Performance Max without burning cash: when to turn it on, how to feed it signals, and how to read what it doesn’t show

PMax is a black box that gets smart with data and dumb without it. Rules:

When to turn it on

Never as the account’s first campaign. First: 30+ conversions in Search/Shopping.
Product with a feed (Merchant) or landing page with a clear conversion. Unqualified leads = PMax will bring in junk.

1 PMax per product or family (not one for everything).
Audience signals: buyer list (customer upload), 30-day visitors, keywords from Search campaigns that convert. Signals don’t limit, they guide.
Asset groups: 1 per angle. 5 short headlines, 5 long headlines, 5 descriptions, 10–20 images (4:5, 1:1, 1.91:1), 3+ videos (without a video, Google creates a horrible one).
Final URL expansion: OFF (otherwise it sends people to the homepage/blog).
Brand: exclude your own brand as a campaign negative (ask support or use a brand list) — otherwise PMax steals cheap conversions from Search and looks like a genius.

“Insights” report → search terms and audiences that convert: use them to launch dedicated Search campaigns (more control, lower CPA).
Channel distribution script (Mike Rhodes PMax script) to see how much goes to Display/YouTube. If > 40% goes to Display with a bad CPA, cut the video and image from the asset group (it becomes almost Shopping).
New customers: turn on “new customers only” with a value if you already have a retargeting base in another campaign.

Start with maximize conversions without a target for 2 weeks. Then set target CPA 10% above what you observed. Make changes 1× per week, no more than 15%.

High conversions and low actual sales: duplicate conversion (tag + GA4) or “page view” marked as a conversion. Audit it.
Great CPA in the first 7 days, then it gets worse: it was eating your brand traffic. See the brand item above.