sinon.assert.match(bids[bid.adUnitCode].bids[0], bid)
-              done();
-            });
-            completeAuction([bid]);
-          })
-        })
-      })
-    })
-
     it('should transfer ttlBuffer to adUnit.ttlBuffer', () => {
       $$PREBID_GLOBAL$$.requestBids({
         ttlBuffer: 123,